WO2024062112A1

WO2024062112A1 - Method for quantification of methane removal

Info

Publication number: WO2024062112A1
Application number: PCT/EP2023/076273
Authority: WO
Inventors: Maarten Van Herpen; Matthew S. Johnson
Original assignee: University Of Copenhagen
Priority date: 2022-09-23
Filing date: 2023-09-22
Publication date: 2024-03-28

Abstract

The present invention relates to a method of quantifying methane removal in the atmosphere. Specifically, the present invention relates to quantifying methane removal by iron-salt aerosols. The present invention also relates to the method for use for the purpose of claiming a carbon credit.

Description

Method for quantification of methane removal

Technical field

Background

In the last decades atmospheric methane levels have continued to increase, with record levels in 2021 [Dlugokencky, 2022], However, methane’s relatively short atmospheric lifetime of about 10 years and strong global warming potential of over 80 times that of CO2 per tonne in a 20 years perspective, make limiting the atmospheric methane concentration one of the greatest opportunities for mitigating climate change in the short term and keeping the world well below 2°C of warming in this century. This could help slow down and reduce the risk of disruptive climate change in the next few decades, e.g. the urgency of mitigating rapid warming in the Arctic.

There are two complementary approaches to achieving such methane reduction: the first is to reduce methane emissions and the second is to remove already emitted methane from the atmosphere.

According to Hoglund-lsaksson [Hoglund-lsaksson, 2020], the maximum global technical potential to limit methane emissions with existing technologies is about 30- 45% by 2030 and 50% by 2050. Thus there is an immediate need in the field for methods that boost removal of methane already present in the atmosphere.

The challenges for atmospheric methane removal are that (1) the chemistry of methane resembles that of a noble gas; it does not dissolve in water to an appreciable extent or form ions and salts and spends very little time on the surfaces of catalysts and adsorbents. In addition, (2) it’s atmospheric concentration at 1.8 ppm is much smaller than that of CO2 at 410 ppm, this high dilution increases the cost of direct air capture, even if there was a good mitigation technology. Iron-salt aerosols (ISA) are a promising solution for atmospheric methane removal [Meyer-Oeste, 2011 , 2012, 2021 a, 2021 b]. Reports of iron-salt aerosols in the laboratory date to 2013 [Zetszsch, 2013; Wittmer, 2015], and the existence of naturally occurring iron-salt aerosol photochemistry has been theorized based on this [Wittmer, 2017], It was demonstrated through smog chamber testing that Cl-producing iron-salt aerosols are formed when iron-oxide is mixed with sea-salt under acidic conditions. This implies that mixing of iron-oxide particles with sea spray aerosols could potentially generate chlorine through iron-salt aerosols.

ISA overcomes the challenge of the low affinity of methane for surfaces by dispersing photocatalytic reaction centres into the gas phase, and overcomes the challenge of the low reactivity of the C-H bond in methane using chlorine radicals which are uniquely suited to the task. Compared to the atmosphere’s main methane oxidant, the hydroxyl radical or OH, the chlorine atom has a much faster reaction with methane. Moreover, it requires less energy to create and the chemical mechanisms are such that it is possible to maintain higher gas-phase concentrations.

While several variations for production of ISA in the atmosphere have been proposed with the aim of reducing global warming by methane reduction [Meyer-Oeste, 2011 , 2012, 2021 a, 2021 b, Oeste, 2017], none of these proposals presents a viable method which allows for the determination of the actual amount of methane removed as a consequence of the produced ISA. There is therefore an immediate need in the field for a method which allows for determination of methane removal by iron-salt aerosols generated in the atmosphere, such as by ex post facto methods after reduction has already taken place.

Summary

The present invention is intended to address the problem of how to quantify the amount of iron-salt aerosols that are produced, or how to quantify the amount of methane that is removed by the application of iron-salt aerosols. The correct quantification is important for the commercialization of ISA generating technology, for example when a company wants to pay for a certain amount of methane removal, and requires validation of the removal amount. The correct quantification of methane removal therefore also has an important role as part of a validation method, related to carbon credits. One aspect of the present invention is a method for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the method comprising the steps: a. Providing a sample collected from within the air volume of the atmosphere, the sample comprising an indicator for the amount of iron- salt aerosol in the sample and/or comprising an indicator in the sample for the amount of methane removal by iron-salt aerosols; b. Quantifying the amount of iron-salt aerosol in the sample and/or of the indicator for the amount of methane removal from the sample by iron- salt aerosols; c. Calculating the total amount of iron-salt aerosols and/or the total amount of the indicator for methane removal by iron-salt aerosols in the air volume of the atmosphere; and d. Correlating the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere.

Another aspect of the present invention is a sensor configured for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the method comprising the steps: a. Providing a sample collected from within the air volume of the atmosphere, the sample comprising an indicator for the amount of iron- salt aerosol in the sample and/or comprising an indicator in the sample for the amount of methane removal by iron-salt aerosols; b. Quantifying the amount of iron-salt aerosol in the sample and/or of the indicator for the amount of methane removal from the sample by iron- salt aerosols; c. Calculating the total amount of iron-salt aerosols and/or the total amount of the indicator for methane removal by iron-salt aerosols in the air volume of the atmosphere; and d. Correlating the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere.

The method and the sensor as described above may be used in the field of claiming carbon credits. As such, a third aspect of the present invention is the method as described herein for use in a method of claiming a carbon credit.

As used herein, the term “carbon credit” refers to a generic term for any tradable certificate or permit representing the “right to emit” one tonne of carbon dioxide or the mass of another greenhouse gas with a carbon dioxide equivalent to one tonne of carbon dioxide. The right to emit one tonne of carbon dioxide may be interpreted as being realized by having paid an amount to have one tonne of carbon dioxide removed from the environment. Conversion of greenhouse gas emissions or reductions into carbon dioxide equivalents is a procedure well-known to persons of ordinary skill and comprises using the global warming potential for CH₄ compared to that of CO2 to convert between CH₄ and CO2 emissions. It is exemplary known in the field that methane has a 28 times greater global warming potential than carbon dioxide on a 100-year timescale, and a 84 times greater potential when the timescale is 20 years. The method of the present disclosure in some embodiments combines determining a chlorine radical concentration generated from iron-salt aerosol emissions (such as by making use of the photo-active iron amount, b¹³C-CO isotopic abundance, or CO:VOC ratio) with a sampling protocol, and modelling to constrain air flow, in order to determine a differential of concentrations for treated (such as in an emissions plume) and untreated (background, ambient) air, and to determine the amount of methane removed specifically by reaction with chlorine produced by e.g. an ISA intervention.

Detailed description

At least three variants for production of ISA have been proposed [Meyer-Oeste, 201 1 , 2012, 2021 a, 2021 b, Oeste, 2017], One variant is the co-combustion of organic iron (including carbonyl iron) additives with liquid or gaseous fuels or heating oils combusted in a ship or jet engines or by oil or gas combustors, preferably above the oceans where it can mix with sea salt spray. A second variant is the injection of vaporous ISA precursor iron compounds such as FeCh into the atmosphere. A third variant is the injection of an ultrasonic nebulized aqueous FeCh solution as an ISA precursor into a carrier gas, in which ISA is generated by water evaporation from the aerosol droplets. However these variants do not address the issue of methane removal quantification.

To quantify the amount of iron-salt aerosols or the associated amount of methane removal, it is not sufficient to only quantify the amount of iron that was present in the precursor that was burned as fuel or injected into the atmosphere. Such a method would be simple, but not sufficiently accurate in predicting the amount of iron-salt aerosols that will be produced for validation purposes, because the amount of ISA and its activity depends on local conditions such as the chemical composition of the emissions plume, humidity, temperature, cloud cover and other weather conditions, wind speed, the presence of sea salt aerosols, past history of the air parcel, particle size, distance from emission source and the chemical composition of the local atmosphere. Moreover, the efficiency for removing methane of the Cl radicals produced by the iron salt aerosol will change with local conditions including the concentrations of NOx, O3 and other volatile organic compounds (VOCs).

Moreover, Fe³⁺ will have different activity for producing gas phase chlorine depending on the chemical state of the aerosol which may range from a dilute aqueous solution such as a fog droplet, to a more concentrated solution such as sea spray aerosol, or when water evaporates from a particle in conditions of low relative humidity such as under 50%, a brine or amorphous wet salt or a salt crystal. The chloride concentration will change, changing the iron chloride chromophore and/or iron chloride containing aerosol. For example, Fe(ll l)Cl_n ^{3 n} where n ranges from 1 to 6. Each of these iron chlorides complexes will have a different rate of absorption of sunlight and quantum yield for chlorine production. In addition, aerosols may become depleted in chloride, and, depending on concentration, chloride and acidity may be replenished by uptake of HCI from the gas phase. Thus, knowledge of the amount of iron that was added must be coupled with a measurement of the amount of methane that has been removed

In the art, there are known techniques to quantify the amount of methane removal from an air mass, for example from measurements of chlorine chemistry in volcano plumes [Baker, 2011], in which chlorine chemistry is quantified using expensive and timeconsuming scientific studies. For example, the radical clock method known to persons of ordinary skill in the art may be used on air samples that are collected using aircrafts. This approach has the disadvantage (without even considering the resources needed, the cost, equipment, logistics, time of personnel) that the time resolution is low, making it hard to accurately locate and characterize the plume, and sample collection is challenging. Another method is to use Direct Optical Absorption Spectroscopy (DOAS) to measure CIO, O3 and NOx levels in the plume, and to combine this with plume modelling [Plane, 2006]. This approach is however not suited for routine measurements to quantify methane removal in a commercial application, because it requires an elaborate field study using expensive DOAS equipment in addition to timeconsuming modelling, validation and data analysis.

During LIV irradiation of iron-salt aerosols, iron(lll) chloride has an intense LIV chargetransfer absorption, and an electron is transferred from chloride to Fe(lll), resulting in photoreduction of Fe(lll) to Fe(ll), and yielding chlorine atoms (Ch) in the liquid phase. Through a number of very fast reactions, two liquid phase chlorine atoms (Ch) eventually lead to degassing of CI2. The acidity of the iron-salt aerosol makes the process catalytic in iron, because the Pourbaix diagram shows that there is a zone of stability for Fe(lll) at acidic pH and oxidizing conditions [Harnung and Johnson, 2012], due to which Fe(ll) will be re-oxidized to Fe(lll) by a variety of processes. As a result, under constant solar intensity, a steady-state is established in which Fe(lll) is photoreduced to Fe(ll) as quickly as Fe(ll) is oxidized to Fe(lll) [Zhu, 1993].

While it has been known in the field that iron-salt aerosols can induce photocatalytic breakdown of methane in the atmosphere, it has not been possible to quantify, based on air samples how much methane has been removed from the sample collection area. However, the present inventors have surprisingly found that it is possible to correlate measurement of photo-active iron from atmospheric samples with iron-salt aerosol concentrations, and further to methane removal. Based on this realization, the present inventors further identified that it is possible to correlate i) measurement of ¹³CO isotope changes, or ii) measurement of CO:ethane ratio, with iron-salt aerosol concentrations, and to methane removal. The method described herein thus utilizes these effects to quantify the iron-salt aerosol amount in the atmosphere and to correlate that to an amount of methane removal. As used herein, the term “photo-active iron” refers to salts, complexes or compounds of Fe(lll) which can take part in photoreductive production of Fe(ll) and simultaneous photooxidation of chloride to chlorine, such as via photoreductive dissolution in a ligand-to-metal charge transfer reaction. In one embodiment of the present disclosure the amount of photo-active iron is measured colorimetrically using UV-VIS spectroscopy.

In another embodiment of the present disclosure, amount of photo-active iron is measured using long path length absorbance spectroscopy (LPAS) in combination with a liquid core waveguide (LCW) flow cell.

A major element of the present invention is thus a method which makes use of a measurement of at least one of: the amount of photo-active iron in an aerosol sample, the change in <5¹³C isotope in CO in an air sample, or the change in CO:ethane ratio in an air sample, and relates this to the amount of iron-salt aerosols in the atmosphere, and/or to relate this to the amount of methane removal in the atmosphere by iron-salt aerosols.

One embodiment of the present disclosure is a method for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the method comprising the steps: a. Providing a sample collected from within the air volume of the atmosphere, the sample comprising an indicator for the amount of iron- salt aerosol in the sample and/or comprising an indicator in the sample for the amount of methane removal by iron-salt aerosols; b. Quantifying the amount of iron-salt aerosol in the sample and/or of the indicator for the amount of methane removal from the sample by iron- salt aerosols; c. Calculating the total amount of iron-salt aerosols and/or the total amount of the indicator for methane removal by iron-salt aerosols in the air volume of the atmosphere; and d. Correlating the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere. in one embodiment, the air volume of the atmosphere sampled is an emissions plume produced by an exhaust. The exhaust may in some embodiments be ejecting and/or emitting at least a source of iron, such as photo-active iron. In one embodiment, one example of an exhaust within the meaning of the present invention is an exhaust from a maritime vessel such as a ship, but may also in other embodiments be a stationary tower.

One embodiment of the present disclosure, is a method for determining an amount of methane removal by iron chloride containing aerosols in an emissions plume produced by an exhaust, the method comprising the steps: a. Providing emissions plume characteristics comprising at least plume dimensions and wind speed relative to emission source, wherein the plume dimensions are provided with a defined plume boundary condition to distinguish the outside of said emissions plume from the inside of said emissions plume; b. providing a first air sample comprising an amount of at least one selected from the group consisting of photo-active iron, CO, or CO:VOC ratio, the first air sample being collected at a first location; c. optionally, providing one or more of a second air sample comprising an amount of at least one selected from the group consisting of photo-active iron, CO, and CO:VOC ratio, the second air sample being collected at a second location; d. measuring, in the first and optionally second air samples, one or more parameters selected from the group consisting of amount of photo-active iron, 513C in CO, and CO:VOC ratio; e. correlating one or more of the measured parameters in each of the first and optionally second air samples to the amount of iron chloride containing aerosols in the air samples; and f. Calculating the total amount of methane removal by the iron chloride containing aerosols in the emissions plume based on the amount of iron chloride containing aerosols in the first and optionally second air samples.

In one embodiment, the amount of photo-active iron in the first and second air samples is averaged and correlated to an average amount of iron chloride containing aerosols in the emissions plume.

In some embodiments of the present disclosure, the first sample location is inside the emissions plume boundary, and the second sample location is outside the emissions plume boundary. In other embodiments of the present disclosure the first and second sample locations are both inside the emissions plume boundary. In one embodiment of the present disclosure, wherein sample collection of the first air sample at the first location and sample collection of the second air sample at the second location is either performed in continuous mode sampling or is sampled separately. When the sampling is performed in continuous mode, this approach may be seen as analogous to providing only one sample wherein the sample location in some embodiments span from a first location inside the plume to a second location also inside the plume.

Thus in one embodiment, the first air sample and second air sample are sampled continuously from the first sample location to the second sample location and the determined concentration averaged over the sampled distance and time.

In one embodiment of the present disclosure, sample collection of the first air sample is performed at a location within the emissions plume boundary, such as less than 100 km from the emission point, such as less than 50 km from the emission point, such as less than 10 km from the emission point, such as less than 5 km from the emission point. Preferably, the first air sample and optionally also the second air sample are collected down wind from an emissions point.

In one embodiment of the present disclosure, the amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols in the sample is calculated based on a model and/or data that includes at least one selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, plume characteristics, and sampling conditions.

In one embodiment of the present disclosure, the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols is correlated to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere based on a model and/or data that includes at least one selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, plume characteristics, and sampling conditions.

When sources of iron are emitted or ejected into the atmosphere, the amount of formed iron-salt aerosols (ISA) and its activity will strongly depend on local conditions such as the chemical composition of the emissions plume, humidity, temperature, cloud cover and other weather conditions, wind speed, the presence of sea salt aerosols, past history of the air parcel, particle size, distance from emission source and the chemical composition of the local atmosphere. Moreover, the efficiency for removing methane of the Cl radicals produced by the iron salt aerosol will change with local conditions including the concentrations of NOx, O3 and other volatile organic compounds (VOCs).

As used herein, the phrase “iron-salt aerosol” or “ISA”, refers to iron chloride containing hydrophilic aerosols composed of particles and/or droplets usually of a size ranging from 0.1 - 2 pm in diameter. The particles and/or droplets are mainly composed of FeCh, but may also comprise an amount of FeCh. In nature, FeCh is generated naturally in the atmosphere by the reaction of iron oxide particles or other iron- containing minerals and acidic sea spray particles, such as hydrogen chloride generated from sea salt spray, or other sources of acidity such as nitric acid or sulfuric acid or formic acid all of which occurs naturally in the atmosphere. Other known sources of iron may in one embodiment be biomass burning smoke or industry. As mentioned herein above iron chloride within the meaning of the present invention refers to a series of salts, compounds and/or complexes with the general formula Fe(l I l)Cl_n ³"ⁿ (n = 1 - 6), who’s concentrations will depend on the local chemical conditions. As used herein the phrases “iron-salt aerosol” and “iron chloride containing aerosol” are used interchangeably. More details on iron-salt aerosols can be found in F. Oeste, 2017.

In one embodiment, the sampling conditions are one or more selected from chemical composition of the emissions plume, humidity, temperature, cloud cover and other weather conditions, wind speed, the presence of sea salt aerosols, past history of the air parcel, particle size, distance from emission source and the chemical composition of the local atmosphere.

In one embodiment of the present disclosure, the sampling conditions are one or more selected from wind-speed, humidity, long itude/latitude coordinates, cloud coverage, degree of dilution and mixing, chemical composition, sunlight intensity, time since emission, and spectral distribution.

In one embodiment of the present disclosure, the plume characteristics are one or more selected from distance from emission source, width, bottom altitude, top altitude, wind speed relative to emission source, degree of dilution and mixing, chemical composition, and volume of air per hour.

In one embodiment of the present disclosure, the plume characteristics further includes at least one measurement and/or input selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, distance from emission source, time since emission, width, bottom altitude, top altitude, degree of dilution and mixing, chemical composition, volume of air per hour, and sampling conditions.

In one embodiment of the present disclosure, said plume characteristics are based on measured data and/or based on a dispersion model such as a gaussian plume dispersion model.

The plume to be sampled for the purpose of determining methane removal by ISA may be both naturally occurring or human in origin, such as e.g., volcano plumes after eruption, sand plumes after desert storms, and dedicated emission towers such as those found in chimneys of factories or exhaust vents on ships. In a preferred embodiment, the air volume of the atmosphere to be sampled is an emissions plume, such as the exhaust of a ship or a land-based emissions tower, preferably an emissions plume downstream and/or down wind (meaning in the direction of the wind) from an emissions point. In a most preferred embodiment, the emissions plume is from a ship.

In one embodiment of the present disclosure, the emissions may be emitted from a vessel such as a ship or floating platform, to create a plume trailing the ship, or a fixed source on land, such as a chimney, tower or air jet, or dust from the surface lofted into the atmosphere, or the emitter may be airborne such as a kite, balloon, dirigible, blimp, zeppelin, airplane or drone.

The temperature-increasing effect of methane, such as the global-warming effect of methane is most pronounced in the layer of the atmosphere known as the troposphere. It is therefore highly desirable to remove methane from these layers in particular. Thus in one embodiment of the present disclosure, the sample is collected from the troposphere, such as from an altitude between 0 m and 15 km above sea level. In more preferred embodiments, the sample is collected at an altitude between 0 m and 2 km above sea level, such as between 0 m and 200 m, such as between 200 m and 400 m, such as between 400 m and 600 m, such as between 600 m and 800 m, such as between 800 m and 1 km, such as between 1 km and 1 .5 km, such as between 1 .5 km and 2 km above sea level.

In situations where samples need to be collected at an altitude, it may be an advantage to collect air samples using a drone, preferably directly in the plume of a ship during normal operation to improve the quality of the quantification. Drones are particularly preferred for their higher sampling resolution compared to e.g., larger planes. The air sampling volume when sampling directly in the plume is reduced especially for shipping plumes because of the higher aerosol mass density, which reduces the power requirement of the drone per sampling and makes sampling via drones more viable. One example of a sampling strategy using unmanned aerial vehicles (i.e. drones) and method for data analysis using a ‘heat map’ is described in US10,416,672B2, and could be adapted for the purpose of the present invention. Similarly, the plume dimensions, variables and concentrations within the plume could be determined using the methodology described in US10,416,672B2. This could in one embodiment comprise the following steps to determine the emissions plume produced by an exhaust during operation, said emissions comprising the presence or concentration of at least one predetermined gas or the count and size of particles:

- identifying the exhaust, its position, bearing and speed;

- determining meteorological conditions in an area cruised by the exhaust;

- providing an aerial vehicle comprising: an electronic control system for controlling the vehicle's flight; at least one sensor for determining emissions in the atmosphere surrounding the vehicle; a data interface of passing signals to an external data collecting unit, said signals comprising at least one of: (a) an output signal of the at least one sensor and (b) data obtained from the output signal of the at least one sensor;

- determining a position and distribution of the emissions plume on the basis of the position, bearing and speed of the exhaust and further on the basis of said meteorological conditions;

- controlling the aerial vehicle to: fly through the emissions plume; determine said emissions in the emissions plume by means of the at least one sensor; and - transmit said signals to an external data collecting unit for further processing thereof, wherein the step of controlling the aerial vehicle comprises determining or adjusting a flight mission, including at least a flight trajectory for the aerial vehicle during flight, and wherein the step of determining or adjusting the flight trajectory is carried out on the basis of meteorological conditions, vessel position, bearing and speed, and sensor data provided by output signals of the at least one sensor in order to navigate the aerial vehicle towards a region a highest determinable gas concentration or particle count within the emissions plume.

Another approach to determine plume dimensions, relevant meteorological conditions and perform sampling measurements is to adapt the methodology used in Chen et al. (2005), which makes use of a plane equipped with with a large suite of instruments to measure gas phase compounds, particle composition, particle size distributions, and meteorological parameters. More detail on specific setup can be found in the methods section of said reference. This methodology may also be expanded to be applied to drones or other aerial vehicles to improve the measurement resolution.

Said exhaust may take the form of either a moving object, such as part of a ship or a plane, or take the form of a stationary object such as part of a factory exhaust outlet or dedicated emissions tower. Said vehicle may take the form of a plane, helicopter, drone, weather balloon or other unmanned aircraft (UAV).

In an alternative embodiment, the air sample may be collected using any one selected from a balloon, free or tethered, or a kite as the sample collection platform, the sample collection platform may in addition further comprise a sample collection unit comprising a filter or membrane, such as a fiberglass filter. The air sample may in one embodiment be a sample collected from inside an emissions plume. In one embodiment the emissions plume must as a prerequisite be a plume by a source of iron aerosols.

The sample collection unit may be configured such that only particles/aerosols or gasses of a certain composition or size are collected by the collection unit or the filter or membrane configured thereon. When fossil fuels are incinerated and burned, the combustion is usually incomplete due to insufficient oxygen levels. As a result carbon monoxide and short-chain hydrocarbons such as ethane, propylene and more are emitted. As such, the filter or membrane of the collection unit may be configured for collection such species for analyses.

In a further embodiment of the present disclosure, only particles and/or aerosols smaller than 2.5 pm are collected. In one embodiment of the present disclosure the particles and/or aerosols collected have a size from 0.1 pm to 0.5 pm, such as from 0.5 pm to 1 .0 pm, such as from 1 .0 pm to 1 .5 pm, such as from 1 .5 pm to 2.0 pm.

In one embodiment of the present disclosure, air sample collection time is between 1 second and 6 hours, such as between 1 second and 10 seconds, such as between 10 seconds and 30 seconds, such as between 30 seconds and 1 minute, such as between 1 minute and 3 minutes, such as between 3 minutes and 10 minutes, such as between 10 minutes and 30 minutes, such as between 30 minutes and 1 hour, such as between 1 hour and 2 hours, such as between 2 hours and 6 hours. Preferably, the sample collection time is between 10 seconds and 3 minutes, but this will depend on air composition and concentration. For baseline/background samples outside shipping plumes containing in the order of 5-10 ng/m³ of photo-active iron, typically 60 minutes is needed for sampling, but in shipping plumes the amount of photo active iron may be up to 10OOx higher depending on distance from the ship, resulting in less sample collection time needed. The volume of the air being sampled is between 0.16 - 500 m³, such as between 0.16 - 1 .0 m³, such as between 1.0 - 5 m³, such as between 5.0 - 20 m³, such as between 20 - 50 m³, such as between 50 - 100 m³, such as between 100 - 500 m³, but preferably between 1 - 20 m³.

In one embodiment of the present disclosure, the iron-salt aerosol source is an exhaust of a dedicated emission tower, and the preferred method for quantification of methane removal is based on measuring photo-active iron or <5¹³C CO isotope changes in an air sample collected from the plume of the emission tower. An advantage with an emission tower is that emissions take place from a fixed position, and this makes the sampling easier, and can also make the local conditions more stable and more accessible. For <5¹³C CO isotope measurements the method could benefit from the fact that such an emission source, such as a tower, may not be emitting CO itself, simplifying analysis. Also, because the background CO level is not elevated close to the emission source, the CO isotope shift will be more intense, making this method that much more preferred. While this method may be preferred in relation to stationary dedicated emissions towers, it will be apparent to the skilled person that this methodology could also be applied to emission plumes generated by non-stationary exhausts, such as those comprised on ships and planes.

The isotope effect of ¹³C versus ¹²C is known to play a role in photochemical reaction of carbon-based gasses and molecules. This is exemplary known from Johnson et al (2002) which is incorporated by reference. Therefore, the delta 13C value in e.g., CO can be linked to the amount of methane that was transformed into CO by the reaction with chloride radicals generated from emission of iron chloride containing aerosols, or ISA.

In the case of very large plumes, such as those originating from desert dust or mineral dust iron-salt aerosols, quantification using the photo-active iron, 5¹³C CO isotopic abundance, or CO:ethane ratio, it will be necessary to combine these measurements with a global atmospheric chemistry model, such as for example the chemistry climate model CAM-Chem (Community Atmospheric Model with chemistry), known to people skilled in the art.

In one embodiment of the present disclosure, air samples may be collected at a certain distance from the point of emission such as between 0 km to 5 km the emission point, such as 5 km to 10 km from the emission point, such as 10 km to 20 km from the emission point, such as 20 km to 50 km from the emission point, such as 50 km to 100 km from the emission point, such as 100 km to 200 km from the emission point. In another embodiment, but other sampling strategies may be also be used in other embodiments. For example, in another embodiment air samples are collected close the center line of the plume with increasing distance from the ship and/or emissions point. Measurements along the center line can be used to calculate the dispersion of the plume and average concentrations within the plume.

The method as disclosed herein can greatly benefit from shipping plume modelling. As explained, modelling can be used to calculate the shape of the plume. In addition, modelling can also reduce the number of samples that are needed, because it can calculate the concentrations of photo-active iron based on a calibration with only a few samples. Thus in one embodiment of the present disclosure, the described method makes use of plume modelling or chemical box modelling. The boundaries of the plume dimension used in the plume model may be set as the plume distance from emission point within which 90% of the iron-salt aerosols are located. As such, the model will determine the amount of methane removed based on 90% of the emitted iron-salt aerosols. Thus in one embodiment of the present disclosure, the method described herein quantifies an amount of methane removed by iron-salt aerosols which may be at least 50% of the total amount, such as at least 60% of the total amount, such as at least 70% of the total amount, such as at least 80% of the total amount, such as at least 90% of the total amount, such as 100% of the total amount.

As used herein, the term “plume dimension” is used interchangeably with “plume concentration field” and/or “plume concentration profile”. In one embodiment, a cut-off value of 90% may be selected, meaning that the plume has a concentration field extending from an emissions point and a boundary is figuratively set at a distance/width/height, within which 90% of emitted particles are located. That is to say, in one embodiment said plume boundary condition is defined by the percentage of emitted particles inside and outside the plume boundary, such as wherein 90% of emitted particles are found inside the plume boundary. The emitted particles could be selected from several known atmospheric trace gasses and aerosols, such as CO, CO2, NO, NO2, NOx, SO2, PM and iron chloride containing aerosols such as iron-salt aerosols.

Determination of such dimensions and/or concentration profiles of plumes can be determined in a number of ways known to the person skilled in the art. One possible way is to use DOAS, such as has been done in Cheng, 2019. DOAS is a spectroscopic method that identifies and quantifies trace gases and some short lived radicals by their unique UV-visible absorption spectrum in the open atmosphere. It is known in the art that DOAS can be used to study trace gases in shipping plumes, such as for example for SO2 and NO2 emissions.

In one embodiment, methods to determine plume concentration fields will either use changes in light transmission, or they may depend on taking air sample measurements to create a map of concentrations of species (such as PM, CO2, NOx, SO2, CO) in the plume, and then using this map to calculate plume concentration profiles. Such air sampling may be done using e.g., drones, helicopters, or weather balloons. In one embodiment wherein the emission source is a ship or a stationary tower with a known position, bearing and speed (for a tower speed and bearing is not relevant making the determination much simpler), the following steps may be taken to perform measurements and determine plume dimensions and/or concentration fields:

1 . Determine position, bearing and speed of the vessel;

2. Determine meteorological conditions, such as wind speed and direction, atmospheric stability and boundary layer height;

3. Compare speed and bearing of vessel with speed and direction of wind to calculate the relative wind speed compared to the vessel;

4. Providing an aerial vehicle such as a drone, plane or helicopter equipped with sensors, such as sensors for measuring CO2 or PM concentration at the location of the aerial vehicle, and including a GPS sensor to determine location of the aerial vehicle;

5. Controlling the aerial vehicle to navigate through the plume of the vessel while staying at the same altitude, such as first navigating in the direction of the wind speed to a certain distance and subsequently navigating in perpendicular direction from the wind speed to make a transect along the width of the plume, and recording the aerial vehicle GPS location and determining the associated local concentration of CO2 using the CO2 sensor;

6. Transmitting the data from the aerial vehicle to a data processing unit;

7. Processing the data to generate a map showing CO2 concentrations for different locations;

8. Use said map to measure the plume width as a function of distance from the vessel;

9. Repeat steps 5 - 8 at different altitudes, to create maps for different altitudes;

10. Compare the maps from different altitudes to determine the bottom and top height of the plume at different distances from the vessel; and

11 . Calculate the area of the plume at a certain distance (height times width), and multiply plume area with relative wind speed at same location to determine the volume of air passing through the plume at several distances from the vessel.

In one alternative embodiment, step 5 here above may be performed by the aerial vehicle navigating through the plume at a specific distance and altitude and in a direction perpendicular to the relative wind speed. In this way, the aerial vehicle first takes measurements outside of the plume, and then enters the plume, navigates through the center line, and out of the plume on the other side. The ‘map’ in step 7 above thereby only comprises a line with data points at one distance from the vessel. This line will show increasing CO2 concentrations inside the plume, and this can be used to calculate the plume width, for example by taking a certain percentage of CO2 above the baseline as borders of the plume, depending on the accuracy of the concentration measurement. By repeating the measurement of in step 9 at different altitudes, the height of the plume can be measured. In one embodiment, step 9 here above may be repeated at least 3 times, such as at least 5 times.

In an alternative embodiment, the plume width may be determined visually using photographs taken from above the plume using exemplary planes, drones and/or satellites. In this case, the method of making such determination could include the following steps:

1 . Taking a photograph from the side of the plume, in which the photograph includes a reference object with known dimensions. For example, the reference could be the ship itself, if it has a known size. The photograph should preferably be taken from a position that looks at the plume in a direction parallel to the wind direction.

2. Determine the relative position of the plume source, camera location and wind direction.

3. Use the reference object, and known relative positions, to add a scale to the photograph, and use this to determine plume height at several distances from the ship.

4. Taking a photograph from above the plume, using an aircraft that is flying above the plume, including a reference object with known dimensions (such as the ship itself).

5. Use the reference object to add a scale to the photograph, and use this to determine plume width at several distances from the ship.

Finally, the plume dimensions may also in some embodiments be determined by modelling based on sample measurements and/or observations fed into an atmospheric dispersion model, such as gaussian plume dispersion models known in the art. Such plume models and diffusion constants are known to practitioners in the field, for example in Seinfeld and Pandis (2016).

The method of the present disclosure makes use of measuring photo-active iron, or CO or other volatile organic compounds (VOC) from within an emission plume, such as from a ship or an emission tower of a factory. These measurements taken from within the plume may also be applied in the methodology for determining plume concentration profiles presented above in order to reduce the total needed amount of aerosol samples which can be costly to both obtain and analyze.

A further embodiment of the present disclosure is a measurement device in which the steps according to method described herein above are automatically performed by a device. Air is sampled and analytes collected on a filter. The filter is inserted in the device. An automatic mechanism uses a solvent such as purified water to extract soluble material from the filter and it is analysed using a solid state fiber optic UV spectrometer in the device. The device then loads a new filter for the next measurement. The device requires distilled water and a water waste tube, and power. The user specifies air sampling volume and sampling frequency, data is stored onboard the device for later read-out.

In one embodiment of the method described herein, methane removal is quantified based on measurements of photo-active iron, 5¹³C CO isotope ratio or CO:VOC ratio. Thus in one embodiment, the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is at least one selected from the group consisting of photo-active iron, 5¹³C CO isotope ratio, and CO:VOC ratio.

In one embodiment of the method described herein, the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is photo-active iron.

In one embodiment of the method described herein, the wherein indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is 513C CO isotopic abundance. <5¹³C is also known as the “delta C thirteen”-value, and can be determined from air samples according to various methods known in the field, exemplary Mak et al (2003) which is incorporated by reference. 513C CO isotopic abundance is used interchangeably herein with 513C CO isotope ratio.

In one embodiment of the method described herein, the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is CO:VOC ratio. The CO:ethane ratio may be determined from air samples and linked to Cl oxidation of methane according to various methods known in the field, exemplary Read et al (2009) which is incorporated by reference.

To quantify the amount of methane removal by iron-salt aerosols, it is necessary to determine the amount of methane removal per gram of iron-salt aerosol per day. Such quantification can be determined through a calibration measurement for the specific local conditions. Thus in one embodiment of the present disclosure, the method as described herein above comprises a dedicated calibration step to determine the removal rate of methane by iron-salt aerosols.

Several methods for performing such calibration measurement can be performed as highlighted in the examples below. In one embodiment of the present disclosure, Differential Optical Absorption Spectroscopy (DOAS) could be used to determine the typical rate of methane removal by iron-salt aerosols in shipping plumes in a local geographical region. DOAS is a spectroscopic method that identifies and quantifies trace gases and some short lived radicals by their unique UV-visible absorption spectrum in the open atmosphere. It is known in the art that DOAS can be used to study trace gases in shipping plumes, such as for example for SO2 and NO2 emissions [Cheng, 2019].

The above calibration will determine the concentration of chlorine radicals, and this can be used to calculate methane oxidation, using known reaction rate constants between Cl and CH₄.

In one embodiment, the calibration for methane removal by chlorine from iron-salt aerosols is estimated in an air parcel having a low background Cl radical concentration, [Cl]bk- If [Cl]bk is high, a measurement needs to be collected both inside and outside the emissions plume to determine the [Cl] generated from ISA.

Another way to calibrate the methane removal from iron-salt aerosols based on photoactive iron leves, is to calculate it through modelling, such as by a photochemical model based on the following equation:

Cl radical production

In which [Fe] is the amount of photo-active iron, i is the reaction rate constant for oxidation of Fe(ll) to Fe(lll), and k₂ is the reaction rate constant for photoreduction of Fe(lll) to Fe(ll) at reference solar intensity Io, and l_(t) is the solar intensity at time t, and photo-active iron and Cl radical production is given in mol. To calculate the total Cl radical production or total methane removal in a day, the above equation is integrated over a full day.

Another example of a calibration uses a combination of the photo-active iron and the <5¹³C CO isotopic abundance variations described in further detail below in the examples of the present application. The photo-active iron measurement provides a value for the amount of iron-salt aerosols in the sample, while the <5¹³C CO isotope measurement can provide a value for the amount of methane removal. Therefore, the combination of these two methods will provide a conversion factor to convert a photoactive iron measurement into an amount of methane removal.

Similarly and as described in further detail below in the examples of the present application, also a combination of photo-active iron measurement and CO:ethane ratio measurement can be used to determine the calibration factor by the same approach.

In one advantageous method according to the present disclosure, a <5¹³C CO isotope measurement is combined with a measurement of photo-active iron concentrations to calibrate the mass or amount of methane removed per mass or amount of Fe in iron- salt aerosols, and subsequently, this calibration factor is used when the method is applied with only photo-active iron measurements.

It is known in the art that the <5¹³C CO isotope value is highly sensitive to even small amounts of CH₄ removal by chlorine radicals [Rockmann, 1999]. Because iron-salt aerosols are removing CH₄ through formation of chlorine radicals, this makes the <5¹³C CO isotope especially suited for quantifying methane removal by iron-salt aerosols. An additional advantage is that the <5¹³C CO isotope change is representing the accumulated CH₄ removal over a distance, thus quantifies the accumulated CH₄ removal by iron-salt aerosols at a distance from the ship. This also means that the <5¹³C CO isotope change quantifies the accumulated atmospheric exposure to iron-salt aerosol chemistry at a distance from the ship, automatically taking into account local conditions such as those related to the sunlight exposure. A further advantage is that the difference between the <5¹³C CO value in the emission plume and the value outside the emission plume, gives a direct measure of the additional chlorine radical chemistry occurring within the plume relative to whatever other chemistry may be occurring in the marine boundary layer. As used herein, the marine boundary layer, or some times boundary layer, has the same meaning as when used in the field, and generally refers to a part of the atmosphere which is in contact and thereby directly influenced by the ociean, such as by exchange of large amounts of heat and moisture e.g., via turbulent transport.

The <5¹³C CO isotope relates to methane removal in the following way.

Due to an exceptionally strong carbon isotope effect in the CH₄ + Cl* reaction [Feilberg et al., 2005], this reaction enriches the remaining CH₄ in ¹³C, and it produces extremely ¹³C-depleted CO. Compared to the reactant CH₄, the isotope effect is relatively larger in the CO product because the ambient CO concentration is more than one order of magnitude lower than the one of CH₄. Rbckmann et al (1999) demonstrated that this isotope effect can be used to detect and quantify low levels of Cl during tropospheric Ozone Depletion Events (ODE) in the Arctic atmosphere [Rbckmann, 1999].

Therefore, when CH₄ is oxidized by chlorine radicals, it eventually leads to the formation of highly <5¹³C depleted CO, with a yield of approximately 90%. By measuring changes in the <5¹³C CO isotope it is therefore possible to quantify changes in methane oxidation by Cl radicals. Because iron salt aerosols generate Cl radicals, this can be used to quantify the amount of methane oxidation resulting from the iron salt aerosols. The approach is elaborated in more detail below.

For CO, the ¹³C/¹²C ratio is reported through the b¹³Cco ratio (hereafter sometimes referred to as b¹³C CO isotope, or b¹³C CO isotope value, b¹³C CO isotope ratio, or b¹³C of CO, or ¹³C in CO or b¹³C CO isotopic abundance) b¹³C = (( [¹³CO] / [¹²CO] ) - Ro) / Ro in which R_o has an accepted value of 0.01 1 18 for the isotope standard, Pedee Belemnite (PDB).

The ‘change in b¹³C CO isotope’ is the difference between the b¹³C CO isotope measurements of a sample compared with a baseline, reference or background concentration, and could be determined using measurements, a model, a model validated by measurements, statistics, or another recognized analysis method. In another embodiment of the present disclosure, the method instead makes use of the CO:ethane ratio to quantify the amount of methane removed by iron-salt aerosols in an emissions plume. This method is based partly on the fact that CO and ethane are coemitted when burning fossil fuels, with a specific ratio between CO and ethane (the ratio being around 50 for fossil fuel emissions), and partly on the fact that CO and ethane are both primarily removed from the atmosphere due to interplay with hydroxyl radicals in the atmosphere, the removal reactions taking place with a similar reaction rate [Read, 2009]. Consequently, the ambient ratio between CO and ethane is normally stable and constant and therefore preferred. However, ethane reacts quickly with Cl radicals while CO reacts very slowly with Cl radicals and due to this, a high concentration of Cl radicals will reduce ethane levels at a different rate than CO and by extensions change the ratio in the air samples. In other embodiments of the present disclosure, additional ethane may be emitted by the emitter into the atmosphere to ensure a sufficiently high ethane background.

While in the examples of the present disclosure, the CO:ethane ratio was used to quantify the amount of methane removal in an air sample, it will be apparent to a person of ordinary skill in the art that this should not be considered as limiting on the utilization of the present method, and that in principle the method can also be performed with other volatile organic compounds (VOCs) instead of ethane, such as for example propane and ethylene. The advantage of other VOCs is that these have different reaction rates with chlorine. For example, when the iron-salt aerosol concentration is very high, it is an advantage to replace ethane with a gas that reacts more slowly with chlorine, so that it does not get depleted too close to the source of iron-salt aerosols. A further advantage is that the other gases may be used to provide additional useful information to supplement, enhance or replace either CO or ethane, for example if conditions do not permit a quality measurement.

It is also possible to modify the method of the present disclosure by using any pair of gases that have a sufficiently different ratio of reaction rate coefficients with Cl and OH radicals. These gases could be selected from the atmospheric trace gases [Harnung and Johnson, 2012], for example, carbon monoxide, ethane, propane, butane, methyl chloride, 1 ,1 ,1 -trichloroethane, dimethyl sulfide, isoprene, and other volatile organic carbon species. In a further refinement the CO2 concentration can be used to determine the degree of dilution of a combustion plume with surrounding air resulting in an improved estimate of the radical concentrations.

As previously described, it may seem desirable and apparent to simply evaluate the iron content of a fuel and assume that all iron present in the fuel is converted into iron- salt aerosols upon combustion and emission into the atmosphere. Such conversion is however highly dependent on local climate and weather conditions. As such, the method of the present disclosure may also be implemented in determining the fraction of iron-additives in a fuel that form iron-salt aerosols upon emission given certain local atmospheric conditions. Following the teachings of in particular Example 1 below, such determination can be readily realized by first determining an amount of methane removal by iron-salt aerosols and subsequently comparing the amount of iron-salt aerosols needed to realize such removal to the amount of iron emissions that took place from the emissions source, having regard to the iron-additive content of the fuel.

Examples

The method of the present invention is illustratively described in the following examples based on a theoretical model and calculation for a ship that is burning iron-containing fuel (for example 25 part per million (ppm) iron in the fuel). As such, when values or properties are described as “measured” in the examples, it should be understood that these values could be measured when performing the method of the present invention using appropriate dedicated equipment, however in the present examples 1 -3, the “measured” values are reasonable theoretical values estimated based on previous empirical observations. The ship is envisioned in a remote location on an ocean near the equator under sunny weather conditions, and the exhaust plume initially rises due to the high temperature at emission, and subsequently attains neutral buoyancy and will become diluted and spread due to atmospheric mixing. Eventually the particles will be washed out of the atmosphere by wet or dry deposition. Before this happens, it will however be possible to identify and analyze individual plumes from specific ships e.g., by measuring particle size distribution as a function of time, possibly in combination with automatic ship identification system (AIS) data and meterological data. Such methodology is well known in the field, exemplary from Kivekas et al (2014) and from Ausmeel et al (2020), both of which make use of scanning mobility particle sizer (SMPS) measurements. The dimensions of the generated shipping plume can be determined through measurements or by modelling. Suitable measurements could be by optical transmission measurements utilizing satellite or drone observations or using measurements of gases emitted by the ship, such as PM, CO2, NOx, SO2, CO. PM here refers to particulate matter, a term known to persons in the field, and generally refers to a mixture of solid particles and liquid droplets in air. One advantage of measuring PM as the input for determining plume dimensions is that the PM measures particles and the iron-salt aerosols and/or iron chloride containing aerosols are also particles, so the measurement is more sensitive to these specific particles opposed to trace gasses.

Modelling can calculate the ship plume’s dimensions given local conditions, for example using Gaussian dispersion models, with inputs such as amount of emissions per time unit, wind speed, ship speed, and other inputs like sources and sinks of the modelled species.

Example 1 - determination of methane removal using measurements of photo-active iron

In this example, the dimensions of the shipping plume, determined by modelling, are shown in Table 1 below, along with the wind speed relative to the moving ship. The plume dimensions in this example are defined as the plume distance within which 90% of the iron-salt aerosols are located. Using the plume dimensions and wind speed, Table 1 also shows the theoretical volume of air that will pass through the plume at a certain distance from the ship within the modelled plume dimensions. For this example a certain turbulent mixing diffusion constant has been assumed, and such a parameter will vary depending on local conditions. Such plume models and diffusion constants are known to practitioners in the field, for example in Seinfeld, J. H., & Pandis, S. N.

(2016). Atmospheric chemistry and physics: from air pollution to climate change. John Wiley & Sons. With the input of the present example, the plume width increases with 20 km/day and the particles descent with a vertical deposition rate between 0.1 - 0.2 cm/s).

Table 1 - shipping plume dimensions and properties in relation to distance from emission source. In this example, the emission source is envisioned as a ship.

Multiple air samples are collected at a certain distance from the ship, for example by drawing air through a fiber filter (for example using a high-volume dichotomous virtual impactor with a typical flow of 335 L/min, and using a particle filter of fiberglass).

Samples are then immediately analyzed after collection to determine the concentration of photo active iron in the samples.

The concentration of photo active iron is determined by first determining the concentration of Fe(ll) colorimetrically by complexation with ferrozine which is then analyzed using standard UV-Vis spectroscopy. Next, the concentration of Fe(lll) is determined by first reducing any Fe(lll) to Fe(ll) and performing the colorimetric analysis as above using ferrozine. The difference between the two Fe(ll) concentration measurements provides a measurement of the amount of reducible Fe(lll) in the sample, and thereby the amount of photo active iron. Such chemical analysis is known to persons skilled in the art and can be performed without undue burden.

Alternatively, the concentration of Fe(ll) is determined using long path length absorbance spectroscopy (LPAS) measurements using an liquid core waveguide (LCW) flow cell [Chen, 2003], also using standard methods known in the field.

The reduction of Fe(lll) to Fe(ll) is performed by either using hydroxylamine hydrochloride reduction, or by exposing the sample to direct sunlight to photo-reduce the Fe(lll). Both approaches will provide the same result [Chen, 2003], and thus both approaches are suitable for the purpose of determining the amount of photo active iron in the sample. In the current example, aerosol sampling is envisioned performed at distances of 3, 10, 20, 50 and 100 km from the ship. Aerosol sampling is done in such a way, that the average concentration of photo-active iron within the emissions plume can be determined. This is achieved by taking multiple samples at a certain distance from the ship and calculating the average concentration of photo-active iron, but could equally be achieved by drawing air through a filter while travelling through the plume, so that the sample to be collected represents an average concentration. Several ways of performing such measurements are known in the art to persons of ordinary skill.

Table 2 below shows the subsequently calculated total emitted iron-salt aerosols (by Fe weight), calculated by multiplying the average concentration of photo-active iron (as determined from the air sample), with the calculated volume of air that is passing through the emissions plume per hour (from Table 1) at the collection site. In this example a value of 25 gram/hour of emitted iron-salt aerosols is found, which is consistent with the input of the present example of a ship that burns 1 .25 tonnes of iron-containing fuel per hour, containing 25 ppm iron in the fuel, and for which the calculation has revealed in this example that 80% of the iron emissions are converted into photo-active iron. In Table 2 it can also be seen that a measurement at a single fixed distance from the ship is sufficient to determine the iron-salt aerosol emission in the present example, because the amount of photo-active iron does not change with distance.

Table 2 - calculated total emitted iron-salt aerosols determined from measurements of photo-active iron at various distances from the ship.

Finally, the amount of methane removed as a result of iron-salt aerosols emitted by a ship can be calculated by multiplying the lifetime of the iron-salt aerosols (in days) with the amount of total emitted iron-salt aerosols (in grams), and with the amount of methane removal per gram of iron-salt aerosol per day.

The lifetime of the iron-salt aerosols will depend on local weather conditions, like wind speed, humidity, altitude and aerosol particle size, and can be estimated taking such conditions into consideration. However, the iron-salt aerosol lifetime can also be determined using only the emissions plume dimensions. In the present example, the particles in the plume are descending with a vertical deposition rate of 0.1 - 0.2 cm/s, starting from an altitude of 500 meters (as seen in the modelled height of the plume, Table 1 ), which gives an average particle lifetime of 3.9 days. To simplify the calculation, it is also possible to use a fixed number for the particle lifetime, for example based on a previous calibration performed under similar weather conditions.

The amount of methane removal per gram of iron-salt aerosol per day can be determined through a calibration measurement for the specific local conditions. Several methods for performing such calibration measurement can be performed as highlighted in the example below. In the present example, the ship is envisioned to be on a remote ocean location close to the equator, under sunny conditions with no cloud coverage. As such, Differential Optical Absorption Spectroscopy (DOAS) could be used to determine the typical rate of methane removal by iron-salt aerosols in shipping plumes in this region to be 22 g CH₄ per gram Fe per day.

DOAS is a spectroscopic method that identifies and quantifies trace gases and some short lived radicals by their unique UV-visible absorption spectrum in the open atmosphere. It is known in the art that DOAS can be used to study trace gases in shipping plumes, such as for example for SO2 and NO2 emissions [Cheng, 2019].

The above calibration will determine the concentration of chlorine radicals, and this can be used to calculate methane oxidation, using the known reaction rate constant between Cl and CH₄.

Another way to calibrate the CH₄ removal by photo-active iron, is to calculate it through modelling. To estimate the number of Cl radicals produced per day, a photochemical model is used in which, under constant solar intensity, a steady-state is established in which Fe(lll) is photo-reduced to Fe(ll) as quickly as Fe(ll) is oxidized to Fe(lll). Assuming a steady state, Cl radical production can then be calculated in the following way:

Cl radical production

In which [Fe] is the amount of photo-active iron, i is the reaction rate constant for oxidation of Fe(ll) to Fe(lll), and k₂ is the reaction rate constant for photoreduction of Fe(lll) to Fe(ll) at reference solar intensity Io, and l_(t) is the solar intensity at time t, and photo-active iron and Cl radical production is given in mol. When the photochemical model includes methane oxidation, the model will calculate methane removal by the Cl radicals that are produced. To calculate the total Cl radical production or total methane removal in a day, the above equation is integrated over a full day.

Using the above equations and integrating over time, a value of methane removal per hour at a certain time of the day can reasonably be converted into a value for methane removal by iron-salt aerosols for a full day. The equation above can also be used to calculate the total removal over the lifetime of a particle. For simplicity, the example will assume that the integrated total for one day is equal to 8 times the hourly amount at peak solar intensity. Thus, the value of 22 gram CH₄/gram Fe/ day from above, corresponds to 2.8 gram CH₄/gram Fe/hour at the maximum solar intensity during the day.

The calculated Cl radical production from iron-salt aerosol can in a preferred method be implemented in a chemical box model, to calculate the amount of CH₄ removal, taking into account local conditions of for example NOx and O3 concentrations. The result from the box model provides a more accurate calibration value to calculate CH₄ removal based on a measurement of photo-active iron.

The accuracy of the calculation may be further improved by taking into account factors such as air sample composition, humidity, long itude/latitude coordinates of the iron-salt aerosols, atmospheric composition, cloud coverage, sunlight intensity, time of year, and spectral distribution, and concentration of sea salt in the air sample. Most of these conditions affect the rate of solar excitation of the iron salt aerosol , and by extension affect the amount of methane removal, and thus by taking these conditions into account it is possible to determine a more accurate amount of methane removal per gram of iron-salt aerosol per day. For example, by taking into account the time of day of the iron emission, it is possible to calculate the total light exposure over the full lifetime of the particles. For example, emitting the iron during night time might lead to a relatively lower total light exposure over the full lifetime of the particle, as opposed to emissions during day time.

The calibration measurement will alternatively be based on the radical clock approach to quantify the role of chlorine radical chemistry inside and outside of an emissions plume. Chlorine radicals react with certain hydrocarbons more quickly than others, and this pattern is different from how hydroxyl radicals, the main atmospheric oxidant, react with the same hydrocarbons. The quantification is realized by collecting air samples for analysis by gas chromatography and measuring the changes in concentrations as a function of extent of reaction. Examples of this methodology include observations of chlorine chemistry in a volcanic plume [Baker, 2011] and in the pollution outflow of Asian cities [Baker, 2016]. This method is not suitable for quantification of methane removal by emitted iron-salt aerosols because the time resolution is too low, however, for a calibration this method is acceptable.

As illustrated in this Example, the calculation found that the theoretical ship was emitting 25 g/hour of iron-salt aerosols (by Fe weight). Using the value of 22 gram CH₄ removal per gram Fe per day, and a lifetime of 3.9 days, the total methane removal by the iron-salt aerosol emissions was calculated to be 2.1 kg CH₄ per hour of continued emissions, based on a fuel consumption of 1 .25 tonnes per hour. This equates to 1 .7 kg CH4 removal per tonne of fuel.

Assuming the ship of the present example consumes 8.800 tonne of fuel in a full year, that corresponds with 14,960 kg of methane removal. In this example, the iron content of the fuel was set at 25 ppm and 80% was calculated to be converted into iron-salt aerosols. If the ship would switch to a fuel with 250 ppm of iron then the amount of methane removal would in turn increase to 149,600 kg per year. It should be noted that for fuels with e.g., ferrocene-containing additives, iron concentrations between 25 ppm and 500 ppm are reported, and an optimal concentration appears to be around 200 ppm to improve engine performance [Elwardany, 2017],

The present example is based on a plume model with a boundary condition that includes 90% of the emitted particles. This means that about 10% of the methane removal has not been taken into account. The method may if desired be adapted to include for example 99% or even 100% of the emitted particles.

In the current examples, the method of the present invention is used to calculate methane removal. However, the invention could also be used to calculate the removal of other species that are oxidized by chlorine radicals, including especially tropospheric ozone and volatile organic compounds (VOCs) which are also known to be powerful climate forcing agents.

Example 2 - determination of methane removal usino measurements of CO isotope Another approach to determining the amount of methane removed by the iron in the ship emissions will be to include the use of measurements of changes in the <5¹³C isotope in CO in the measured air sample. Changes as referred to here are either in relation to the background air, i.e., measured outside the plume boundary; or in relation to another plume sample measured at another distance from the ship. In the present Example, the background concentration of CO is assumed to be 60 ppb. The delta C thirteen value <5¹³C can be determined from air samples according to various methods known in the field, exemplary Mak et al (2003) which is incorporated by reference.

Example 2 as described here is based on the same theoretical conditions as for Example 1 , however the ship is assumed to be burning fuel containing 120 ppm iron. In the present example, an average <5¹³C CO isotope change is calculated by comparing the average isotope value assumed in the plume with the assumed background level, and the results are correlated to the emitted iron-salt aerosols and methane oxidation caused by chloride radicals.

Sampling is envisioned done by filling flasks with air, and determining the <5¹³C CO isotope ratio in a dedicated laboratory. Flasks may for example be filled using drones, airplanes, kites or balloons, from ships or from ground based stations. In the current example, the air sampling is envisioned at distances of 3, 10, 20 and 100 km from the ship, and is performed both within the emission plume boundary, and outside the plume. Sampling is done in such a way, that the average <5¹³C CO isotope ratio within the plume can be determined at a certain distance from the ship and compared to the <5¹³C CO isotope ratio outside of the plume at the same distance. This will be achieved by taking multiple samples at a certain distance from the ship and calculating the average 5¹³C CO isotope ratio, or for example by sampling air while travelling through the plume (so that the collected sample represents an average amount).

Table 3 below shows the data that is assumed collected when working the method according to the present example. In Table 3 it can be seen that the CO level assumed in the plume is elevated relative to the assumed background due to the CO emissions by the ship engine, causing a CO concentration of above 500 ppb at 3.3 km from the ship, in this example, and as the plume disperses the CO level drops back to the ambient level of 60 ppb. This is relevant for the CO isotope, because the shift in isotope due to Cl radicals from iron-salt aerosols depends on the CO background level. In addition, the CO emitted by the ship itself also causes a change in the 513C CO isotope ratio compared to ambient air (air outside of the emissions plume), because the 5¹³C CO isotope from burning fuel will be different than the 5¹³C CO isotope from the ambient air.

To determine the 513C CO isotope effect due to iron-salt aerosols, it is therefore necessary to also determine the 513C CO isotope impact of the CO emitted by the ship. One way to do this would be to use the measured CO levels in the ship plume close to the emissions point, because this is a measure of the amount of CO emitted by the ship.

If the ¹³C isotope value of the CO in the ship exhaust is known, the background CO concentration is used to calculate the CO isotope effect of the ship exhaust, and separate it from the 513C CO isotope effect caused by emission of iron-salt aerosols. If however, as assumed in the present Example, the ¹³C isotope value of the CO of the ship exhaust is unknown, this will be solved by combining 513C CO isotope ratio measurements at two distances.

In the present example, it is utilized that CO from the emission exhaust is added to the emissions plume only at the emission location, while the CO generated by oxidation of methane due to Cl radicals from ISA increases as time progresses, and therefore also with distance from emission point. Consequently, by taking flask samples at two distances from the ship, it is possible to calculate the amount of extra CO that was generated between these two distances, and relate that to oxidation of methane due to ISA. The total mass of added CO in the shipping plume can be represented as follows in an equation:

Am = Am + Am , . ISA ship

In which AmisA is the mass of CO created through ISA, and Am_Shi_P is the mass of CO that was emitted by the ship. Because Am_Shi_P does not change with the distance to the ship, the difference of Am at two distances is therefore:

1 2 1 2 to - to = n_miSA - n_miSA

The concentration of CO and the abundance of ¹³C in the CO should be determined for ambient air, for pure ship exhaust, and for air some distance downstream and/or down wind, meaning in the direction of the wind. Additional information could be obtained from additional sampling points and/or by constructing a plume model. If an alternative method is available to establish the degree of dilution, such as measurement of CO2 or modelling, then the accuracy could be further increased. In addition, a determination of the time from emission, from modelling or measurement, and sampling at two or more points, would mean that a characterization of 5¹³C-CO and CO from the ship are not needed. It is thereby evident that one could use the increase in the abundance of ¹³C in CO between up and downstream positions to calculate the ISA impact, without knowing exactly the abundance of ¹³C in CO from the ship. Further we will validate below, the assumption/approximation that the 5¹³C-CO of the ship exhaust is the same as ambient. Table 3 below shows how in a first step the average 5¹³C-CO abundance is converted into an average amount of added 5¹³C depleted CO assuming it is fully caused by Cl radical methane removal. This is calculated using an isotope change of - 100%o divided by the total CO level in the plume (mixing ratio co in ppb), for every ppb of CO that is added. Next, the density of CO (pco = 1140 g/m³ at standard temperature and pressure) is used to convert ppb into g/m³, and this value is multiplied by the volume of air (V) that passes the plume at the specified distance per hour from Table 2 to calculate how much CO is added to the plume from the Cl radical CH₄ oxidation reaction, in g/hour at a specified distance:

The calculated CO added from Cl radical oxidation of CH₄, is an assumed value because this calculation does not take into account the impact of the CO emission in the ship exhaust on the ¹³C isotope change. For example, in Table 3 at the data point of 3.3 km, the extra CO is calculated at 7.7 ppb, but in reality substantially more CO is added to the atmosphere at this point, since the CO concentration increase from 60 ppb in the ambient background to 534 ppb in the plume at this distance. However, because the isotope ratio in the ship exhaust is close to the background isotope ratio, the isotope change caused by the fuel emitted CO is relatively small for the CO that was added through the ship exhaust.

By taking a measurement at two distances, it is possible to calculate the CO produced from ISA, independent from the CO in the ship exhaust. This is visible in the calculated extra CO in g/hour, which increases with distance from the ship. The g/hour notation denotes the accumulated CO in the total air volume that passes at the specified distance in an hour.

Table 3 - data obtained from calculation based on measured CO and <5¹³C CO isotope values as described above .

Table 4 below shows that by comparing measurements taken at two different distances from the ship, it is possible to calculate the amount of methane that is oxidized by iron- salt aerosols as the plume travelled from one distance to the other distance. In the current example, the measurement at 3.3 km distance is used as baseline, and measurements at further distances are compared to this baseline.

Using the travel time for the plume between two measurements, the mass of CO added by the Cl radical oxidation of CH₄ per hour of emissions can be calculated. This is converted into an amount of CH₄ removal using the yield of CH₄ to CO (in this example 90% is used, and this estimate could be further refined using a model and appropriate conditions).

Finally, the CH₄ removed per hour from Cl radical oxidation is related to the amount of iron-salt aerosols in the envisioned air sample, using a known removal rate (in this example a value of 2.7 gram CH₄/gram Fe/hour is used for hours with maximum solar irradiation, which is consistent with the value per day in the previous example of 22 g CH₄/g Fe/day). The resulting calculated emission of iron-salt aerosols by the ship was found to be 125 g/hour, which indeed corresponds with the expected emission based on the fuel.

Table 4 - methane removal and iron-salt aerosol emission parameters calculated by comparing measurements taken at two distances from the ship.

Now that the amount of iron-salt aerosols emitted by the ship has been quantified, the amount of methane removal by the emitted iron-salt aerosols can be calculated by multiplying the lifetime of the iron-salt aerosols (in days) with the mass of total emitted iron-salt aerosols (in grams), and with the mass of methane removal per gram of iron- salt aerosol per day. This is done in the same way as described before for Example 1 . Similarly, the total methane removal by the ship during a year is calculated in the same way as described before for Example 1 , using the total fuel consumption.

The method described here uses a pre-calibrated value for methane removal per gram Fe in iron-salt aerosols (per hour, and per day). These values can be determined in the same way as described before.

One advantage of utilizing the change in 5¹³C-CO according to the method of the present invention is that it is actually not necessary to calculate the amount of iron-salt aerosols emitted by the ship in order to be able to quantify the amount of methane removal by the ship, because the 513C CO isotope ratio method allows methane removal to be quantified first, and the iron-salt aerosol amount to be calculated (if desired) based on the methane removal. This therefore makes the method independent of the pre-calibrated value for methane removal per gram of Fe in iron-salt aerosols.

As illustrated above, in this example, the methane removal as caused by Cl radical oxidation was quantified at 343 g/hour during peak solar intensity for 1 hour of shipping emissions (Table 4). Based on this, the total methane removal per day can be approximated by adding up the methane removal for a full day for the iron-salt aerosols that were emitted in that 1 hour of emissions, using the solar intensity as scaling parameter. As mentioned before, in our examples the value for a full day is 8 times the value during the hour of peak solar intensity, which means that methane removal is 8 x 343 = 2.7 kg methane/day for 1 hour of shipping emission.

Using a lifetime of 3.9 days for the iron-salt aerosols results in a total removal of 10.7 kg methane, for 1 hour of fuel consumption (1 .25 tonne fuel). In example 1 , a value of 2.1 kg per hour was found for 1 hour of fuel consumption, which is 5 times less, and this is in agreement with the 5 times lower iron concentration in the fuel in the example 1 (25 ppm in Example 1 instead of 125 ppm as in the present Example).

In conclusion, in this example we calculate 8.6 kg methane removal per 1 .0 tonnes of fuel consumed. If the ship consumes 8800 tonnes of fuel in a full year, that corresponds to approximately 75,000 kg of methane removal. Thus, in this example using changes in CO isotopes, it is possible to quantify total methane removal without quantifying exactly how many iron-salt aerosols were formed. However, it is required to quantify what amount of methane removal could be attributed to iron-salt aerosols, and also the lifetime of the iron-salt aerosols as was determined in Example 1 , before the total methane removal by iron-salt aerosols emitted from the ship could be quantified. By also including the time of emission of the iron-salt aerosols, it would be possible to quantify total methane removal even more accurately as briefly described in Example 1 .

It is advantageous to make use of the <5¹³C CO isotope ratio, because it also can be used to quantify the amount of methane removal as a stand-alone method, and this can be combined with the previously described determination of photo-active iron to compute the amount of methane removal per amount of iron per time unit.

For example, using a value of 125 g/hour of iron-salt aerosol emission determined through photo-active iron measurements, and using the value of 343 g/hour CH₄ removal determined through <5¹³C CO isotope ratio, leads to a value of 2.74 gram CH₄ removal per gram Fe per hour during full sunlight conditions. The result may be combined with total sunlight exposure during a full day to calculate the methane removal per day (which was 22 gram CH₄/g Fe per day in the current example).

An advantage of the <5¹³C CO isotope ratio is that it is very sensitive to small changes in CH₄, and it is very specific for Cl radical removal of CH₄ (which is how iron-salt aerosols work). Another advantage is that this method is based on measuring levels of a product of methane oxidation, and in that way provides a direct measurement of methane removal rather than an indirect measurement.

Example 3 - determination of methane removal using measurements of CO:ethane ratio

As another alternative, this example describes an approach to determining an amount of removed methane that uses the ratio of CO to ethane (C2H₆) concentrations identified in collected air samples. In this method, we envision that air samples are collected and the CO and ethane concentrations determined in a laboratory. Alternatively measurements can be done in real-time in the field, for example using spectroscopic sensors mounted on a drone or unmanned aerial vehicle, or for example using a spectroscopic method like DOAS, MAX-DOAS, limb-sounding, LIDAR, infrared camera.

The present example makes use of the same situation as described earlier, of a ship in a remote ocean near the equator under sunny conditions, and this time using the low- iron fuel (25 ppm). Table 5 below shows a theoretical example of CO and ethane concentrations in air samples at difference distances from the ship.

In this example, the background air contains a CO:ethane ratio of 200, and the fossil fuel by the ship is emitting a CO:ethane ratio of 50. The background CO level is 60 ppb and the ethane background level is 0.3 ppb. Table 5 illustrates that the CO and ethane levels are elevated close to the ship due to its own CO and ethane emissions. The CO:ethane ratio close to the ship is around 50, because it is driven by the ship’s own emissions. Further away from the ship, the CO:ethane ratio increases, eventually towards the background ratio of 200. With high iron-salt aerosol concentrations, the CO:ethane ratio can reach much higher values than the background (above 200 is estimated in this example). At very far distances the CO:ethane ratio is expected to be above the background level, but at that distance the ethane concentration in the plume will have dropped to such a low level that an accurate measurement may be difficult, such as if the ethane concentration drops below the detection limit of the ethane sensor.

Table 5 - data obtained from calculations based on a theoretical level of CO and ethane concentrations in air samples at difference distances from the ship as described above.

To calculate the concentration of iron-salt aerosols and CH₄ removal, one could calculate the expected CO:ethane ratio based on only dispersion of the plume, and in this way calculate the amount of extra ethane that was removed due to the chlorine produced by the iron-salt aerosols. The calculated depletion of ethane can be used to calculate the concentration of chlorine atoms that was generated from ISA, and this can be used to calculate the amount of methane removal and iron-salt aerosols emitted by the ship.

To illustrate, between the distances of 3.33 km and 10.00 km, the CO concentration decreased from 534 to 167 ppb. The background CO level outside the plume is 60 ppb at both distances, which means that at 10.00 km distance the air was mixed with 77.5% ambient air of 60 ppb, and 22.5% air of 534 ppb, resulting in a new concentration of 22.5% x 534 ppb + 77.5% x 60 ppb = 167 ppb. Using the same mixing for ethane (which has a background concentration of 0.3 ppb), the ethane concentration at 10 km distance should be 22.5% x 9.77 ppb + 77.5% x 0.3 ppb = 2.4 ppb. However, the measured ethane concentration at 10 km distance is 1 .35 ppb, which means that ethane depletion due to Cl radicals is a factor of 1 .35 ppb / 2.4 ppb = 0.56. The total exposure time between 3.3 km and 10 km is 1 .3 hours (see table 4), and combined with the reaction rate constant for the reaction between Cl radical and ethane (k = 5.9 10^-11 /s/cm³), it is possible to calculate the average concentration of Cl radicals between distances of 3.3 and 10 km:

2.0 ■ 10⁶ atoms / cm3.

Using the reaction rate constant for the reaction between methane and Cl radical (k = 1 ■ 10 ¹³ /s/cm³) the removal of methane during this time is 0.1%, which corresponds to 1 .8 ppb at an assumed 1800 ppb background methane level. Divided by the 1 .3 hours of time between 3.3 km and 10 km distance, this means 1 .4 ppb methane removal per hour.

Using the volume of air at 10 km distance of 0.123 km³ (see table 2), the total methane removed between 3.3 and 10 km distance is 107 gram CH4 per hour of shipping emission. In this example, the efficiency for CH4 removal was relatively high compared to the previous examples, because the local atmospheric conditions used in the model were more favourable for CH4 removal, which resulted in 4.3 gram CH4 per gram Fe per hour (compared to 2.74 g CH₄ / g Fe in the previous two examples). The resulting emission of iron salt aerosols is calculated to be 25 g/hour, which is the same as in example 1 (this is expected, because the same fuel was used). To calculate the total methane removal by the ship based on the fuel consumption, it is possible to use approaches similar to examples 1 and 2.

Alternatively ship plume modelling may be combined with air sample measurements to quantify the amount of iron-salt aerosols and methane removal. Table 6 below shows what the output of such a plume model might look like, in which the model calculates the theoretical concentration of ethane and CO at different positions from the ship, taking local conditions into account, and fits this to the ethane and CO levels in air samples that will be collected at the same positions, in order to calculate the amount of iron-salt aerosols that has been generated by the ship emissions. The model calculates the local ethane depletion on top of what is expected from dilution of the plume (in which the fraction 0.9 in this example means that 10% of the ethane is depleted per hour). Based on the local depletion, the model calculates the local Cl radical concentration, and the model then calculates the local Cl radical production by iron-salt aerosols, which is used to calculate the local concentration of iron-salt aerosols. The calculation of methane removal is done analogous to the other examples.

Table 6 - output of plume modelling taking local atmospheric conditions into account as described above.

The advantage of the CO:ethane method is that is can potentially be applied using real time ethane and CO sensors in the field, depending on whether the ethane concentration is within the range of currently developed real-time ethane sensors. This makes it possible to take many measurements, and no time is lost in waiting for laboratory measurements. The CO:ethane ratio is also very sensitive to small chlorine concentrations, and therefore is especially useful for ships with for example low amounts of ISA emission. Another advantage is that the CO:ethane ratio can also be used to calculate CH₄ removal directly, without the need to calibrate the CH₄ removal by iron-salt aerosols (similar to the 513C CO isotope ratio method described in Example 2). It is however a prerequisite of the CO:ethane ratio method that the ethane background signal is not at such a low concentration that most sensors cannot take an accurate measurement.

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1 . A method for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the method comprising the steps: a. Providing a sample collected from within the air volume of the atmosphere, the sample comprising an indicator for the amount of iron- salt aerosol in the sample and/or comprising an indicator in the sample for the amount of methane removal by iron-salt aerosols; b. Quantifying the amount of iron-salt aerosol in the sample and/or of the indicator for the amount of methane removal from the sample by iron-salt aerosols; c. Calculating the total amount of iron-salt aerosols and/or the total amount of the indicator for methane removal by iron-salt aerosols in the air volume of the atmosphere; and d. Correlating the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere.

2. The method according to item 1 , wherein the air volume is an emissions plume, such as the exhaust of a ship or a land-based emissions tower, preferably an emissions plume downstream from an emissions point.

3. The method according to any one of items 1 to 2, wherein the amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols in the sample is calculated based on a model and/or data that includes at least one input/data selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, plume characteristics, and sampling conditions.

4. The method according to any one of items 1 to 3, wherein the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols is correlated to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere based on a model and/or data that includes at least one input/data selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, plume characteristics, and sampling conditions. 5. The method according to any one of items 3 to 4, wherein the sampling conditions are one or more selected from wind-speed, humidity, long itude/latitude coordinates, cloud coverage, degree of dilution and mixing, chemical composition, sunlight intensity, and spectral distribution.

6. The method according to any one of items 3 to 5, wherein the plume characteristics are one or more parameters selected from the group of: distance from emission source, width, bottom altitude, top altitude, wind speed relative to emission source, degree of dilution and mixing, chemical composition and volume of air per hour.

7. The method according to any one of items 1 to 6, wherein the sample is collected from the troposphere.

8. The method according to any one of items 1 to 7, wherein the indicator for the amount of iron-salt aerosol in the sample is at least one selected from the group consisting of photo-active iron, 513C CO isotope ratio, and CO:VOC ratio.

9. The method according to any one of items 1 to 8, wherein the indicator for the amount of methane removal by iron-salt aerosols in the sample is at least one selected from the group consisting of photo-active iron, 513C CO isotope ratio, and CO:VOC ratio.

10. The according to any one of items 1 to 9, wherein the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is photo-active iron.

11 . The according to any one of items 1 to 9, wherein the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is 513C CO isotope ratio.

12. The according to any one of items 1 to 9, wherein the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is CO:VOC ratio. 13. The method according to item 12, wherein the VOC is selected from the group consisting of ethane, propane, butane, methyl chloride, 1 ,1 ,1 -trichloroethane, dimethyl sulfide, and isoprene, preferably ethane.

14. The method according to any one of items 3 to 13, wherein the model is a plume model or a chemical box model.

15. The method according to any one of items 1 to 14, wherein the sample is collected using at least one selected from the group consisting of a drone, a balloon, and a kite, each individually selected as configured with or without a floating platform.

16. The method according to item 15, wherein each member of the group is characterized by comprising a sample collection unit, wherein the sample collection unit comprises a filter, such as a fiberglass filter.

17. The method according to claim any one of items 1 to 16, wherein the amount of methane determined is the total amount of methane removed by iron-salt aerosols.

18. The method according to any one of items 1 to 17 for use in claiming a carbon credit.

19. The method according to item 18, further comprising a step of calculating an amount of CO2 equivalents based on the calculated amount of methane removal by iron-salt aerosols in the air volume of the atmosphere.

20. A sensor configured for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the sensor comprising a sampling unit, an analysis unit, and a data processing unit configured to carry out the method according to any of items 1 -19. Items 2

1 . A method for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the method comprising the steps: a. Providing a sample collected from within the air volume of the atmosphere, the sample comprising an indicator for the amount of iron- salt aerosol in the sample and/or comprising an indicator in the sample for the amount of methane removal by iron-salt aerosols; b. Quantifying the amount of iron-salt aerosol in the sample and/or of the indicator for the amount of methane removal from the sample by iron- salt aerosols; c. Calculating the total amount of iron-salt aerosols and/or the total amount of the indicator for methane removal by iron-salt aerosols in the air volume of the atmosphere; and d. Correlating the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron-salt aerosols to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere.

3. The method according to any one of items 1 to 2, wherein the amount of iron- salt aerosols and/or of the indicator for methane removal by iron-salt aerosols in the sample is calculated based on a model and/or data that includes at least one input/data selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, plume characteristics, and sampling conditions.

4. The method according to any one of items 1 to 3, wherein the calculated total amount of iron-salt aerosols and/or of the indicator for methane removal by iron- salt aerosols is correlated to the amount of methane removal by iron-salt aerosols in the air volume of the atmosphere based on a model and/or data that includes at least one input/data selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, plume characteristics, and sampling conditions. The method according to any one of items 3 to 4, wherein the sampling conditions are one or more selected from wind-speed, humidity, longitude/latitude coordinates, cloud coverage, degree of dilution and mixing, chemical composition, sunlight intensity, and spectral distribution. The method according to any one of items 3 to 5, wherein the plume characteristics are one or more parameters selected from the group of: distance from emission source, width, bottom altitude, top altitude, wind speed relative to emission source, degree of dilution and mixing, chemical composition and volume of air per hour. The method according to any one of items 1 to 6, wherein the indicator for the amount of iron-salt aerosol in the sample is at least one selected from the group consisting of photo-active iron, 5¹³C CO isotope ratio, and CO:VOC ratio. The method according to any one of items 1 to 7, wherein the indicator for the amount of methane removal by iron-salt aerosols in the sample is at least one selected from the group consisting of photo-active iron, 5¹³C CO isotope ratio, and CO:VOC ratio. The according to any one of items 1 to 8, wherein the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is photo-active iron. The according to any one of items 1 to 8, wherein the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is b¹³C CO isotope ratio. The according to any one of items 1 to 8, wherein the indicator for the amount of iron-salt aerosol in the sample, and/or for the amount of methane removal by iron-salt aerosols in the sample is CO:VOC ratio. 12. The method according to item 11 , wherein the VOC is selected from the group consisting of ethane, propane, butane, methyl chloride, 1 ,1 ,1 -trichloroethane, dimethyl sulfide, and isoprene, preferably ethane.

13. The method according to any one of items 1 to 12, wherein the sample is collected using at least one selected from the group consisting of a drone, a balloon, and a kite, each individually selected as configured with or without a floating platform.

14. The method according to any one of items 1 to 13 for use in claiming a carbon credit, the method further comprising a step of calculating an amount of CO2 equivalents based on the calculated amount of methane removal by iron-salt aerosols in the air volume of the atmosphere.

15. A sensor configured for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the sensor comprising a sampling unit, an analysis unit, and a data processing unit configured to carry out the method according to any of items 1-14.

Claims

1 . A method for determining an amount of methane removal by iron chloride containing aerosols in an emissions plume produced by an exhaust, the method comprising the steps: a. Providing emissions plume characteristics comprising at least plume dimensions and wind speed relative to emission source, wherein the plume dimensions are provided with a defined plume boundary condition to distinguish the outside of said emissions plume from the inside of said emissions plume; b. providing a first air sample comprising an amount of at photo-active iron, the first air sample being collected at a first sample location; c. providing one or more of a second air sample comprising an amount of photo-active iron, the second air sample being collected at a second sample location; d. measuring, in the first and second air samples, the amount of photoactive iron; e. correlating the amount of photo-active iron in the first and second air sample to the amount of iron chloride containing aerosols in the first and second air samples; f. correlating the amount of iron chloride containing aerosols in the first and second air samples to the amount of methane removed by iron chloride containing aerosols by using said emissions plume characteristics; wherein the first sample location is different from the second sample location.

2. A method for determining an amount of methane removal by iron chloride containing aerosols in an emissions plume produced by a vessel exhaust, the method comprising the steps: a. Providing emissions plume characteristics comprising at least plume dimensions and wind speed relative to emission source, wherein the plume dimensions are provided with a defined plume boundary condition to distinguish the outside of said emissions plume from the inside of said emissions plume; b. providing a first air sample comprising an amount of CO, the first air sample being collected at a first sample location; c. providing one or more of a second air sample comprising an amount of CO, the second air sample being collected at a second sample location; d. measuring the 5¹³C in CO in each of the first and second CO-containing air samples; e. correlating the change and/or difference in 5¹³C in CO between the first and second air samples to an amount of extra 5¹³C depleted CO generated from methane oxidation by Cl atoms in the first and second air samples by using said emissions plume characteristics; f. correlating the amount of extra 5¹³C depleted CO generated from methane oxidation by Cl atoms to the amount of methane removal by iron chloride containing aerosols; wherein the first sample location is different from the second sample location. A method for determining an amount of methane removal by iron chloride containing aerosols in an emissions plume produced by a vessel exhaust, the method comprising the steps: a. Providing emissions plume characteristics comprising at least plume dimensions and wind speed relative to emission source, wherein the plume dimensions are provided with a defined plume boundary condition to distinguish the outside of said emissions plume from the inside of said emissions plume; b. providing a first air sample comprising a CO:VOC ratio, the first air sample being collected at a first sample location; c. providing one or more of a second air sample comprising a CO:VOC ratio, the second air sample being collected at a second sample location; d. measuring the CO:VOC ratio in each of the first and second air samples; e. quantifying the change and/or difference in CO:VOC ratio in the first and second air sample and correlate this difference to a depletion in VOC; and f. correlating said depletion in VOC to the amount of methane removal by iron chloride containing aerosols in the emissions plume using said emissions plume characteristics; wherein the first sample location is different from the second sample location. The method according to any one of the preceding claims, wherein the exhaust is part of an object selected from a ship, an emissions tower, an exhaust tower, and a plane. The method according to any one of the preceding claims, wherein the exhaust provides at least one source of iron, such as photo-active iron. The method according to any one of the preceding claims, wherein said plume boundary condition is defined by the percentage of emitted particles inside and outside the plume boundary, such as wherein 90% of emitted particles are found inside the plume boundary. The method according to claim 1 , wherein the amount of photo-active iron in the first and second air samples is averaged and correlated to an average amount of iron chloride containing aerosols in the emissions plume. The method according to any one of the preceding claims, wherein the first sample location is inside the emissions plume boundary, and the second sample location is outside the emissions plume boundary. The method according to any one of claims 1 to 7, wherein the first and second sample locations are inside the emissions plume boundary. The method according to any one of the preceding claims, wherein said plume boundary condition provide a plume wherein at least 70%, such as at least 80%, such as at least 90% of emitted particles are within the plume boundary. The method according to any one of the preceding claims, wherein sample collection of the first air sample at the first location and sample collection of the second air sample at the second location is either performed in continuous mode sampling or is sampled separately

12. The method according to any one of the preceding claims, wherein the first air sample and second air sample are sampled continuously from the first sample location to the second sample location and the determined concentration averaged over the sampled distance and time.

13. The method according to any one of the preceding claims, wherein sample collection of the first air sample is performed at a location within the emissions plume boundary, such as less than 100 km from the emission point, such as less than 50 km from the emission point, such as less than 10 km from the emission point, such as less than 5 km from the emission point.

14. The method according to claim 1 , wherein the amount of photo-active iron is measured colorimetrically using UV-VIS spectroscopy.

15. The method according to claim 1 , wherein the amount of photo-active iron is measured using long path length absorbance spectroscopy (LPAS) in combination with a liquid core waveguide (LCW) flow cell.

16. The method according to any one of the preceding claims, wherein the first and second air samples are collected down wind from an emissions point.

17. The method according to any one of the preceding claims, wherein the plume characteristics are based on measured data and/or based on a dispersion model such as a gaussian plume dispersion model.

18. The method according to any one of the preceding claims, wherein the plume characteristics further includes at least one measurement and/or input selected from the group consisting of aerosol particle size, aerosol and/or gas chemical composition, distance from emission source, time since emission, width, bottom altitude, top altitude, degree of dilution and mixing, chemical composition, volume of air per hour, and sampling conditions. The method according to claim 18, wherein the sampling conditions are one or more selected from wind-speed, humidity, longitude/latitude coordinates, cloud coverage, degree of dilution and mixing, chemical composition, sunlight intensity, time since emission and spectral distribution. The method according to claim 3, wherein the VOC is selected from the group consisting of ethane, propane, butane, methyl chloride, 1 ,1 ,1 -trichloroethane, dimethyl sulfide, and isoprene, preferably ethane. The method according to any one of the preceding claims, wherein the first and second air samples are collected using at least one selected from the group consisting of a drone, a balloon, plane, helicopter and a kite, each individually selected as configured with or without a floating platform. The method according to any one of the preceding claims, wherein the step of providing emissions plume characteristics comprises the steps: i. determine position, bearing and speed of the exhaust; ii. determine meteorological conditions, such as wind speed and direction, atmospheric stability and boundary layer height; ill. compare speed and bearing of exhaust with speed and direction of wind to calculate the relative wind speed compared to the vessel; iv. providing an aerial vehicle such as a drone, plane or helicopter equipped with sensors, such as sensors for measuring CO2 or PM concentration at the location of the aerial vehicle, and including a GPS sensor to determine location of the aerial vehicle; v. controlling the aerial vehicle to navigate through the plume of the exhaust while staying at the same altitude, such as first navigating in the direction of the wind speed to a certain distance and subsequently navigating in perpendicular direction from the wind speed to make a transect along the width of the plume, and recording the aerial vehicle GPS location and determining the associated local sensor output such as concentration of CO2 using the CO2 sensor; vi. transmitting the data from the aerial vehicle to a data processing unit; vii. processing the data to generate a map showing sensor output such as CO2 concentrations for different locations; viii. use said map to measure the plume width as a function of distance from the vessel; ix. repeat steps v - viii at different altitudes, to create maps for different altitudes; x. compare the maps from different altitudes to determine the bottom and top height of the plume at different distances from the exhaust; and xi. calculate the area of the plume at a certain distance (height times width), and multiply plume area with relative wind speed at same location to determine the volume of air passing through the plume at several distances from the vessel. A sensor configured for determining an amount of methane removal by iron-salt aerosols in an air volume of the atmosphere, the sensor comprising a sampling unit, an analysis unit, and a data processing unit configured to carry out the method according to any of claims 1 to 22.