CN115015210B

CN115015210B - Method for detecting content of hydroxyl radicals in gas

Info

Publication number: CN115015210B
Application number: CN202210936053.9A
Authority: CN
Inventors: 左澎; 朱彤
Original assignee: Peking University
Current assignee: Peking University
Priority date: 2022-08-05
Filing date: 2022-08-05
Publication date: 2022-11-01
Anticipated expiration: 2042-08-05
Also published as: CN115015210A

Abstract

The application provides a method for detecting the content of hydroxyl radicals in gas, which can be used in the technical field of active radical detection, in particular to the technical field of atmospheric active radical detection. The method comprises the following steps: by using the central wavelength falling on the hydroxyl radical

Transition of state to

Irradiating the gas flow to be detected in the fluorescence detection cavity by using the ultrashort pulse laser beam on the absorption band of the state so as to enable hydroxyl radicals in the gas flow to be detected to emit fluorescence signals; detecting a fluorescent signal within the fluorescent detection chamber; and determining the hydroxyl radical content in the airflow to be detected according to the detected fluorescent signal. The method for detecting the content of the hydroxyl radicals in the gas, provided by the embodiment of the application, utilizes the ultrashort characteristic of the ultrashort pulse laser, avoids the interference of secondary hydroxyl radicals caused by ozone molecules and water molecules in the atmosphere, and enables the detection of the hydroxyl radicals in the gas flow to be detected to be more accurate.

Description

Method for detecting content of hydroxyl radicals in gas

Technical Field

The application relates to the technical field of active free radical detection, in particular to the technical field of atmospheric active free radical detection, and specifically relates to a method for detecting the content of hydroxyl free radicals in gas.

Background

The atmospheric active free radicals are very reactive and therefore have very low concentrations and short residence times, but play a key role in atmospheric chemical reactions, being important sources of organic acids, carbonyl compounds, etc., which in turn generate secondary particulates. For example, hydroxyl radicals (OH) have a half-life in the atmosphere of only a few seconds and are present in extremely low concentrations, typically only 10 molecules per cubic centimeter of air⁶One (billion). And due to the diversity of atmospheric components, the complexity of atmospheric chemical reaction processes, it is difficult to accurately measure the atmospheric active free base. Therefore, how to monitor the concentration change of these trace species in real time in atmospheric observation has been a great challenge facing atmospheric chemistry research.

The technology for measuring the OH free radicals in the atmosphere needs to meet the requirements of high time resolution (preferably not more than one minute), high selectivity, in-situ measurement and single-point measurement. At present, three measuring methods capable of simultaneously meeting the requirements of the points are Laser Induced Fluorescence (LIF), long-range differential absorption spectroscopy (LP-DOAS) and Chemical Ionization Mass Spectrometry (CIMS). The detection sensitivity of CIMS is the highest (up to 10)⁵molec./cm³Below), but the required sampling time is long. The LP-DOAS method does not need calibration and has simple device structure, but has poor detection sensitivity, high requirement on the stability of an optical system and complex data processing. Compared with CIMS and LP-DOAS, LIF is the best OH free radical detection means with comprehensive performance. As shown in FIG. 1, LIF is typically measured by exciting OH molecules at 282nm

Transition to

The state of the optical disk is changed into a state,

relaxation of the state via an internal transition pathway to

State, then returned to the ground state via radiative transition

Fluorescence is emitted (in the 308nm region), so that excitation is at 282nm and a fluorescence signal is collected in the 308nm region.

The LIF method is initially applied to the detection of OH free radicals in a stratosphere, has high detection sensitivity and good selectivity, and the detection limit can reach 5 multiplied by 10⁴molec./cm³. However, when OH is measured by using this technique in the troposphere (troposphere: a part of the atmosphere from the earth's surface to the height of 10-12 km and stratosphere: a part of the atmosphere from the height of 10-50 km), the ozone molecule O is used₃And water molecule H₂The interference of O may cause a trouble.

Disclosure of Invention

In view of the problems in the prior art, embodiments of the present application provide a method for detecting the content of hydroxyl radicals in a gas, which can at least partially solve the problems in the prior art.

In one aspect, an embodiment of the present application provides a method for detecting a content of hydroxyl radicals in a gas, including:

using a central wavelength falling on a hydroxyl radical

Transition of state to

Irradiating the airflow to be detected in the fluorescence detection cavity by using the ultrashort pulse laser beam on the state absorption band so as to enable hydroxyl radicals in the airflow to be detected to emit a fluorescence signal;

detecting a fluorescent signal within the fluorescent detection chamber;

and determining the hydroxyl radical content in the airflow to be detected according to the detected fluorescent signal.

Optionally, the ultrashort pulse laser beam is obtained by performing frequency conversion processing on a laser beam emitted by a preset laser.

Optionally, the utilization center wavelength falls on the hydroxyl radical

Transition of state to

The ultrashort pulse laser beam on the absorption band of the state irradiates the airflow to be detected in the fluorescence detection cavity, and the ultrashort pulse laser beam comprises: controlling the central wavelength to fall in the hydroxyl radical

Transition of state to

And irradiating the gas flow to be detected in the fluorescence detection cavity by using the ultrashort pulse laser beam on the absorption band of the state at a preset irradiation frequency, wherein the flow speed of the gas flow to be detected is greater than the product of the preset irradiation frequency and the diameter of the ultrashort pulse laser beam.

Optionally, the air pressure in the fluorescence detection cavity is 0.001 to 1 atmosphere.

Optionally, the air pressure in the fluorescence detection cavity is 0.01-0.05 atmosphere.

Optionally, the detecting the fluorescence signal in the fluorescence detection cavity includes:

collecting a first optical signal within the fluorescence detection chamber;

filtering the collected first optical signal to obtain a second optical signal, wherein the second optical signal comprises a fluorescent signal;

detecting a fluorescent signal in the second optical signal.

Optionally, the filtering the collected first optical signal to obtain a second optical signal includes: and filtering stray light with the wavelength of 306 nm-312 nm in the first optical signal to obtain a second optical signal with the wavelength of 306 nm-312 nm.

Optionally, the detecting the fluorescent signal in the second optical signal includes: and after the ultrashort pulse laser beam irradiates the airflow to be detected in the fluorescence detection cavity, detecting the fluorescence signal in the second optical signal after delaying for a preset time.

Optionally, when the second optical signal includes a resonance raman scattering signal of the ozone molecule, the preset time length is greater than or equal to a duration of the resonance raman scattering signal of the ozone molecule.

Optionally, the method further includes: detecting a background light noise signal of the fluorescent signal after detecting the fluorescent signal;

the determining the hydroxyl radical content in the gas flow to be detected according to the detected fluorescent signal comprises: and determining the content of hydroxyl radicals in the airflow to be detected according to the detected fluorescent signal and the background light noise signal.

Optionally, the detection duration of the fluorescence signal is determined according to the lifetime of the fluorescence signal, and the detection duration of the background light noise signal is equal to the detection duration of the fluorescence signal.

The method for detecting the content of the hydroxyl radicals in the gas provided by the embodiment of the application utilizes the fact that the central wavelength falls on the hydroxyl radicals

Transition of state to

Irradiating the airflow to be detected in the fluorescence detection cavity by using the ultrashort pulse laser beam on the state absorption band so as to enable hydroxyl radicals in the airflow to be detected to emit a fluorescence signal; detecting a fluorescent signal within the fluorescent detection chamber; and determining the hydroxyl radical content in the airflow to be detected according to the detected fluorescent signal. Therefore, by using the ultrashort characteristic of the ultrashort pulse laser, the interference of secondary hydroxyl radicals caused by ozone molecules and water molecules in the atmosphere is avoided, and the detection of the hydroxyl radicals in the airflow to be detected is more accurate.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts. In the drawings:

FIG. 1 is a diagram of hydroxyl radical (OH) energy levels.

FIG. 2 is a schematic diagram of a fluorescence process of exciting OH free radicals by a nanosecond narrow-band pulse laser in the prior art.

Fig. 3 is a schematic diagram of the effect of the photomultiplier tube high voltage power switch on the gain effect.

Fig. 4 is a schematic flowchart of a method for detecting OH free radical content in a gas according to an embodiment of the present disclosure.

FIG. 5 shows OH radical absorption spectrum and ozone (O)₃) Absorption spectrum, femtosecond laser pulse spectrogram.

FIG. 6 shows 282nm femtosecond laser pulse excitation O₃/H₂And (3) a schematic diagram of a kinetic process simulation result of generating secondary OH free radicals by an O reaction.

Fig. 7 is a schematic diagram of an example provided in the present application, in which a laser beam emitted by a predetermined laser is frequency-converted to obtain a 282nm ultrashort pulse laser.

Fig. 8 is a schematic diagram of a 282nm ultra-short pulse laser obtained by frequency conversion of a laser beam emitted by a predetermined laser according to another embodiment of the present disclosure.

Fig. 9 is a schematic top view of a system for detecting a content of hydroxyl radicals in a gas according to an embodiment of the present disclosure.

FIG. 10 is a schematic diagram of a side view of a system for detecting the content of hydroxyl radicals in a gas according to an embodiment of the present disclosure.

Fig. 11 is a partial schematic flow chart of a method for detecting the content of hydroxyl radicals in a gas according to an embodiment of the present disclosure.

Fig. 12 is a schematic diagram illustrating a filtering effect of the optical filter 1 according to an example provided in the present application.

FIG. 13 shows the OH radical fluorescence spectrum and nitrogen molecule (N)₂）、O₃Raman scattering signal spectrum of (a).

FIG. 14 is a graph showing the comparison of fluorescence signal intensity to the A-gate and B-gate sampling times of a photon counter according to one example provided herein.

FIG. 15 is a graph showing the spectrum of the OH radical fluorescence signal detected in one example provided herein and the spectrum of the background light noise signal (baseline).

Detailed Description

To make the objects, technical solutions and advantages of the embodiments of the present application more clearly understood, the embodiments of the present application are further described in detail below with reference to the accompanying drawings. The exemplary embodiments and descriptions of the present application are provided to explain the present application and should not be interpreted as limiting the present application. It should be noted that the embodiments and features of the embodiments in the present application may be arbitrarily ordered with each other without conflict.

The terms "first," "second," "8230," "8230," and the like as used herein do not particularly denote any order or sequence, nor are they used to limit the present application, but rather are used to distinguish one element from another element or operation described in the same technical language.

As used herein, the terms "comprising," "including," "having," "containing," and the like are open-ended terms that mean including, but not limited to.

As used herein, "and/or" includes any and all permutations of the described things.

For a better understanding of the present invention, the following detailed description is made of the research background of the present application:

the existing Laser Induced Fluorescence (LIF) method for detecting the atmospheric hydroxyl free radical (OH) generally uses pulse laser with narrow spectral line, the time domain pulse width is tens of nanoseconds (ns), the time of the action with sample molecules is long, and ozone (O) in the atmosphere is caused₃) Also has obvious absorption at 282 nanometers (nm) and decomposes to generate atomic oxygen O (in an excited state)¹D)，O(¹D) With water molecules (H) in the atmosphere₂O) will generate OH free radical, and the secondary OH free radical will have the opportunity to react with laser pulse, be excited and then generate fluorescence signal. From atmospheric O₃The concentration of secondary OH radicals generated by decomposition is much higher than the original OH radicals in the atmosphere, which can seriously interfere with the measurement of the original OH radicals. The reaction process is as follows:

；

。

in the atmosphere of the stratosphere, albeit O₃Absorb 282nm light decompose to release O: (¹D) But stratosphere H₂The concentration of O is extremely low, the concentration of secondary OH free radicals is also extremely low, and the measurement of the original OH free radicals cannot be influenced. The spectral study shows that O₃Will decompose to produce O after absorbing photons with wavelength less than 320nm¹D)。

In order to avoid O₃/H₂Interference of O, it is necessary to avoid O as much as possible₃Strong absorption band of (2). For this purpose, OH radicals in atmospheric samples are excited with a 308nm laser during troposphere observation, which greatly reduces O₃Decomposing to produce O: (¹D) However, secondary OH free radicals still influence the measurement result, and the interference still needs to be eliminated by a method during observation. Further, since it is necessary to collect a fluorescence signal in a wavelength band of 308nm which is the same as the excitation wavelength, and the intensity of the excitation light scattering signal is much stronger than that of the fluorescence signal, it is very difficult to suppress the interference of the scattering light. To avoid this interference, it is often necessary to provide complex gating switches in the high voltage circuit of the photomultiplier tube for detecting the fluorescence signal, discarding the fluorescence signal for the first time (depending on the duration of the excitation laser pulse) after the sample is excited, which results in a severe loss of useful signal (see fig. 2), resulting in a decrease in LIF detection sensitivity. As shown in fig. 3, the response of the photomultiplier tube to the voltage change is relatively slow, when the high voltage is recovered, the gain efficiency of the photomultiplier tube can only be recovered to about 75% within 50 nanoseconds, and it takes several microseconds (μ s) to completely recover to 100% gain, that is, the acquisition of the fluorescence signal is completed in the process of slowly recovering the gain efficiency of the photomultiplier tube, which may distort the signal. In addition, when the background noise signal is collected, a high-voltage power supply of the photomultiplier does not need to be switched on or off, the background signal is collected under the condition that the gain efficiency is 100%, and the background noise signal is different from the condition of collecting the fluorescence signal and also needs to be corrected.

In view of the above problems in the prior art, embodiments of the present application provide a method for detecting the content of hydroxyl radicals in a gas, which can at least partially solve the above problems in the prior art.

Fig. 4 is a schematic flow chart of a method for detecting a content of hydroxyl radicals in a gas according to an embodiment of the present application, and as shown in fig. 4, the method for detecting a content of hydroxyl radicals in a gas according to an embodiment of the present application includes:

s101, utilizing the center wavelength falling in hydroxyl radical

Transition of state to

Irradiating the gas flow to be detected in the fluorescence detection cavity by using the ultrashort pulse laser beam on the absorption band of the state so as to enable hydroxyl radicals in the gas flow to be detected to emit fluorescence signals;

in step S101, the fluorescence detection cavity is a hollow cavity structure, the airflow to be detected passes through the fluorescence detection cavity, and the central wavelength falls on OH free radicals

Transition of state to

The ultrashort pulse laser beam on the state absorption band is used as an excitation light source (the pulse laser with the pulse width less than 20 picoseconds (ps) is called as ultrashort pulse laser, such as picosecond pulse and femtosecond fs pulse, 1ns = 1000ps, 1ps = 1000 fs), and the OH free radicals in the air flow to be measured are excited. For example, broadband ultrashort pulse laser with a central wavelength of 282nm is used as an excitation light source to excite OH free radicals in the gas flow to be measured, so that the OH free radicals in the gas flow to be measured emit fluorescence (in a region of 308 nm).

Compared with nanosecond laser pulse, the time domain width of the ultrashort pulse laser pulse is extremely short, and the action time of the ultrashort pulse laser pulse with a sample to be detected is extremely short. The secondary OH free radical has no chance to act with the exciting light, and the fluorescence signal of the original OH free radical can not be interfered naturally. Therefore, ultrashort laser pulses are used as exciting light of the LIF method, and interference of secondary OH free radicals can be effectively avoided. As shown in FIG. 5, the absorption spectrum of OH radicals contains many lines with extremely narrow widths, and the line widths are less than 0.1 cm at normal temperature^-1Typically, when the excitation light is selected, the excitation light has a line that coincides with the line of the molecule to be measured, i.e. resonates.

As shown in fig. 5, the spectral bandwidth of the femtosecond pulse laser (picosecond laser can also be used), the absorption spectrum of the OH radical contains many spectral lines, and the broad-spectrum femtosecond laser is used to excite the OH radical, so that all the spectral lines of the absorption spectrum of the OH radical can be excited simultaneously. Femtosecond pulsesThe time of interaction with the air flow to be measured is only dozens of femtoseconds, and the excited state O₃Decompose to form O: (¹D) The time constant of the process is about 150fs, and even if OH free radicals are generated finally, the OH free radicals do not have the opportunity to act with laser pulses to emit fluorescent signals, so that the measurement of the original OH in the airflow to be measured cannot be disturbed. The results of numerical simulations indicate that this approach is feasible. For example, in O₃The hot summer afternoon with high concentration and air humidity (one atmosphere, 35 ℃, humidity 60%, O)₃ Concentration 100 ppb), an air sample was excited using a laser pulse of 68fs (center wavelength 282nm, single pulse energy 20 μ J). As shown in fig. 6, finally pass through O₃/H₂The concentration of the secondary OH free radical generated by the reaction of O is about 4X 10⁸ mole./cm³This value is much higher than the concentration of OH radicals originally contained in air. However, at the instant when the femtosecond laser pulse passes through the sample, the amount of secondary OH radicals generated is extremely small, and thus the interference with the fluorescence measurement is extremely small. The relative concentrations of secondary OH radicals at different times during the reaction are shown in table 1 below:

table 1:

s102, detecting a fluorescence signal in the fluorescence detection cavity;

in step S102, the fluorescence signal can be detected in the region of 306nm to 312nm by a detection device, for example, a spectrometer is used to record a spectrum, or a photomultiplier tube is used to record a photocurrent, etc.

S103, determining the content of the hydroxyl free radicals in the airflow to be detected according to the detected fluorescent signal.

In step S103, the content (number and/or concentration) of OH radicals in the gas flow to be detected can be determined according to the detected fluorescence signal in the fluorescence detection cavity.

Transition of state to

Irradiating the gas flow to be detected in the fluorescence detection cavity by using the ultrashort pulse laser beam on the absorption band of the state so as to enable hydroxyl radicals in the gas flow to be detected to emit fluorescence signals; detecting a fluorescent signal within the fluorescent detection chamber; and determining the hydroxyl radical content in the airflow to be detected according to the detected fluorescent signal. Thus, the ultrashort characteristics of the ultrashort pulse laser are utilized to avoid O in the atmosphere₃/H₂And the interference of secondary OH free radicals caused by O enables the detection of the OH free radicals in the airflow to be detected to be more accurate.

In an optional embodiment of the present application, the ultrashort pulse laser beam is obtained by performing frequency conversion processing on a laser beam emitted by a preset laser. Specifically, the ultrashort laser pulse required by the present application may be generated by nonlinear optical wavelength conversion, for example, when the central wavelength of the ultrashort pulse laser beam is 282nm, as shown in fig. 7, a most commonly used titanium sapphire ultrashort pulse laser amplifier may be used as a seed light source (preset laser), and the laser pulse with the required wavelength may be generated through frequency doubling and sum frequency conversion. Specifically, the output center wavelength of the titanium sapphire laser is adjustable within a certain range of about 800nm, and a titanium sapphire laser amplifier with the center wavelength of 846nm is selected (pulse repetition frequency is 5KHz, 10KHz and the like, is determined according to actual working requirements, the repetition frequency is matched with the flow velocity of air flow to be detected in fluorescence detection, and the repetition frequency is selected as much as possible on the premise of no repeated irradiation, so that the signal acquisition efficiency is improved). The 846nm seed light generates 423nm frequency doubling light after passing through an optical nonlinear frequency doubling crystal (such as beta barium borate and beta-BBO), the 423nm frequency doubling light is mixed with 846nm fundamental frequency light in a sum frequency nonlinear crystal (such as beta-BBO), and finally, a triple frequency laser pulse with the central wavelength of 282nm is generated. The optical path compensation method can use optical path compensation materials, such as calcite crystals, alpha-BBO crystals, etc., and can also use the optical translation stage shown in fig. 8 to adjust the compensation optical path. Alternatively, the 282nm femtosecond laser pulses may be generated by other methods, for example, wavelength conversion using an optical parametric amplifier, or the like. If a laser capable of directly outputting 282nm ultrashort laser pulses exists, 282nm ultrashort laser pulses emitted by the laser capable of directly outputting 282nm ultrashort laser pulses can also be directly used for irradiating the airflow to be measured.

In an alternative embodiment of the present application, the utilization of the central wavelength falling within the hydroxyl radical is performed by

Transition of state to

The ultrashort pulse laser beam on the absorption band of state, the air current that awaits measuring of shining fluorescence detection intracavity includes: controlling the central wavelength to fall in the hydroxyl radical

Transition of state to

Ultrashort pulse laser beam on the absorption band of attitude shines with predetermineeing the frequency of shining the air current that awaits measuring in the fluorescence detection intracavity, wherein, the velocity of flow of the air current that awaits measuring is greater than predetermine the frequency of shining with the product of the diameter of ultrashort pulse laser beam. The laser pulse repeated irradiation frequency is matched with the flow velocity of the air flow to be detected in the fluorescence detection process, so that the air flow to be detected is prevented from being repeatedly irradiated by the laser. For example, the preset irradiation frequency is 1KHz, which means 1000 laser pulses per second, i.e. one laser pulse per millisecond (ms), in order to avoid that the same portion of the gas flow to be measured is irradiated 2 times by the laser pulses. For example, if the diameter of the laser beam is 5mm and the repetition frequency is 1KHz, the distance that the gas flow to be measured flows in 1ms is more than 5mm.

In an optional embodiment of the present application, the pressure in the fluorescence detection chamber is 0.001 to 1 atmosphere. The fluorescence emission efficiency of the excited OH free radical is influenced by the molecular collision of the environmentThe excited OH radical molecule is exposed to oxygen (O) in the atmosphere₂）、N₂Equimolecular collisions lose activity. The loss of collision deactivation is related to atmospheric pressure, and when the atmospheric pressure is high, the air molecule density is high, the collision loss is also high, and the fluorescent signal attenuation speed is also high, namely the fluorescent lifetime tau is short. Reducing the gas pressure can increase the efficiency of excited OH radical fluorescence emission (increase quantum yield), while reducing the gas pressure can reduce the number of excited OH radical molecules in a single volume of gas, weakening the fluorescence signal. The fluorescence signal intensity is determined by the fluorescence emission efficiency and the concentration of OH free radicals, and the fluorescence lifetime and the relative fluorescence signal intensity of OH free radicals under different air pressure conditions are listed in the following table 2. As can be seen from the data in Table 2, the fluorescence signal intensity did not change much and the fluorescence lifetime changed much between one atmosphere and 0.01 atmosphere. When the atmospheric OH free radicals are actually detected, the optimal sampling condition can be determined according to specific conditions, for example, the air pressure in the fluorescence detection cavity is controlled to be 0.01-0.05 atmospheric pressure, under the air pressure condition, the service life of the fluorescence signal is remarkably prolonged, the intensity loss of the fluorescence signal is small, and the detection of the fluorescence signal is facilitated.

Table 2:

the air pressure in the fluorescence detection cavity can be controlled by selecting the aperture of the air flow sample inlet hole to be detected on the fluorescence detection cavity and the pumping speed of the air flow air pump to be detected. The OH free radicals have high chemical activity, and a sampling pipe is not used in the airflow sampling device to be tested as far as possible because the OH free radicals have serious loss on the pipe wall. In one embodiment of the present invention, the sample inlet of the fluorescence detection chamber is directly exposed to the gas to be detected (e.g., exposed to the outdoor atmosphere), so that the loss of OH radicals can be minimized. The outdoor air pressure is one atmosphere, and the air pressure in the cavity is lower than one atmosphere.

In an alternative embodiment of the present application, as shown in fig. 9 and 10, the excitation laser beam passes through the fluorescence detection chamber through the front and rear windows, which may be sealed quartz glass. The fluorescence detection cavity can be made of metal materials, and the inner wall of the cavity is subjected to anti-reflection treatment (such as oxidation blackening treatment, coating of light absorption materials and the like). The air flow to be detected vertically flows in from the sample inlet hole of the fluorescence detection cavity, the position of the bottom of the cavity, which is right opposite to the sample inlet hole, is an air outlet, the air outlet is externally connected with an air pump, and the air flow to be detected is pumped into the fluorescence detection cavity by the air pump. The airflow to be detected is orthogonal to the excitation laser beam, OH free radicals in the airflow to be detected irradiated by the laser beam can emit fluorescence, and a fluorescence signal is transmitted to the periphery by taking a luminous point as a sphere center. Thus, the laser-induced OH free radicals emit fluorescent signals. Of course, the manner in which the laser beam and the airflow to be detected enter the fluorescence detection cavity, the relative position relationship between the laser beam and the airflow to be detected, and the structure of the fluorescence detection cavity are not limited to these, and may be adjusted accordingly according to the specific application scenario.

As shown in fig. 11, optionally, the detecting the fluorescence signal in the fluorescence detection cavity includes:

s1021, collecting a first optical signal in the fluorescence detection cavity;

in step S1021, since the fluorescent signal emitted by the OH radical in the airflow to be detected propagates around the light-emitting point as the center of the sphere, the first optical signal in the fluorescence detection cavity can be collected, where the first optical signal includes the fluorescent signal; specifically, as shown in fig. 9, a concave mirror and a light collecting lens may be installed in the left and right horizontal directions of the fluorescence detection cavity, the concave mirror and the lens are disposed opposite to each other to collect fluorescence signals in the left and right horizontal directions perpendicular to the excitation laser beam, and the back mirror is used to reflect the fluorescence signals emitted in the opposite direction to the light collecting lens back to the light collecting lens, so as to increase the collection amount of the fluorescence signals. In an example of the present application, the optical axes of the excitation laser beam, the air flow to be measured, and the fluorescence signal collecting optical path are orthogonal to each other. In addition, a device such as an aspherical mirror disposed in the fluorescence detection cavity may be used to collect the fluorescence signal, which is not limited in this embodiment.

S1022, filtering the collected first optical signal to obtain a second optical signal, wherein the second optical signal comprises a fluorescent signal;

in step S1022, the collected first optical signal includes not only the fluorescence signal but also other stray light. And filtering the first optical signal to obtain a second optical signal containing the fluorescent signal, wherein the second optical signal may only include the fluorescent signal, or may include both the fluorescent signal and other optical signals.

And S1023, detecting a fluorescent signal in the second optical signal.

In step S1023, when only the fluorescent signal is included in the second optical signal, the fluorescent signal in the second optical signal may be directly detected. When the second optical signal includes both the fluorescent signal and the other optical signal, the fluorescent signal in the second optical signal may be detected by using a corresponding detection strategy according to respective characteristics of the fluorescent signal and the other optical signal.

In an optional embodiment of the present application, the filtering the collected first optical signal in step S1022 to obtain a second optical signal includes: and filtering stray light with the wavelength of 306 nm-312 nm in the first optical signal to obtain a second optical signal with the wavelength of 306 nm-312 nm. Specifically, the filtering device may be a set of optical filters, and the optical filters may filter out stray light with a wavelength of 306nm to 312nm in the first optical signal, where the optical filters are used to filter out scattered light of the excitation laser and raman scattering signals (mainly N in air) from the airflow to be measured₂、O₂) And O excited by laser pulses₃The fundamental frequency and the frequency-doubled raman scattering signal, etc.

For example, an edge-pass filter may be used to filter stray light with a wavelength greater than or less than the OH free radical fluorescence signal and filter out fluorescence signals with a wavelength between 306nm and 312 nm. The side-pass filter has small manufacturing difficulty, good filtering effect and small loss on useful signals, the conventional OH free radical fluorescence detection device adopts a band-pass filter to simultaneously filter out stray light signals with the wavelength of less than 306nm and more than 312nm, the manufacturing difficulty of the band-pass filter is large, and the band-pass filter is used for filtering out the stray light signals in the region of 306nm to 312nmThe transmission efficiency of the fluorescence signal is low (usually, only about 20%). As shown in FIG. 9, most of the 282nm excitation laser scattered light and O are filtered by a long-wave side-pass filter 1₂The cut-off depth of the filter 1 to 282nm light is more than 3 (the residual signal is less than one thousandth), the spectral diagram shown in fig. 12 shows the filtering effect of the filter used in the example of the application, and the extinction effect of the filter to 282nm light can reach 3.8 (10)^-3.8One-half of 6300) while applying N to 302nm region₂Has little effect on the raman scattering signal. The cut-off position of the optical filter for the optical signal can be designed and manufactured according to actual requirements.

The long-wave side-pass filter set 2 is a set of filters with slightly different performances, and the filters are installed on a sliding type porous installation seat for installing the filters or a filter rotating wheel and can be switched as required. One of the functions of the long-wave side-pass filter set 2 is to reduce the intensity of the residual 282nm scattered signal light again, and the 282nm scattered light intensity after secondary extinction is lower than one million of the original value, so that the requirement of OH free radical fluorescence detection is met. Each filter pair N in filter set 2₂The extinction effect of the raman scattering signal is different and can be selected according to the requirement. N is a radical of₂The wavelength of the Raman scattering signal is 302 nm-303 nm, the intensity is stable, the reproducibility is good, the Raman scattering signal can be used as an internal standard for light path adjustment, and the Raman scattering signal can be adjusted according to N during system debugging₂The Raman scattering signal intensity of the light path is judged to optimize the effect of the light path. When the system is debugged, an optical filter which can transmit light of 302 nm-303 nm in the optical filter group 2 is selected; when detecting the OH free radical fluorescence signal, an optical filter which can filter light with the wavelength of 302nm to 303nm in the optical filter group 2 is selected. A short-pass filter 3 is arranged behind the long-pass filter group 2 and is used for filtering out all stray light (from sunlight and O) with the wavelength of more than 312nm₃High frequency-doubled raman scattering signal, etc.). The optical signal passing through these filters (i.e. the second optical signal) contains fluorescent signals from OH radicals with wavelengths of 306nm to 312nm, and the second optical signal can be focused on the detector through the focusing lens.

In an optional embodiment of the present application, the detecting the fluorescent signal in the second optical signal comprises: and after the ultrashort pulse laser beam irradiates the airflow to be detected in the fluorescence detection cavity, delaying for a preset time length and then detecting the fluorescence signal. Since the second optical signal may not only include the fluorescence signal, when other optical signals are included in the second optical signal, the occurrence time and the lifetime of each optical signal may be combined, and a corresponding delay strategy may be set to detect the fluorescence signal in the second optical signal at an optimal time period.

For example, when the second optical signal includes a resonance raman scattering signal of stretching vibration of ozone molecules, the preset time period is greater than or equal to O₃The duration of the resonant raman scattering signal. Containing O in the atmosphere or the like₃Of the air flow to be measured, O in the air flow to be measured₃The concentration is at the level of parts per billion, much greater than the concentration of OH radicals. O is₃The absorption cross section at 282nm is relatively large, and a resonance raman scattering signal is generated, as shown in fig. 13, the triple frequency raman scattering signal of the stretching vibration is overlapped with the fluorescence signal of the OH free radical, and cannot be eliminated through the optical filter. Since this portion of the raman scattering signal results from scattering of the incident laser light, the raman scattering signal lasts for the same time as the excitation light pulse passes through the gas stream under measurement. For example, the excitation light source used in the embodiment of the present application is an ultrashort pulse laser with a pulse width of 250fs (half peak height), the time of passing through the gas flow to be measured and the time of the scattered light signal passing through the reflected light path of the back reflector are less than 100ps seconds, O₃The raman scattering signal lasts for a period of 100 ps. Therefore, can be avoided by means of time delay₃The raman scattering signal of the molecule, only the fluorescence signal of the OH radicals is recorded.

When atmospheric OH free radical detection is carried out, the concentration of atmospheric OH free radicals is extremely low, and the concentration of atmospheric OH free radicals is usually only 10 per cubic centimeter of air⁶One part (billion) with very weak fluorescence signal. As shown in FIG. 9, the OH radical fluorescence signal can be detected by using a high-gain photoelectric conversion device, such as a photomultiplier tube (PMT, CPM), a multi-channel plate (MCP), etc.,and the weak light signal is converted into a current signal, then the current signal is amplified by a preamplifier and converted into a voltage signal, and then the voltage signal is recorded by adopting a photon counter to complete the detection of the fluorescence signal. Among them, the photon counter is an instrument dedicated to recording very weak optical signals. Including O in the second optical signal₃When the triple frequency Raman scattering signal of the stretching vibration is carried out, the sampling interval (gate control) of the photon counter can be set to avoid O₃The triple frequency Raman scattering signal of the (1) is obtained, and only the fluorescence signal of OH free radicals is recorded. Specifically, a pulse delay generator can be used as an external trigger signal source to control the sampling time of the photon counter. The pulse delay generator can be controlled by a synchronization signal from the ultrashort pulse laser system, synchronized with the laser pulse.

In an optional embodiment of the present application, the method further comprises: detecting a background light noise signal of the fluorescent signal after detecting the fluorescent signal; the determining the hydroxyl radical content in the gas flow to be detected according to the detected fluorescent signal comprises: and determining the content of the hydroxyl radical in the airflow to be detected according to the detected fluorescent signal and the background light noise signal. Specifically, a photon counter with two channels (A-gate, B-gate) may be used to simultaneously acquire the fluorescence signal and the background light noise signal. When the kit is used, A-gate is used for collecting OH free radical fluorescence signals, and B-gate is used for collecting background noise signals. As shown in FIG. 9, the working state of the photon counter can be controlled by the pulse delay generator, and the time when the laser pulse irradiates the air flow to be measured is set as the time starting point t₀，O₃The duration of the resonant raman scattering signal at t, at which the pulse delay generator may be₀And triggering the photon counter to start working after the time delta t. As shown in FIG. 14, the A-gate channel of the photon counter is at t₀Starting after the time delta t, recording a fluorescence signal, matching the working time length (gate width) of the A-gate with the fluorescence lifetime tau of the OH free radical in a sampling state (namely determining the detection time length of the fluorescence signal according to the lifetime of the fluorescence signal), for example, setting the sampling time length to be 5 tau. The B-gate channel of the photon counter is spaced for a period after the A-gate channel is closedAnd starting after a time delta, wherein the time delta is used for recording background light noise signals, the time interval delta is usually set to be in the level of several microseconds to dozens of microseconds, and the working time of the B-gate channel is the same as that of the A-gate channel (namely the detection time of the background light noise signals is equal to that of the fluorescence signals). In one example of the present application, the spectrum of the detected OH radical fluorescence signal and the spectrum of the background light noise signal (baseline) are shown in fig. 15. Wherein, the wavelength of the OH free radical fluorescence signal is in the interval of 306 nm-312 nm, and the fluorescence signal detected by the photon counter is the sum of the fluorescence spectrum signals in the range of 306 nm-312 nm shown in figure 15.

The precision of the pulse delay generator for adjusting the delay can reach 1ps, and delta t is limited by the response speed of a photoelectric conversion device (such as a photomultiplier) in practical use. The single photon rising time of the fastest photomultiplier at present is about 2-3 ns, deltat is determined according to the actual situation during sampling, and O is completely avoided₃On the premise of Raman scattering signals, the delta t value is as small as possible, so that the fluorescence signals can be collected as much as possible. O at one atmospheric pressure₃The intensity of the Raman scattering signal is equivalent to that of the fluorescence signal of OH free radicals, and the photomultiplier is not damaged even if the Raman scattering signal is irradiated to the working photomultiplier without treatment (in a high-voltage electrified state). Therefore, unlike the prior device for exciting the 308nm nanosecond laser pulse as described above, the photomultiplier tube does not need to turn off the high-voltage power supply, so that the gain efficiency of the photomultiplier tube is kept stable and the signal is not distorted.

In the description herein, reference to the description of the terms "one embodiment," "a particular embodiment," "some embodiments," "for example," "an example," "a particular example," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the application. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

The above-mentioned embodiments are provided to further explain the objects, technical solutions and advantages of the present application in detail, and it should be understood that the above-mentioned embodiments are only examples of the present application and are not intended to limit the scope of the present application, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present application should be included in the scope of the present application.

Claims

1. A method for detecting the hydroxyl radical content of a gas, comprising:

by using the central wavelength falling on the hydroxyl radical

Transition of state to

Irradiating the gas flow to be detected in the fluorescence detection cavity by using the ultrashort pulse laser beam on the state absorption band so as to enable hydroxyl radicals in the gas flow to be detected to emit fluorescence signals, wherein the gas pressure in the fluorescence detection cavity is 0.01-0.05 atmospheric pressure;

detecting a fluorescent signal within the fluorescent detection chamber;

determining the content of hydroxyl radicals in the airflow to be detected according to the detected fluorescent signal;

the detecting the fluorescent signal within the fluorescent detection chamber comprises: collecting a first optical signal in the fluorescence detection cavity; filtering the collected first optical signal to obtain a second optical signal, wherein the second optical signal comprises a fluorescence signal; detecting a fluorescent signal in the second optical signal;

the detecting the fluorescent signal in the second optical signal comprises: and after the ultrashort pulse laser beam irradiates the airflow to be detected in the fluorescence detection cavity, detecting the fluorescence signal in the second optical signal after delaying a preset time, wherein when the second optical signal comprises a resonance Raman scattering signal of an ozone molecule, the preset time is longer than or equal to the duration of the resonance Raman scattering signal of the ozone molecule.

2. The method as claimed in claim 1, wherein the ultra-short pulse laser beam is obtained by frequency-converting a laser beam emitted from a predetermined laser.

3. The method of claim 1, wherein the using the center wavelength falls on a hydroxyl radical group

Transition of state to

The ultrashort pulse laser beam on the absorption band of the state irradiates the airflow to be detected in the fluorescence detection cavity, and the ultrashort pulse laser beam comprises:

controlling the central wavelength to fall in the hydroxyl radical

Transition of state to

4. The method of claim 1, wherein filtering the collected first optical signal to obtain a second optical signal comprises:

and filtering stray light with the wavelength of 306 nm-312 nm in the first optical signal to obtain a second optical signal with the wavelength of 306 nm-312 nm.

5. The method of claim 1, further comprising:

detecting a background light noise signal of the fluorescent signal after detecting the fluorescent signal;

the determining the hydroxyl radical content in the gas flow to be detected according to the detected fluorescent signal comprises: and determining the content of the hydroxyl radical in the airflow to be detected according to the detected fluorescent signal and the background light noise signal.

6. The method of claim 5, wherein the detection duration of the fluorescent signal is determined based on the lifetime of the fluorescent signal, and the detection duration of the background light noise signal is equal to the detection duration of the fluorescent signal.