WO2002018911A1

WO2002018911A1 - Method and installation for determining organic particles floating in the air

Info

Publication number: WO2002018911A1
Application number: PCT/NL2001/000639
Authority: WO
Inventors: Arja Even; Rene Paul Otjes
Original assignee: Stichting Energieonderzoek Centrum Nederland
Priority date: 2000-08-31
Filing date: 2001-08-30
Publication date: 2002-03-07
Also published as: AU2001296085A1; NL1016065C2

Abstract

The present invention relates to a method for determining organic particles in the air. This determination takes place by means of three steps: mixing the air with steam in order to allow aerosol to grow into droplets, separating generating droplets from the stream and determining the quantity of organic matter in the droplets. A step in which interfering gases are collected can optionally be added. The invention also comprises an installation for determining organic particles in the air.

Description

Method and installation for determining organic particles floating in the air

Aerosols are extremely fine mists of small solid or liquid particles in the atmosphere consisting of inorganic substances (ammonia, sodium chloride and other salts) or organic substances; the latter (POC: particulate organic carbon) have usually passed into the atmosphere as a result of human activity. Aerosols have a substantial influence on the quality of the environment, both because of their chemical, toxic and mutagenic properties and because of their heat-absorbing or radiation-reflecting effect. They thus have significant consequences for life on Earth and for public health as well. According to the state of the art, determination of the organic fraction in aerosols can be carried out using an ambient carbon particulate monitor (ACPM). This monitor functions on the basis of conventional sampling using an impactor, that is to say a sampling method with which a stream of air is accelerated and the particles are then collected on a plate that is perpendicular to the stream of air, and analysis consisting of volatilisation, oxidation and detection of evolved CO2 (see G. Rupprecht et al., A New Automated Monitor for the Measurement of Particulate Carbon in the Atmosphere, presented at "Particulate Matter: Health and Regulatory Issues ", Pittsburgh, PA, 4-6 April, 1995). Disadvantages of the impactor used in the abovementioned reference are the adsorption of gases and the fact that aerosols smaller than 0.14 μm are not sampled. Consequently, the correct quantity of POC is not determined. Use is also made of fine mesh filters in conventional sampling for POC (Hering et al., (1990) Aerosol Sc. and Technol. 12, 200-213). However, in this case also gas adsorption takes place and, on the other hand, lighter organic components volatilise, so that the results are not reliable.

In addition to these disadvantages, the known methods for the determination of POC have the disadvantage that they are slow: a determination takes several hours up to one day.

It has now been found that organic aerosols (POC) can be collected virtually quantitatively (without loss of volatile components) from air samples if use is made of steam injection when sampling. As a result of the quantitative and undisturbed sampling, the content of organic components can then be determined quickly and accurately. The invention therefore relates to a method and an installation for collecting organic aerosols in air, wherein:

(a) the air is brought into contact with a jet of steam in a mixing vessel, such that the aerosol present grows, (b) droplets generated are separated from the stream of air,

(c) the quantity of organic matter in the droplets collected is determined in an organic substance analyser.

Organic aerosols that can be determined using a method and installation according to the invention are, inter alia, aerosols emitted by combustion, heating, traffic, cooking, industrial processes, wear on tyres and roads, ground dust that blows up or decomposing plants, such as, inter alia, higher n-alkanes, mono- and dicarboxylic acids, terpenes, polycyclic aromatic hydrocarbons and humus acids.

The installation according to the invention comprises: an air inlet for introducing an air sample; - preferably, means, such as a denuder or filter, for separating water-soluble organic gases and particles; a mixing vessel that is connected to the denuder and that is provided with a capillary for admitting steam; a cyclone for separating air and condensed material; - an organic substance analyser.

An example of the installation according to the invention is shown in appended Figure 1. The air inlet (1) is preferably connected to a denuder (2). The function of a denuder is to collect water-soluble organic gases, for example ethanol, formic acid, acetic acid, isoprene, benzene or toluene, so that these gases can be separated from organic particles and the determination is not disturbed by gaseous compounds. The principle of separation used by the denuder is the difference in the rate of diffusion of gases and particles. The rate of diffusion is a measure of the displacement speed of gases or particles in the direction perpendicular to the stream of air. The rate of diffusion of gases and particles is 2*10^" m²/s and 5*10^"8 m²/s, respectively, so that the difference is a factor of approximately 400. In the abovementioned example of the installation, which must not be regarded as limiting, use is made of a Wet annular denuder. However, an installation making use of other denuders, such as a parallel plate denuder with plates made of active carbon or plates made of XAD resins, is also part of the invention. If a denuder is not used, a correction will usually have to be made for the adsorption of water-soluble gases. This can be effected, for example, by making use of one or more filters. These can be used to collect particles from the air sample. With this arrangement the particles remain behind on the filter and gases flow through. Filters that can be used according to the invention are, for example, fibre filters or pore filters. The filter must meet two requirements for correction measurements. Firstly, the filter must collect virtually all particles in the order of size of nanometres to micrometres and secondly the filter must not collect any gases that are relevant for the correction measurement, that is to say organic gases. The majority of commercially available filters meet the first condition. Not all filters meet the second condition. For instance, it is known, for example, that quartz fibre filters can absorb organic gases, whilst PTFE filters virtually do not do this. Filters made of the latter material are therefore also preferred, which does not preclude filters made of other materials with the same characteristics also being able to be employed. A correction measurement comprises a measurement with and a measurement without filter. The organic gases collected are determined during the measurement with filter. The measurement without filter can then be corrected for this. The measurement with filter takes place in parallel with the measurement without filter, but the measurements can also take place separately before or after one another. In a particular embodiment of the invention, a filter is used in combination with a denuder. In this embodiment a filter is positioned upstream of the denuder in order to check whether the denuder is functioning well. If it is not functioning well, a correction for the absorption of gases can be made as described above.

As shown in Figure 1, liquid such as demineralised water or a combination of buffers and/or oxidation liquids is fed from a liquid reservoir (3) via a liquid inlet (4) into the denuder. The liquid is brought into contact with the air sample in counter- current and the water-soluble gases are absorbed and retained in the liquid and then discharged together with the liquid. The abovementioned liquid is present in a small amount in the bottom of the denuder and is usually preferably discharged at the start of the denuder. Furthermore, the denuder can comprise one or more rotating tubes. The air then flows further from the wet denuder, via a connector (5), to the mixing vessel (6). The mixing vessel is preferably placed at an angle, for example at an angle of 45° (in general between 20° and 70°) for better mixing of air and steam and in order to allow the condensate to run through. Steam can be introduced from a steam generator (8) via a steam inlet (7). The steam inlet consists of a capillary. Such a capillary allows steam to pass through only when a small overpressure has been built up. This method of steam feed leads to better mixing of steam and air than does injection without overpressure. The mixing vessel, in which, as a result of the supersaturation with steam, aerosol particles grow into droplets having a diameter greater than 2 μm, is connected at the bottom via a connector (9) to a coil followed by a cyclone (10), in which the gas stream is separated from the condensed particles and is discharged via a gas outlet (11). The coil present serves only to prolong the growth time of the aerosol particles and if desired can be omitted or replaced by a straight tube. The particles collected are collected in a cyclone (10) and from there are fed via a coupling vessel (12) to one or more analysers (13) and a liquid flow meter (14). There are three reasons for using a coupling vessel. Firstly, it ensures that the sample is transported unchanged from the steam jet aerosol collector (SJAC) to the total organic carbon analyser (TOC analyser). Secondly, it effects the necessary removal of air from the sample and thirdly it provides a constant liquid stream to the TOC analyser. A debubbler (15) can optionally be positioned between the coupling vessel and a liquid flow meter. This debubbler serves to remove the air from a second partial stream. This second partial stream can be measured together with the liquid stream that is fed through the TOC analyser and the total liquid stream can be thus be determined. In the model 820 Turbo TOC analyser the sample is drawn in at a constant rate and a variable quantity of H₃PO₄ (1.5-6 mmol/ml) and an oxidising agent, preferably ammonium persulphate (0-0.03 % (m/m)), are then added. The mixture formed is then split into two streams. One of the streams is fed to the detection unit for the determination of inorganic carbon (IC). The CO₂ liberated by acidification passes through a CO₂ selective membrane here, where the CO2 content is determined by determining the conductivity. The second stream is exposed to JV light. The organic substances present are oxidised to CO2 by this means. The CO2 content is then determined in a detection unit that is identical to the unit that has just been described. The TOC content can then be determined from the difference between the total amount of carbon and the amount of inorganic carbon (IC) (for the. mode of operation of the 800/810/820 Turbo TOC analyser see Operation and Service Manual, Manual Revision J, Sievers Instruments Inc., 1997).

The use of steam injection, as described above, in the collection of aerosol in air samples is known per se. For the determination of inorganic aerosols (NH₄ ⁺, SO₄ ^2", NO3^", Cl^"), Khlystov et al. (1995) Atmospheric Environment 29, 2229, used a steam jet aerosol collector (SJAC) in which the particles/air to be determined are mixed with a jet of steam that causes the aerosol particles to grow to such a size (greater than 1 μm) that these can be separated off using a cyclone, collected and analysed. In the above- mentioned reference the use of a mixing reservoir, a steam pot and a double cyclone is described. In the present invention, use is made of only one cyclone because the collection efficiency of the cyclone has been improved by angling the mixing reservoir at, for example, 45° and by the coil. This cyclone is preceded by a coil, the purpose of which is to prolong the growth time of the aerosol droplets. In (1997) J. Aerosol Sci. 28 Suppl. 1, S443 and S445 the same authors reported the elimination of artefacts and the online analysis for this determination of inorganic aerosols.

A steam injector in accordance with the abovementioned steam jet aerosol collector is found to perform well in the method according to the invention. However, contrary to the known application of the SJAC for inorganic substances, measures have to be taken to prevent organic substances being eliminated from the analysis by adsorption between the collector and the analyser. These measures relate in particular to the choice of material for the lines between the collector and the analyser. Preference is given to materials that do not contain carbon, such as glass and metal. Preferably, no materials such as membranes and other materials with rough surfaces are used and the use of flexible tubes such as are used in peristaltic pump tubes is not desirable.

For analysis of the organic components, use can be made of a stand-alone analyser, for example a total organic carbon (TOC) analyser (for reference see above) or an online TOC 5000 from Shimadzu (see Instruction Manual Total Organic Carbon Analyzer Model 5000A, version 4.00, Shimadzu Corporation, Environmental Analysis Instruments Plant, Environmental Instruments Division, Kyoto, Japan).

If desired, inorganic aerosol components, such as, for example, the inorganic aerosol components mentioned in Khlystov et al. (1995) Atmospheric Environment 29, 2229, can also be determined by determining the quantity of inorganic matter in the collected droplets before or after step (c). Analysis of inorganic components can be carried out using conventional equipment as described, for example, in the abovementioned article by Khlystov et al.

The following tests were carried out to determine the suitability of the steam jet aerosol collector (SJAC) for determination of the organic aerosol fraction: 1. determination of the collection efficiency for hydrophobic aerosol

2. determination of the throughput of hydrophobic aerosol

3. coupling of the TOC analyser and the SJAC

4. determination of the detection efficiency of this TOC analyser 5. determination of the collection efficiency for semi- volatile material

6. the reduction of adsorption of gases by the S AC with a denuder

7. reduction of the instrument blank.

These tests are briefly described below. The SJAC was found to be suitable for determination of the organic fraction in respect of points 1, 2, 5 and 6. The TOC analyser used was found to be suitable for coupling to the SJAC (point 4). However, the flow in this instrument was reduced. The coupling between the two instruments was modified compared with previous versions to restrict losses of insoluble material (point 3).

Brief description of the tests 1. The collection efficiency for hydrophobic aerosol

The steam jet aerosol collector (SJAC) can be used for sampling aerosol. The organic aerosol fraction can consist of both hydrophobic and water-soluble aerosol. Although the SJAC has already previously been used for water-soluble aerosol (see Khlystov et al., 1995), we have found that this method can also be employed for hydrophobic aerosol.

The test was set up in the following way. A CPC (condensation particle counter) was placed downstream of the SJAC in the air stream to the pump. In the case of complete collection, this air would have to contain no particles. The test aerosol used was bis(2-ethylhexyl) sebacate, a hydrophobic substance with the chemical formula C₂₆H₅oO . The concentration of the test aerosol was 3*10 cc^" and the median diameter was less than 100 nm. The particle count was first determined without switching on the steam generation of the SJAC, so that 100 % of the aerosol count was determined. Steam generation was then switched on. The particle count fell to below 1 %. The collection efficiency was 99+1 %. Collection is based on the following physical principle: with oversaturation of a few hundred percent, water vapour condenses on all available surfaces. At this high oversaturation, the chemical composition has no influence on the occurrence of condensation and droplet growth. The aerosol grows into droplets having a diameter greater than 2 μm and can be collected using a cyclone.

2. Throughput of hydrophobic aerosol

After collection, the aerosol comes into contact with a large surface, first with the glass of the cyclone and then with the Teflon tubes. Losses, especially of insoluble material, can occur on all of these surfaces. This was tested using a suspension of insoluble organic material: soot. This suspension was dripped into the SJAC, flowed through the entire cyclone and was then collected at the bottom. The concentration in this liquid was compared with the initial concentration. This experiment was repeated with a number of different concentrations, comparable with the concentrations that occur in outside air samples, 100, 200 and 300 ppb TOC. The throughput efficiency of the material was 1 lO±lO %.

After some time a black deposit was found in the Teflon tubes and especially in the silicone coupling pieces. The silicone tubes in the peristaltic pump can give rise to losses. These materials are therefore not used in the SJAC and were not used in the test described. Only glass and metal can be used in contact with the sample liquid.

3. Coupling of the TOC analyser and the SJAC

In the original embodiment the SJAC was coupled to the analytical instrument using a peristaltic pump. Because of losses of insoluble material this was replaced in such a way that the sample liquid does not come into contact with this pump. See the diagram in Figure 1. Contact with the debubblers must also be avoided because contamination of the sample occurs in these.

4. Detection efficiency of the TOC analyser for the organic aerosol fraction

An analyser suitable for determining total organic carbon (TOC) in the sample is needed for on-line analysis of the sample. We used the model 820 Turbo TOC analyser from Sievers. Because this monitor (sic) had not previously been used for the analysis of the organic aerosol fraction, it was tested for this purpose. Two types of test components, water-soluble and suspended material, were used to test the detection of organic components. The components chosen were relevant for aerosol in the outside air.

Water-soluble test components were organic acids, aromatic hydrocarbons, humus O acid (6*10 UAM) and a persistent substance, EDTA. 100 % of these components was detected.

The suspended material consisted of polystyrene (PS) particles having a diameter of 90, 200 and 365 nm and soot black. All of the 90-nm PS particles were detected; only 20 % of the larger particles was detected. Tests with outside air aerosol should show whether such large insoluble particles are relevant for outside air aerosol. None of the soot black was detected, so that the basic premise remains the determination of the organic fraction and not the total carbon-containing fraction.

Finally, the analysis of outside air samples using the Sievers instrument was compared with an offline instrument from Shimadzu. The latter measures total carbon- containing aerosol. The result of this comparison gave a recovery of 75 % of the total carbon-containing material with the Sievers. Because it is known that the soot black concentration makes up 20-25 % of the total carbon-containing aerosol, it can be concluded that the total organic aerosol fraction has been determined. It can also be concluded that there were no insoluble particles larger than 90 nm.

5. The collection efficiency for semi- volatile material

One of the problems with conventional sampling methods for the organic aerosol fraction is volatilisation of semi-volatile components. Because there is no reference method for this aerosol fraction, a different semi-volatile component was used for which there is a reference method, specifically ammonium nitrate. The reference method is a denuder filter pack. Comparison of the sampling results from the SJAC with the reference method gives a sampling efficiency of 100 %.

It is thus demonstrated that the SJAC prevents underestimation as a result of volatilisation during sampling, which is guaranteed by enclosure of the aerosol in water droplets and immediate collection of the condensate.

6. Reduction of adsorption of gases: denuder

Another problem with conventional sampling methods is the adsorption of gaseous organic compounds in the sample. A wet annular denuder is used in order to prevent this. The efficiency of this denuder was tested by comparison of the measured concentrations with and without denuder in particle-free air. Use of a denuder lowered the measured concentrations by more than 60 %.

Water-soluble gases are collected in the denuder, so that they can no longer be absorbed in the SJAC. Hydrophobic gases are not collected by the denuder, but will also not be absorbed in the SJAC.

7. Reduction of the instrument blank

The instrument blank was reduced by replacing all silicone tubes in the instrument by tubes based on polypropylene. The debubblers were also removed from the sample stream and only ultra-pure water with a TOC content of less than 50 ppb is (sic) used.

Figure 1: Schematic diagram of the steam jet aerosol collector for organic carbon (1) air inlet

(2) denuder

(3) liquid reservoir

(4) liquid inlet

(5) connector (6) mixing vessel

(7) steam inlet

(8) steam generator

(9) connector

(10) cyclone (11) gas outlet

(12) coupling vessel

(13) analyser(s)

(14) liquid flow meter

(15) debubbler (16) liquid pump

(17) denuder controller

(18) liquid reservoir containing ultra-pure water

(19) controller for the steam generator

The straight lines indicate liquids; the broken lines indicate a signal and the undulating lines indicate current.

Claims

Claims:

1. A method for determining the organic aerosol fraction in the air, wherein:

(a) the air is brought into contact with a jet of steam in a mixing vessel in such a way that the aerosol present grows into droplets,

(b) generated droplets are separated from the gas stream,

(c) the quantity of organic matter in the collected droplets is determined.

2. A method according to Claim 1, wherein the mixing vessel and any lines with which the sample comes into contact are made of non-carbon-containing material, in particular of glass or metal.

3. A method according to any one of the preceding claims, wherein in (b) the droplets formed are separated by means of a cyclone.

4. A method according to any one of the preceding claims, wherein inorganic components in the air are also determined by determining the quantity of inorganic matter in the collected droplets before or after step (c).

5. A method according to any one of the preceding claims, wherein the quantity of organic matter is determined on-line.

6. A method according to any one of the preceding claims, wherein the water-soluble organic gases are collected before step (a).

7. Installation for determining the organic aerosol fraction in the air, which comprises: an air inlet for introducing an air sample; a vessel that is connected to the air inlet and that is provided with a capillary for admitting steam; a cyclone for separating air and condensed material; an analyser, characterised in that the analyser is an organic substance analyser.

8. Installation according to Claim 7, characterised in that the cyclone and any other connections between mixing vessel and analyser are made of metal or glass.

9. Installation according to Claim 7 or 8, characterised in that a denuder is placed between the air inlet and the mixing vessel.