Mid-infrared three-wavelength differential absorption lidar and atmospheric benzene pollution source detection method

Abstract

This invention discloses a mid-infrared three-wavelength differential absorption lidar, comprising a system control and processing platform, a laser input and collimation system, a telescope receiving system, and a beam splitting and detection system connected sequentially along the optical path. The system control and processing platform connects the laser input and collimation system and the beam splitting and detection system. The lidar's method for detecting atmospheric benzene pollution includes: adjusting the output wavelengths of three lasers using the system control and processing platform, calculating the laser emission power of the three wavelengths, measuring the echo signal power of the three wavelengths, calculating and correcting the concentrations of toluene and benzene, and then calculating and analyzing the ratio of these concentration values. This invention overcomes the limitation of existing technologies that can only detect small areas and compensates for the deficiency of differential absorption lidar technology in detecting benzene pollution sources in the mid-infrared band.

Description

Technical Field

[0001] The present invention relates to the technical field of laser active remote sensing detection, and in particular to a mid-infrared three-wavelength differential absorption laser radar. The present invention also relates to a method for detecting atmospheric benzene pollution sources using the radar. Background Art

[0002] In recent years, with the continuous development of social economy, people's living standards have been significantly improved, but there are also many atmospheric environmental problems such as dust aerosol, acid rain, sandstorms, etc., which have brought serious troubles and harm to people's lives. In the analysis of pollution sources, atmospheric volatile organic compounds (VOCs) are one of the important sources. Volatile organic compounds (VOCs) are the formation of fine particulate matter (PM 2.5 ), ozone (O3) and other secondary pollutants, which in turn cause atmospheric environmental problems such as haze and photochemical smog. Benzene series (BTEX) is an important component of VOCs. Therefore, exploring the content of BTEX in the atmosphere is of great significance in environmental protection. BTEX, also often called benzene and BTEX, has an absolute number of more than 10 million in the full sense. Among them, benzene (Benzene), toluene (Toluene), ethylbenzene (Ethylbenzene) and xylene (Xylene) are the representative substances, so they are referred to as BTEX. BTEX produced by the production and manufacturing of chemical products, the use of BTEX reagents, forest fires, wood burning, oil production, and transportation vehicles has a serious negative impact on the regional, especially urban atmospheric environment, which makes it particularly important to detect the concentration and pollution sources of BTEX in the atmosphere.

[0003] Some people have proposed to measure the concentration ratio of benzene to toluene to determine the source of benzene pollution. TB =ρ T / ρ B )<2, it means that this monitoring point is mainly affected by motor vehicle exhaust emissions; when R TB When R >2, it means that this monitoring point is affected by the use of industrial solvents in addition to the exhaust emissions from motor vehicles. TB The larger the ratio is, the greater the impact of solvent use sources is, and the smaller the impact of motor vehicle exhaust emissions is.

[0004] At present, the traditional detection methods for benzene series are mainly divided into two types: gas chromatography-mass spectrometry and ionization-mass spectrometry. Both methods have high sensitivity. However, the existing technology usually requires single-point sampling and detection in an offline manner. This method can only detect small areas and is not suitable for real-time detection of benzene series concentrations in larger areas. Summary of the invention

[0005] The purpose of the present invention is to provide a mid-infrared three-wavelength differential absorption laser radar, which has two schemes: coaxial and heteroaxial (non-coaxial). The system can use three wavelengths of mid-infrared to detect the concentration of benzene and toluene, and can detect concentration in a large range, solving the problem that the traditional detection method of benzene series in the prior art can only detect a small area.

[0006] Another object of the present invention is to provide a method for detecting atmospheric benzene pollution sources using the above-mentioned laser radar.

[0007] The technical solution adopted by the present invention is a mid-infrared three-wavelength differential absorption laser radar, including a system control and processing platform, a laser input and collimation system, a telescope receiving system and a spectroscopic and detection system connected in sequence along the optical path, wherein the system control and processing platform is connected to the laser input and collimation system and the spectroscopic and detection system through a connecting cable.

[0008] The coaxial system of the present invention is characterized in that the laser input and collimation system includes three groups of optical paths arranged in parallel, wherein the first group of optical paths includes a first inter-band cascade laser, a first collimation system, a second beam splitter and a first beam splitter arranged in sequence along the optical path, and the second beam splitter also splits the optical path into a side branch optical path to a fourth photodetector; the second group of optical paths includes a second inter-band cascade laser, a second collimation system, a fourth beam splitter and a third beam splitter arranged in sequence along the optical path, and the fourth beam splitter also splits the optical path into a side branch optical path to a fifth photodetector; the third group of optical paths includes a third inter-band cascade laser, a third collimation system, a fifth beam splitter and a fourth reflector arranged in sequence along the optical path, and the fifth beam splitter also splits the optical path into a side branch optical path to a sixth photodetector, and the fourth photodetector, the fifth photodetector and the sixth photodetector are respectively connected to the system control and processing platform through connecting cables.

[0009] The heteroaxial (non-coaxial) system of the present invention is characterized in that the laser input and collimation system includes three groups of optical paths arranged accordingly, wherein the first group of optical paths includes a first inter-band cascade laser, a first collimation system and a second spectroscope arranged in sequence along the optical path, and the second spectroscope further divides the optical path into a side branch optical path to a fourth photodetector; the second group of optical paths includes a second inter-band cascade laser, a second collimation system and a fourth spectroscope arranged in sequence along the optical path, and the fourth spectroscope further divides the optical path into a side branch optical path to a fifth photodetector; the third group of optical paths includes a third inter-band cascade laser, a third collimation system and a fifth spectroscope arranged in sequence along the optical path, and the fifth spectroscope further divides the optical path into a side branch optical path to a sixth photodetector, and the fourth photodetector, the fifth photodetector and the sixth photodetector are respectively connected to the system control and processing platform through connecting cables.

[0010] In the telescope receiving system, the laser after the laser input and collimation system is emitted to the diffuse reflection target, and the diffuse reflection target reflects the laser to the telescope with an aperture of 200mm and a focal length of 100mm. The telescope transmits the laser to the spectroscopic and detection system through the third lens and the second lens.

[0011] The spectrometry and detection system includes a lens, a blazed grating and a reflector connected to a telescope receiving system. The light received by the telescope system is emitted to the blazed grating after passing through a third lens and a second lens. After being spectrally split by the blazed grating, the light is received by a first photodetector, a second photodetector and a third photodetector respectively to realize photoelectric detection. The first photodetector, the second photodetector and the third photodetector are all connected to an amplifying circuit system through a connecting cable, and the amplifying circuit system is connected to a system control and processing platform through a first connecting cable.

[0012] Another technical solution adopted by the present invention is a method for detecting atmospheric benzene pollution sources by mid-infrared three-wavelength differential absorption laser radar, comprising the following steps:

[0013] Step 1: Use the system control processing platform to adjust the three output laser wavelengths of the first inter-band cascade laser, the second inter-band cascade laser, and the third inter-band cascade laser to λ1, λ2, λ 3， The voltage values ​​detected by the fourth photodetector, the fifth photodetector, and the sixth photodetector represent the laser emission powers P0(λ i ), where i = 1, 2, 3;

[0014] Step 2: The continuous pulse signals of the three wavelengths in step 1 are injected into the atmosphere through the laser input and collimation system, and then received by the telescope receiving system and transmitted to the spectroscopic and detection system to measure the echo signal power P(R, λ) of the three wavelengths in step 1. i );

[0015] Step 3: Use the echo signal power P(R,λ) obtained in step 2 i ), calculate and correct the concentration of toluene and benzene N T (R) and N B (R);

[0016] Step 4: Substitute the N obtained in step 3 T (R) and N B (R) Calculate the ratio and analyze it.

[0017] In step 1, among the three output laser wavelengths, λ1 is 3304.8 nm, λ2 is 3282.4 nm, and λ3 is 3198.8 nm.

[0018] The calculation method in step 3 is:

[0019] According to the laser radar principle and Beer-Larmbert theorem, the continuous wave diffuse reflection target reflection laser radar equation can be expressed as

[0020]

[0021] Where P(0) is the transmitting power of the laser, A is the effective receiving area of ​​the telescope, η is the optical efficiency of the system, R is the path length, β is the diffuse reflectivity of the diffuse reflective target at a distance R from the laser radar, and α(r) is the atmospheric extinction coefficient at a distance r;

[0022] The extinction coefficient is divided into two parts, one is the extinction coefficient α1(r) caused by the absorption of the sensitive gas being measured, and the other is the extinction coefficient α0(r) caused by factors other than the absorption of the sensitive gas being measured. The extinction coefficient can be expressed as

[0023] α(r)=α1(r)+α0(r)=N(r)·σ1+α0(r) (2)

[0024] Where N(r) is the molecular density of the measured gas at a distance r, and σ1 is the absorption cross-sectional area of ​​the measured gas;

[0025] Differential absorption lidar uses a strong absorption wavelength λ on and weak absorption wavelength λ off , according to the two-way backscattered light signal P(R,λ on ), P(R,λ off ) ratio to calculate the material concentration; Since the differential absorption lidar shares an optical system and λ on and λ off The atmospheric extinction except for the measured gas is basically the same, that is, α0(r, λ on )≈α0(r,λ off ).

[0026] P0(λ i ) and P(R,λ i ) into formula (1), and combine formula (1) with formula (2), that is,

[0027]

[0028] Assuming that the number density N(r) of the measured gas is uniformly distributed in the detection path R, the number density N(R) of the measured gas can be expressed as

[0029]

[0030] The value P0(λ i ), P(R,λ i) is substituted into formula (3), and P(R, λ1) / P(R, λ2) and P(R, λ2) / P(R, λ3) are calculated respectively. Then, the (average / column) concentrations of benzene and toluene can be obtained;

[0031]

[0032] In formula (5), when σ(λ1) is substituted as the absorption cross section of toluene, N1(R) is represented as the gas number density of toluene, denoted as N 1T (R), σ(λ3) is substituted as the absorption cross section of benzene, N1(R) is expressed as the gas number density of benzene, recorded as N 1B (R); Similarly, the gas number density N of toluene in formula (6) is obtained 2T (R) and N represents the gas number density of benzene 2B (R), and find N 1T (R) and N 2T The average value of (R) T (R), N 1B (R) and N 2B The average value of (R) B (R).

[0033] The correction method in step 3 is:

[0034] Since the absorption spectra of benzene and toluene overlap, the absorption cross sections at wavelengths λ1, λ2, and λ3 contain both benzene and toluene components. The absorption cross sections σ(λ1), σ(λ2), and σ(λ3) are divided into σ B (λ1),σ T (λ1),σ B (λ2),σ T (λ2),σ B (λ3),σ T (λ3), assuming that the gas number density of toluene is N T (R), the gas number density of benzene is N B (R), and then substituting it into formula (3), we get

[0035]

[0036] The calculation method in step 4 is:

[0037] The N obtained in step 3 T (R) and N B (R) Calculate and analyze the ratio. If N T (R) / N B When (R)<2, it means that this monitoring point is mainly affected by motor vehicle exhaust emissions; if N T (R) / N B(R) < 2, indicating that this monitoring point is affected by the use of industrial solvents in addition to the exhaust emissions from motor vehicles. N T (R) / N B The larger the ratio (R), the greater the impact of solvent use sources, and the smaller the impact of motor vehicle exhaust emissions.

[0038] The beneficial effect of the present invention is that the method for detecting atmospheric benzene pollution sources by the laser radar of the present invention improves the deficiency that the traditional detection method of benzene series can only detect small areas, and utilizes the characteristic of the inter-band cascade laser that can emit mid-infrared band laser to make up for the defect of differential absorption laser radar in detecting benzene series pollution sources in the mid-infrared band. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the coaxial distribution structure of the mid-infrared three-wavelength differential absorption laser radar of the present invention;

[0040] Figure 2 It is a schematic diagram of the heteroaxial (non-coaxial) distribution structure of the mid-infrared three-wavelength differential absorption laser radar of the present invention;

[0041] Figure 3 It is a right view of the placement of the inter-band cascade laser at the telescope system of the heteroaxial (non-coaxial) distributed structure of the mid-infrared three-wavelength differential absorption laser radar of the present invention;

[0042] Figure 4 It is a schematic diagram of the absorption characteristics of benzene series in the detection method of the present invention;

[0043] Figure 5 It is a schematic diagram of the absorption cross section of benzene and toluene in the detection method of the present invention.

[0044] In the figure, 1. system control processing platform, 2. first connecting cable, 3. amplifying circuit system, 4. second connecting cable, 5. third connecting cable, 6. fourth connecting cable, 7. first photodetector, 8. second photodetector, 9. third photodetector, 10. first reflector, 11. second reflector, 12. blazed grating, 13. first lens, 14. second lens, 15. third lens, 16. telescope, 17. echo signal, 18. third reflector, 19. diffuse reflection target, 20. continuous laser beam, 21. first spectroscope, 22. second spectroscope , 23. fourth photodetector, 24. first collimation system, 25. fifth connecting cable, 26. first inter-band cascade laser, 27. sixth connecting cable, 28. third spectroscope, 29. fourth spectroscope, 30. fifth photodetector, 31. second collimation system, 32. seventh connecting cable, 33. second inter-band cascade laser, 34. eighth connecting cable, 35. fourth reflector, 36. fifth spectroscope, 37. sixth photodetector, 38. third collimation system, 39. ninth connecting cable, 40. third inter-band cascade laser, 41. tenth connecting cable. DETAILED DESCRIPTION

[0045] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0046] The mid-infrared three-wavelength differential absorption laser radar of the present invention has a coaxial distribution structure as follows Figure 1 As shown, the system control processing platform 1 is connected to three groups of optical paths arranged in parallel, wherein the first group of optical paths is: the system control processing platform 1 is connected to and controls the first inter-band cascade laser 26 through the sixth connecting cable 27 to emit laser light which passes through the first collimation system 24, the second beam splitter 22 and the first beam splitter 21 in sequence, the second beam splitter 22 also splits the laser light and bypasses it to the fourth photodetector 23, and the fourth photodetector 23 is connected to the system control processing platform 1 through the fifth connecting cable 25; the second group of optical paths is: the system control processing platform 1 is connected to and controls the second inter-band cascade laser 33 to emit laser light which passes through the second collimation system 24, the second beam splitter 22 and the first beam splitter 21 in sequence, the second beam splitter 22 also splits the laser light and bypasses it to the fourth photodetector 23, and the fourth photodetector 23 is connected to the system control processing platform 1 through the fifth connecting cable 25; The straight system 31, the fourth beam splitter 29 and the third beam splitter 28, the fourth beam splitter 29 also splits the laser bypass to the fifth photodetector 30, the fifth photodetector 30 is connected to the system control and processing platform 1 through the seventh connecting cable 32; the third group of optical paths is: the system control and processing platform 1 is connected through the tenth connecting cable 41 and controls the third inter-band cascade laser 40 to emit laser through the third collimation system 38, the fifth beam splitter 36 and the fourth reflector 35 in sequence, the fifth beam splitter 36 also splits the laser bypass to the sixth photodetector 37, the sixth photodetector 37 is connected to the system control and processing platform 1 through the ninth connecting cable 39.

[0047] The light of the third light path reflected by the fourth reflector 35 merges with the light of the second light path at the beam splitter 28, and then merges with the laser of the first light path at the first beam splitter 21, enters the third reflector 18, and is emitted to the diffuse reflection target 19 in the form of a continuous laser beam 20, and its echo signal 17 is received by the telescope 16.

[0048] The received optical signal is emitted to the blazed grating 12 after passing through the third lens 15 and the second lens 14, and is transmitted to the first lens 13 after being spectrally split by the blazed grating 12, and is then received by the first photodetector 7, the second photodetector 8 and the third photodetector 9 respectively to realize photoelectric detection. The detection signal enters the amplifying circuit system 3 through the second connecting cable 4, the third connecting cable 5 and the fourth connecting cable 6 respectively for amplification, and is then sent to the system control processing platform 1 through the first connecting cable 2 for analysis.

[0049] The mid-infrared three-wavelength differential absorption laser radar of the present invention has a heteroaxial (non-coaxial) distribution structure as follows Figure 2 As shown, the system control processing platform 1 is connected to three groups of optical paths arranged in sequence, wherein the first group of optical paths is: the system control processing platform 1 is connected through the sixth connecting cable 27 and controls the first inter-band cascade laser 26 to emit laser light in sequence through the first collimation system 24 and the second beam splitter 22, the second beam splitter 22 also splits the laser light and bypasses it to the fourth photodetector 23, and the fourth photodetector 23 is connected to the system control processing platform 1 through the fifth connecting cable 25; the second group of optical paths is: the system control processing platform 1 is connected through the eighth connecting cable 34 and controls the second inter-band cascade laser 33 to emit laser light in sequence through Through the second collimation system 31 and the fourth beam splitter 29, the fourth beam splitter 29 also bypasses the laser to the fifth photodetector 30, and the fifth photodetector 30 is connected to the system control and processing platform 1 through the seventh connecting cable 32; the third group of optical paths is: the system control and processing platform 1 is connected through the tenth connecting cable 41 and controls the third inter-band cascade laser 40 to emit laser through the third collimation system 38 and the fifth beam splitter 36 in sequence, the fifth beam splitter 36 also bypasses the laser to the sixth photodetector 37, and the sixth photodetector 37 is connected to the system control and processing platform 1 through the ninth connecting cable 39.

[0050] The continuous laser beams 20 emitted by the three groups of optical paths are directed toward the diffuse reflection target 19 in a non-coaxial (non-coaxial) distribution manner, and the echo signals 17 thereof are received by the telescope 16 .

[0051] The received optical signal is emitted to the blazed grating 12 after passing through the third lens 15 and the second lens 14, and is transmitted to the first lens 13 after being spectrally split by the blazed grating 12, and is then received by the first photodetector 7, the second photodetector 8 and the third photodetector 9 respectively to realize photoelectric detection. The detection signal enters the amplifying circuit system 3 through the second connecting cable 4, the third connecting cable 5 and the fourth connecting cable 6 respectively for amplification, and is then sent to the system control processing platform 1 through the first connecting cable 2 for analysis.

[0052] The placement of the inter-band cascade laser in the telescope system of the heteroaxial (non-coaxial) distribution structure is shown in the right view. Figure 3 shown.

[0053] The absorption coefficients of benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene are as follows: Figure 4 As shown, according to the information described in the figure, benzene and toluene are selected as the pollutant concentration detection sources of this invention.

[0054] like Figure 5 As shown, three wavelengths are selected and one wavelength λ2 is a universal wavelength, λ1 = 3304.8 nm, λ2 = 3282.4 nm, and λ3 = 3198.8 nm.

[0055] The working principle of the atmospheric benzene pollution source detection method of the mid-infrared three-wavelength differential absorption laser radar of the present invention is as follows: the wavelength of the laser emitted by the inter-band cascade laser is in the mid-infrared band of 3000nm to 6000nm. For example, in a coaxial system, the third inter-band cascade laser 40 emits a continuous laser beam with a wavelength of 3282.4nm and a power of 10mw. The laser beam passes through the third collimation system 38 to form a continuous laser beam with a divergence angle of 0.2mrad, and passes through the fifth beam splitter 36, the fourth reflector 35, the third reflector 36, the fifth reflector 37, the fifth reflector 38, the fifth reflector 39, the fifth reflector 31, the fifth reflector 32 and the fifth reflector 33 in sequence. The laser beam is then directed toward the diffuse reflection target 19 through the three-beam mirror 28, the first beam splitter 21, and the third reflector 18. The third reflector 18 can adjust the direction of the laser beam to achieve that the continuous laser beam 20 is parallel to the optical axis of the telescope 16. A small portion of the energy of the continuous laser beam is reflected by the fifth beam splitter 36 and directed toward the sixth photodetector 37 to achieve photoelectric conversion. The voltage value displayed by the sixth photodetector represents the initial emission power of the third inter-band cascade laser 40. In addition, the system control processing platform 1 also provides working power for the amplifying circuit system 3.

[0056] During the transmission process, the continuous laser beam 20 will interact with atmospheric matter to form atmospheric scattering, and part of the backscattered light is collected by a telescope 16 with a diameter of 200 mm and a focal length of 100 mm; the backscattered light collected by the telescope 16, that is, the laser radar echo signal, is emitted to the blazed grating 12 after passing through the third lens 15 and the second lens 14, and is transmitted to the first lens 13 after spectral splitting by the blazed grating 12, and then is received by the first photodetector 7, the second photodetector 8 and the third photodetector 9 respectively to realize photoelectric detection, and the detection signals enter the amplification circuit system 3 through the second connecting cable 4, the third connecting cable 5 and the fourth connecting cable 6 respectively, and are amplified, and then sent to the system control processing platform 1 through the first connecting cable 2 for analysis.

[0057] The method for detecting the concentration of benzene-based pollutants using the above-mentioned mid-infrared three-wavelength differential absorption laser radar includes the following steps:

[0058] Step 1: Using the system control processing platform, the three output laser wavelengths of the first inter-band cascade laser, the second inter-band cascade laser, and the third inter-band cascade laser are adjusted to λ1=3304.8nm, λ2=3282.4nm, and λ3=3198.8nm, and the laser emission powers P0(λ i ), where i = 1, 2, 3;

[0059] Step 2: The continuous pulse signals of the three wavelengths in step 1 are injected into the atmosphere through the laser input and collimation system, and then received by the telescope receiving system and transmitted to the spectroscopic and detection system to measure the echo signal power P(R, λ) of the three wavelengths in step 1. i );

[0060] Step 3: Use the echo signal power P(R,λ) obtained in step 2 i ), calculate and correct the concentration of toluene and benzene N T (R) and N B (R);

[0061] Step 4: Substitute the N obtained in step 3 T (R) and N B (R) Calculate the ratio and analyze it.

[0062] The calculation method in step 3 is:

[0063] According to the laser radar principle and Beer-Larmbert theorem, the continuous wave diffuse reflection target reflection laser radar equation can be expressed as:

[0064]

[0065] Where P(0) is the transmitting power of the laser, A is the effective receiving area of ​​the telescope, η is the optical efficiency of the system, R is the path length, β is the diffuse reflectivity of the diffuse reflective target at a distance R from the laser radar, and α(r) is the atmospheric extinction coefficient at a distance r;

[0066] The extinction coefficient is divided into two parts. One part is the extinction coefficient α1(r) caused by the absorption of the sensitive gas being measured, and the other part is the extinction coefficient α0(r) caused by factors other than the absorption of the sensitive gas being measured. The extinction coefficient can be expressed as:

[0067] α(r)=α1(r)+α0(r)=N(r)·σ1+α0(r) (2)

[0068] Where N(r) is the molecular density of the measured gas at a distance r, and σ1 is the absorption cross-sectional area of ​​the measured gas;

[0069] Differential absorption lidar uses a strong absorption wavelength λ on and weak absorption wavelength λ off , according to the two-way backscattered light signal P(R,λ on ), P(R,λ off ) ratio to calculate the material concentration; Since the differential absorption lidar shares an optical system and λ on and λ off The atmospheric extinction except for the measured gas is basically the same, that is, α0(r, λ on )≈α0(r,λ off ).

[0070] P0(λ i ) and P(R,λ i ) into formula (1), and combine formula (1) with formula (2), that is:

[0071]

[0072] Assuming that the number density N(r) of the measured gas is uniformly distributed in the detection path R, the number density N(R) of the measured gas can be expressed as:

[0073]

[0074] The value P0(λ i ), P(R,λ i ) is substituted into formula (3), and P(R, λ1) / P(R, λ2) and P(R, λ2) / P(R, λ3) are calculated respectively. Then, the (average / column) concentrations of benzene and toluene can be obtained;

[0075]

[0076] In formula (5), when σ(λ1) is substituted as the absorption cross section of toluene, N1(R) is represented as the gas number density of toluene, denoted as N 1T (R), σ(λ3) is substituted as the absorption cross section of benzene, N1(R) is expressed as the gas number density of benzene, recorded as N 1B (R); Similarly, the gas number density N of toluene in formula (6) is obtained 2T (R) and N represents the gas number density of benzene 2B (R), and find N 1T (R) and N 2T The average value of (R) T (R), N 1B (R) and N 2B The average value of (R) B (R).

[0077] The correction method in step 3 is:

[0078] Since the absorption spectra of benzene and toluene overlap, the absorption cross sections at wavelengths λ1, λ2, and λ3 contain both benzene and toluene components. The absorption cross sections σ(λ1), σ(λ2), and σ(λ3) are divided into σ B (λ1),σ T (λ1),σ B (λ2),σ T (λ2),σ B (λ3),σ T (λ3), assuming that the gas number density of toluene is N T (R), the gas number density of benzene is N B (R), and then substituting it into formula (3), we get:

[0079]

[0080] The calculation method in step 4 is:

[0081] The N obtained in step 3 T (R) and N B (R) Calculate and analyze the ratio. If N T (R) / N B When (R)<2, it means that this monitoring point is mainly affected by motor vehicle exhaust emissions; if N T (R) / N B (R) < 2, indicating that this monitoring point is affected by the use of industrial solvents in addition to the exhaust emissions from motor vehicles. N T (R) / N B The larger the ratio (R), the greater the impact of solvent use sources, and the smaller the impact of motor vehicle exhaust emissions.

Claims

1. Mid-infrared three-wavelength differential absorption laser radar, characterized in that: A system control processing platform (1), a laser input and collimation system, a telescope receiving system, and a spectroscopic and detection system are sequentially connected along the optical path, wherein the system control processing platform (1) is connected to the laser input and collimation system and the spectroscopic and detection system via a connecting cable, and the detection system runs an algorithm for detecting atmospheric benzene pollution sources, and the algorithm for detecting atmospheric benzene pollution sources comprises the following steps: Step 1: Use the system control processing platform to adjust the three output laser wavelengths of the first inter-band cascade laser, the second inter-band cascade laser, and the third inter-band cascade laser to λ 1, λ 2, λ 3， The laser emission powers of the three wavelengths are calculated according to the voltage values ​​displayed by the fourth photodetector, the fifth photodetector, and the sixth photodetector. P 0( λ i ) ， Where i=1, 2, 3; Step 2: The continuous pulse signals of the three wavelengths in step 1 are injected into the atmosphere through the laser input and collimation system, and then received by the telescope receiving system and transmitted to the spectroscopic and detection system to measure the echo signal power of the three wavelengths in step 1. P ( R,λ i ); Step 3: Use the echo signal power obtained in step 2 P ( R,λ i ), calculate and correct the concentrations of toluene and benzene N T ( R )and N B ( R ); Step 4: N T ( R )and N B ( R ) to calculate and analyze the ratio; The calculation method in step 3 is specifically as follows: According to the laser radar principle and Beer-Larmbert theorem, the continuous wave diffuse reflection target reflection type laser radar equation can be expressed as: （1） In the formula, P (0) is the emission power of the laser, A is the effective receiving area of ​​the telescope, η is the optical efficiency of the system, R It's a long path. β is the distance laser radar R The diffuse reflectivity of the diffuse reflective target, α(r) It is the distance r Atmospheric extinction coefficient at ; The extinction coefficient is divided into two parts. One part is the extinction coefficient caused by the absorption of the sensitive gas being measured. α 1( r ), and the other part is the extinction coefficient caused by factors other than the absorption of the sensitive gas being measured. α 0( r ), the extinction coefficient can be expressed as: （2） In the formula, N ( r ) is the distance r is the molecular density of the gas being measured, σ1 is the absorption cross-sectional area of ​​the gas being measured; Differential absorption lidar uses strong absorption wavelengths λ on and weak absorption wavelength λ off , according to the two-way backscattered light signal , The material concentration is calculated by the ratio of λ on and λ off The atmospheric extinction except for the measured gas is basically the same, that is, ; Will P 0( λ i )and P ( R,λ i ) into formula (1), and combine formula (1) with formula (2), that is, (3) Assume that the number density of the measured gas is N ( r ) in the detection path R The number density of the measured gas is uniformly distributed inside. N ( R ) can be expressed as: (4) Set the value when i=1, 2, 3 P 0( λ i ), P ( R,λ i ) into formula (3), and calculate P ( R,λ 1) / P ( R,λ 2), P ( R,λ 2) / P ( R,λ 3), at this time, the (average / column) concentrations of benzene and toluene can be obtained; (5) (6) Among them, in formula (5), When the absorption cross section of toluene is substituted, N 1( R ) is the gas number density of toluene, recorded as N 1T ( R ), When the absorption cross section of benzene is substituted, N 1( R ) is the gas number density of benzene, recorded as N 1B ( R ); Similarly, the gas number density of toluene in equation (6) is obtained N 2T ( R ) and represents the gas number density of benzene N 2B ( R ), and find N 1T ( R )and N 2T ( R ) N T ( R ), N 1B ( R )and N 2B ( R ) N B ( R ); The correction method in step 3 is: Since the absorption spectra of benzene and toluene overlap, the absorption cross sections at wavelengths of λ1, λ2, and λ3 contain both benzene and toluene components. , , Divided into , , , , , , assuming that the gas number density of toluene is N T ( R ), the gas number density of benzene is N B ( R ), and then substitute into formula (3): (7) (8)。 2. The mid-infrared three-wavelength differential absorption laser radar according to claim 1, characterized in that: In step 1, among the three output laser wavelengths, λ 1 is 3304.8 nm, λ 2 is 3282.4 nm, λ 3 is 3198.8 nm.

3. The mid-infrared three-wavelength differential absorption laser radar according to claim 1, characterized in that: The calculation method in step 4 is: The obtained in step 3 N T ( R )and N B ( R ) to calculate and analyze the ratio. If N T ( R ) / N B ( R )<2, it means that this monitoring point is mainly affected by motor vehicle exhaust emissions; if N T ( R ) / N B ( R )<2, indicating that this monitoring point is affected by the use of industrial solvents in addition to the exhaust emissions from motor vehicles; N T ( R ) / N B ( R ) is larger, the greater the impact of solvent use sources is, and the smaller the impact of motor vehicle exhaust emissions is.

4. The mid-infrared three-wavelength differential absorption laser radar according to claim 1, wherein the coaxial structure distribution is characterized in that the laser input and collimation system includes three groups of optical paths arranged in parallel, wherein the first group of optical paths includes a first inter-band cascade laser (26), a first collimation system (24), a second beam splitter (22) and a first beam splitter (21) arranged in sequence along the optical path, and the second beam splitter (22) further splits the optical path into a side branch optical path to a fourth photodetector (23); the second group of optical paths includes a second inter-band cascade laser (33), a second collimation system (31), a fourth beam splitter arranged in sequence along the optical path (29) and a third beam splitter (28), the fourth beam splitter (29) further splits the optical path into a side branch optical path to a fifth photodetector (30); the third optical path group includes a third inter-band cascade laser (40), a third collimation system (38), a fifth beam splitter (36) and a fourth reflector (35) arranged in sequence along the optical path, the fifth beam splitter (36) further splits the optical path into a side branch optical path to a sixth photodetector (37), and the fourth photodetector (23), the fifth photodetector (30) and the sixth photodetector (37) are respectively connected to the system control processing platform (1) through connecting cables.

5. The mid-infrared three-wavelength differential absorption laser radar of claim 1, wherein the heteroaxial structure distribution is characterized in that the laser input and collimation system includes three groups of optical paths arranged correspondingly, wherein the first group of optical paths includes a first inter-band cascade laser (26), a first collimation system (24) and a second beam splitter (22) arranged in sequence along the optical path, and the second beam splitter (22) further splits the optical path into a side branch optical path to a fourth photodetector (23); the second group of optical paths includes a second inter-band cascade laser (33), a second collimation system (31) and a second photodetector (22) arranged in sequence along the optical path. Four beam splitters (29), wherein the fourth beam splitter (29) further splits the optical path into a side branch optical path to a fifth photodetector (30); the third optical path group comprises a third inter-band cascade laser (40), a third collimation system (38) and a fifth beam splitter (36) arranged in sequence along the optical path, wherein the fifth beam splitter (36) further splits the optical path into a side branch optical path to a sixth photodetector (37), and the fourth photodetector (23), the fifth photodetector (30) and the sixth photodetector (37) are respectively connected to the system control processing platform (1) via connecting cables.

6. The mid-infrared three-wavelength differential absorption laser radar according to claim 4 or 5, characterized in that: The laser wavelength emitted by the first inter-band cascade laser (26), the second inter-band cascade laser (33) and the third inter-band cascade laser (40) is 3000 nm-6000 nm and the power is 3-200 mW. The collimation system trims the laser into a continuous laser beam with a divergence angle of 0.2 mrad.

7. According to claim 1, the mid-infrared three-wavelength differential absorption laser radar, its coaxial structure distribution is also characterized in that the telescope receiving system includes a third reflector (18) for receiving laser input and emitting laser from the collimation system, the third reflector (18) reflects the laser to the diffuse reflection target (19), the diffuse reflection target (19) reflects the laser to the telescope (16), the telescope (16) transmits the laser to the blazed grating (12) through the third lens (15) and the second lens, and the telescope (16) has an aperture of 200 mm and a focal length of 100 mm.

8. The mid-infrared three-wavelength differential absorption laser radar according to claim 1, characterized in that: The spectroscopic and detection system comprises a third lens (15) connected to a telescope receiving system. The third lens (15) transmits laser light to a blazed grating (12) via a second lens (14). The blazed grating (12) performs spectral spectrometry on the laser light and transmits the light to a first photodetector (7), a second photodetector (8) and a third photodetector (9) via a first lens (13). The first photodetector (7), the second photodetector (8) and the third photodetector (9) are all connected to an amplifying circuit system (3) via a connecting cable. The amplifying circuit system (3) is connected to a system control processing platform (1) via a first connecting cable (2).

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