Atmospheric carbon dioxide background concentration detection method based on laser radar

Abstract

The invention discloses an atmospheric carbon dioxide background concentration detection method based on a laser radar, and the method comprises the steps: receiving a corresponding echo signal based on a plurality of groups of detection lasers with range resolution transmitted to a target atmosphere by the laser radar; the detection laser comprises first laser and second laser, and the absorption coefficient of the first laser to carbon dioxide is higher than that of the second laser to carbon dioxide; determining an absorption coefficient difference of the same group of detection laser under different distances based on the echo signals; determining an absorption coefficient difference mean value based on the absorption coefficient difference corresponding to each group of detection laser; and determining the background concentration of the atmospheric carbon dioxide based on the mean value of the absorption coefficient difference. The detection method can realize large-range measurement so as to provide wide spatial distribution data, does not need to perform physical contact on the sample, and is simple to operate and free of interference.

Description

Technical Field

[0001] The present invention relates to the field of laser radar detection technology, and in particular to a method for detecting atmospheric carbon dioxide background concentration based on laser radar. Background Art

[0002] Carbon dioxide is one of the main greenhouse gases in the atmosphere. Its main sources include natural processes such as mantle movement, biological respiration, and ocean release, as well as human activities such as fossil fuel combustion, cement production, and land use change. By 2023, the global atmospheric carbon dioxide concentration has exceeded 420ppm, a significant increase from the pre-industrial 280ppm. The increase in atmospheric carbon dioxide concentration will not only lead to a series of ecological and environmental problems such as global temperature rise, sea level rise, and changes in precipitation distribution, but will also affect the radiation balance of the earth-atmosphere system, which is closely related to global climate change. Therefore, it is necessary to detect the background of carbon dioxide in the atmosphere to monitor changes in the content of carbon dioxide in the atmosphere.

[0003] Atmospheric background refers to the long-term average state and changing characteristics of atmospheric composition and its related properties on a global or regional scale. Detecting the background level of carbon dioxide in the atmosphere is of great significance for understanding climate change, formulating environmental policies, and evaluating the global carbon cycle. At present, common methods for detecting carbon dioxide background include bottle sampling, canister sampling, and field observation. Bottle sampling and canister sampling collect air samples and send them to the laboratory for analysis. Gas chromatography separates and quantitatively analyzes carbon dioxide in the gas through a gas chromatograph, which can provide accurate concentration data. However, due to its complex and time-consuming process, it is usually used for periodic background detection rather than real-time monitoring. The ground monitoring network consists of multiple monitoring stations, which provide long-term carbon dioxide observation data on a global or regional scale to help analyze the background concentration differences and long-term change trends in different locations.

[0004] However, these traditional methods have certain limitations in spatial resolution and remote monitoring. They can usually only measure at specific points or in a small range, and it is difficult to provide extensive spatial distribution data. In addition, many traditional measurement methods require physical contact or handling of samples, which increases the complexity of the operation and may introduce contamination or interference. Summary of the invention

[0005] The purpose of the present invention is to overcome the above-mentioned defects or problems existing in the background technology and to provide a method for detecting the background concentration of atmospheric carbon dioxide based on lidar, which can realize large-scale measurement to provide a wide range of spatial distribution data, and does not require physical contact with the sample, and the operation is simple and non-interfering.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] Technical Solution 1: A method for detecting the background concentration of atmospheric carbon dioxide based on laser radar, comprising: transmitting multiple groups of detection lasers with distance resolution to the target atmosphere based on the laser radar, and receiving corresponding echo signals; the detection lasers include a first laser and a second laser, and the absorption coefficient of the first laser to carbon dioxide is higher than the absorption coefficient of the second laser to carbon dioxide; based on the echo signals, determining the absorption coefficient difference of the same group of detection lasers at different distances; based on the absorption coefficient differences corresponding to each group of detection lasers, determining the average of the absorption coefficient differences; based on the average of the absorption coefficient differences, determining the background concentration of atmospheric carbon dioxide.

[0008] Technical solution 2 based on technical solution 1: before calculating the mean value of the absorption coefficient difference, the absorption coefficient differences corresponding to each group of detection lasers are subjected to outlier elimination processing, and the absorption coefficient differences whose outlier amplitudes are greater than a preset outlier threshold are eliminated, and the absorption coefficient difference mean value is determined based on the absorption coefficient differences corresponding to each group of detection lasers after the outlier elimination processing.

[0009] Technical solution three based on technical solution two: the outlier amplitude is calculated by the median absolute deviation method.

[0010] Technical solution 4 based on technical solution 3: the outlier threshold is 2.5 to 3.5 times the median of the median absolute deviation value.

[0011] Technical solution five based on technical solution two: in the detection laser, the first laser and the second laser have the same absorption coefficient for H2O in the target atmosphere.

[0012] Technical solution 6 based on technical solution 5: the absorption coefficient difference is calculated by the following formula:

[0013] Wherein, the subscript on represents the first laser, and the subscript off represents the second laser; N s is the number of returned photons; E is the pulse energy, in J; x i is the laser wave number, in cm -1 ; R is the distance in meters; h is the Planck constant; η0 is the optical efficiency of the transmitted signal; η q is the quantum efficiency; A t is the area of ​​the telescope in m 2 ; O(R) is the geometric overlap factor at distance R; c is the speed of light; τ is the pulse width, in seconds; β is the Mie volume backscattering coefficient, in meters -1 ; T r is the transmittance function; α a is the aerosol extinction coefficient, in m -1 ; α s is the molecular extinction coefficient, expressed as α s=α+α m , where α is the absorption coefficient of carbon dioxide, α m is the extinction coefficient of other molecules in the target atmosphere; i represents the distance i×ΔR from the emission point of the detection laser.

[0014] Technical solution 7 based on technical solution 6: the outlier amplitude is calculated by the following formula:

[0015] Wherein, MAD is the median of the absolute deviation values ​​of each data point to the median; X is a data point, and its subscript represents the difference in absorption coefficient corresponding to the detection laser at different distances; k is a proportional factor, erfcinv is the inverse function of the error function; Th is the outlier threshold; n is a multiple related to the outlier threshold, and its value is 2.5-3.5; when performing outlier removal processing, if |X i -median(X)|>Th, the absorption coefficient difference corresponding to the data point will be eliminated.

[0016] Technical Solution 8 based on Technical Solution 7: The atmospheric carbon dioxide background concentration is calculated by the following formula: Where N is the molecular number density in mol / cm 3 ; is the mean value of the absorption coefficient difference; σ is the absorption cross section, in cm 2 / mol; Δσ=σ on -σ off is the difference between the absorption cross sections of the first laser and the second laser; P is the atmospheric pressure, in Pa; V is the gas volume, in m 3 ; n is the number of moles of gas; T is the ambient temperature, in K; R is the universal gas constant, which is 8.314 J·mol -1 ·K -1 ; N air is the number density of moist air molecules, in mol / m 3 ; N A is Avogadro's constant, which is 6.0225×10 23 mol -1 ;k b is the Boltzmann constant, which is 1.380649×10 -23 J.K. -1 ; is the background concentration of atmospheric carbon dioxide.

[0017] It can be seen from the above description of the present invention that, compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention provides a method for detecting atmospheric carbon dioxide background concentration based on laser radar, wherein the detection method uses multiple groups of detection lasers with distance resolution to detect the atmosphere, and the detection lasers include a first laser and a second laser with different wavelengths, the difference between the two being that the absorption of carbon dioxide by the first laser is stronger than that by the second laser, so that the contrast of the echo signal can be improved through the differential absorption technology, the accurate calculation of the carbon dioxide absorption coefficient can be achieved, the measurement accuracy can be improved, and the measurement error can be reduced; based on the above-mentioned echo signal, the absorption coefficient difference of the two echo signals corresponding to each group of detection lasers at different distances can be obtained, and then the absorption coefficient difference mean of the absorption coefficient differences is calculated, and then the atmospheric carbon dioxide background concentration is calculated based on the absorption coefficient difference mean. The detection method can measure the target atmosphere over a large range through laser detection to provide a wide range of spatial distribution data, and because the laser detection method is adopted, no physical contact with the sample is required, the operation is simple and interference-free, and the accuracy of atmospheric carbon dioxide background concentration detection can be effectively improved.

[0019] Among them, since the detection method can measure the target atmosphere over a large range, it is easily affected by the carbon dioxide source and sink area within the detection range, making the final measured atmospheric carbon dioxide background concentration inaccurate. For this reason, in the detection method provided by the present invention, the absorption coefficient difference corresponding to each group of detection lasers is further processed by outlier elimination. After the outlier elimination process, the affected data points can be eliminated to reduce the impact of these data on the overall measurement.

[0020] When performing outlier removal processing, the outlier amplitude can be calculated by the median absolute deviation method, and the outlier amplitude can be compared with the preset outlier threshold. If the outlier amplitude is greater than the preset outlier threshold, it means that the data point is abnormal and needs to be removed. The data for the final calculation of the mean absorption coefficient difference will only include data with outlier amplitudes less than or equal to the preset outlier threshold. The mean absorption coefficient difference obtained based on these processed data can better fit the actual background concentration of carbon dioxide in the target atmosphere, eliminating the influence of the carbon dioxide source and sink areas in the target atmosphere.

[0021] Among them, the outlier amplitude can be calculated by the median absolute deviation method, and the outlier threshold is set to the median of 2.5 to 3.5 times the median absolute deviation value. The selection range of the outlier threshold can not only reduce the impact of the carbon dioxide source and sink areas in the target atmosphere on the mean absorption coefficient difference, but also will not cause the absorption coefficient difference that slightly deviates from the median value to be eliminated. It can achieve a good balance between reducing the impact of abnormal samples and ensuring the number of samples.

[0022] In addition, the absorption coefficients of the first laser and the second laser in the detection laser to H2O in the target atmosphere are made the same. When processing and analyzing the echo signal, the influence of H2O in the target atmosphere on the detection result can be ignored, which reduces the calculation difficulty and improves the detection accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 A schematic diagram of a flow chart of a method for detecting atmospheric carbon dioxide background based on a laser radar provided in an embodiment of the present invention;

[0025] Figure 2 A schematic diagram of the working method of atmospheric carbon dioxide background detection based on laser radar provided in an embodiment of the present invention;

[0026] Figure 3 In the embodiment of the present invention, 6360 cm -1 A wavelength selection diagram of the first laser and the second laser taking the wavelength band as an example;

[0027] Figure 4 In the embodiment of the present invention, on =6359.962cm -1 ,λ off =6360.513cm -1 For example, on With λ off Schematic diagram of signal-to-noise ratio at 0-6km;

[0028] Figure 5 In the embodiment of the present invention, on =6359.962cm -1 ,λ off =6360.513cm -1 For example, the difference in absorption coefficient at different distances of 0-6km is α on -α off Schematic diagram of

[0029] Figure 6 In the embodiment of the present invention, on =6359.962cm -1 ,λ off =6360.513cm -1 For example, the outlier amplitude of each data point at different distances of 0-6km|X iSchematic diagram of -median(X)| and outlier threshold Th. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are preferred embodiments of the present invention and should not be regarded as excluding other embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] In the claims, description and the above-mentioned drawings of the present invention, unless otherwise clearly defined, the use of terms such as "first", "second" or "third" etc. are for distinguishing different objects rather than for describing a specific order.

[0032] In the claims, specification and the above-mentioned drawings of the present invention, unless otherwise explicitly defined, directional words, such as the terms "center", "lateral", "longitudinal", "horizontal", "vertical", "top", "bottom", "inside", "outside", "up", "down", "front", "back", "left", "right", "clockwise", "counterclockwise", etc., indicating directions or positional relationships are based on the directions and positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction or be constructed and operated in a specific direction, and therefore cannot be understood as limiting the specific protection scope of the present invention.

[0033] In the claims, specification and the above drawings of the present invention, unless otherwise clearly defined, if the term "fixed connection" or "fixed connection" is used, it should be understood in a broad sense, that is, any connection method without a displacement relationship and relative rotation relationship between the two, that is to say, including non-detachable fixed connection, detachable fixed connection, integrated connection and fixed connection through other devices or elements.

[0034] In the claims, description and drawings of the present invention, if the terms "include", "have" and their variations are used, they are intended to mean "including but not limited to".

[0035] Example

[0036] An embodiment of the present invention provides a method for detecting the background concentration of atmospheric carbon dioxide based on a laser radar. The detection method is used to detect the background concentration of carbon dioxide in a target atmosphere through a laser radar. The laser radar used is a differential absorption laser radar, which can simultaneously emit two laser beams to detect the target atmosphere.

[0037] Reference Figure 1, which shows the process of the atmospheric carbon dioxide background concentration detection method based on laser radar provided in this embodiment. Specifically, the detection method includes the following steps:

[0038] S001: Based on the laser radar, multiple groups of detection lasers with distance resolution are emitted to the target atmosphere, and corresponding echo signals are received;

[0039] S002: Determine the difference in absorption coefficient of the same group of detection lasers at different distances based on the echo signal;

[0040] S003: determining a mean value of the absorption coefficient difference based on the absorption coefficient difference corresponding to each group of detection lasers;

[0041] S004: Determine the atmospheric carbon dioxide background concentration based on the mean value of the absorption coefficient difference.

[0042] In step S001, the detection laser includes a first laser and a second laser, and the absorption coefficient of the first laser to carbon dioxide is higher than the absorption coefficient of the second laser to carbon dioxide; and the first laser and the second laser selected by the detection laser have the same absorption coefficient to H2O in the target atmosphere. During detection, multiple groups of detection lasers are emitted to the target atmosphere through the laser radar, and the detection distance of each group of detection lasers is different, thereby forming an echo signal with distance resolution.

[0043] In step S003, the absorption coefficient differences are processed using the following steps: before calculating the mean of the absorption coefficient differences, the absorption coefficient differences corresponding to each group of detection lasers are subjected to outlier elimination processing, and the absorption coefficient differences whose outlier amplitudes are greater than a preset outlier threshold are eliminated, and the mean of the absorption coefficient differences is determined using the absorption coefficient differences corresponding to each group of detection lasers after the outlier elimination processing.

[0044] In addition, in step S003, the outlier amplitude is calculated by the median absolute deviation method. The outlier threshold is 2.5 to 3.5 times the median absolute deviation value.

[0045] The method for detecting the atmospheric carbon dioxide background concentration based on laser radar provided in this embodiment uses multiple groups of detection lasers with distance resolution to detect the atmosphere, and the detection lasers include a first laser and a second laser with different wavelengths. The difference between the two is that the absorption of carbon dioxide by the first laser is stronger than that by the second laser, so that the contrast of the echo signal can be improved through the differential absorption technology, and the accurate calculation of the carbon dioxide absorption coefficient can be achieved, the measurement accuracy can be improved, and the measurement error can be reduced; based on the above echo signals, the absorption coefficient difference of the two echo signals corresponding to each group of detection lasers at different distances can be obtained, and then the absorption coefficient difference mean of these absorption coefficient differences is calculated, and then the atmospheric carbon dioxide background concentration is calculated based on the absorption coefficient difference mean. The detection method can measure the target atmosphere over a large range by laser detection to provide a wide range of spatial distribution data, and because the laser detection method is adopted, it does not require physical contact with the sample, the operation is simple and interference-free, and the accuracy of atmospheric carbon dioxide background concentration detection can be effectively improved.

[0046] Among them, since the detection method can measure the target atmosphere over a large range, it is easily affected by the carbon dioxide source and sink area within the detection range, making the final measured atmospheric carbon dioxide background concentration inaccurate. For this reason, in the detection method provided by the present invention, the absorption coefficient difference corresponding to each group of detection lasers is further processed by outlier elimination. After the outlier elimination process, the affected data points can be eliminated to reduce the impact of these data on the overall measurement.

[0047] When performing outlier removal processing, the outlier amplitude can be calculated by the median absolute deviation method, and the outlier amplitude can be compared with the preset outlier threshold. If the outlier amplitude is greater than the preset outlier threshold, it means that the data point is abnormal and needs to be removed. The data for the final calculation of the mean absorption coefficient difference will only include data with outlier amplitudes less than or equal to the preset outlier threshold. The mean absorption coefficient difference obtained based on these processed data can better fit the actual background concentration of carbon dioxide in the target atmosphere, eliminating the influence of the carbon dioxide source and sink areas in the target atmosphere.

[0048] Among them, the outlier amplitude can be calculated by the median absolute deviation method, and the outlier threshold is set to 2.5 to 3.5 times the median absolute deviation value. The selection range of the outlier threshold can not only reduce the impact of the carbon dioxide source and sink areas in the target atmosphere on the mean absorption coefficient difference, but also will not cause the absorption coefficient difference that slightly deviates from the median value to be eliminated. It can achieve a good balance between reducing the impact of abnormal samples and ensuring the number of samples.

[0049] In addition, the absorption coefficients of the first laser and the second laser in the detection laser to H2O in the target atmosphere are made the same. When processing and analyzing the echo signal, the influence of H2O in the target atmosphere on the detection result can be ignored, which reduces the calculation difficulty and improves the detection accuracy.

[0050] The following further describes the specific detection process of the atmospheric carbon dioxide background concentration detection method based on laser radar provided in this embodiment. In the following description, the effectiveness of the above detection method is verified through a specific experiment. Figure 2 In this experiment, the background concentration of carbon dioxide in the target atmosphere is 450ppm, the concentration of H2O is 1.7%, the temperature is 297K, the pressure is 101325Pa, and there is interference from factory exhaust emissions at a distance of 4 kilometers from the laser radar emission point. Factory exhaust emissions will cause the background concentration of carbon dioxide in the atmosphere of this area to rise.

[0051] In this embodiment, a differential absorption laser radar is used to detect the target atmosphere. The detection laser emitted by the differential absorption laser radar includes a first laser and a second laser. The laser radar equation is expressed by the following formula (1) and formula (2):

[0052]

[0053] In equations (1) and (2), the subscript on represents the first laser, and the subscript off represents the second laser; N s is the number of returned photons; E is the pulse energy, in J; x i is the laser wave number, in cm -1 ; R is the distance in meters; h is the Planck constant; η0 is the optical efficiency of the transmitted signal; η q is the quantum efficiency; A t is the area of ​​the telescope in m 2 ; O(R) is the geometric overlap factor at distance R; c is the speed of light; τ is the pulse width, in seconds; β is the Mie volume backscattering coefficient, in meters -1 ; T r is the transmittance function; α a is the aerosol extinction coefficient, in m -1 ; α s is the molecular extinction coefficient, expressed as α s =α+α m , where α is the absorption coefficient of carbon dioxide, α m is the extinction coefficient of other molecules in the target atmosphere; i represents the distance i×ΔR from the emission point of the detection laser.

[0054] Reference Figure 3 In this embodiment, we select λ on=6359.962cm -1 ,λ off =6360.513cm -1 , that is, the wavelength of the first laser is 6359.962cm -1 , the wavelength of the second laser is 6360.513cm -1 In this embodiment, the wavelengths of the two lasers have the same absorption coefficient for H2O in the target atmosphere, so the effect of water vapor on the CO2 detection result can be ignored.

[0055] Reference Figure 4 , which shows the signal-to-noise ratio results of the first laser and the second laser in the distance range of 0-6km.

[0056] At the same time, since the wavelength difference between the first laser and the second laser in the detection laser is small, and the wavelength change is within 0.2pm, when the emission energy of the first laser and the second laser is the same, it can be considered that β and α a and α m It is not sensitive to wavelength, so it can be obtained from the lidar equation:

[0057]

[0058] According to formula (3), we can get:

[0059]

[0060] Among them, α on -α off That is, the absorption coefficient difference corresponding to each group of detection lasers. In this embodiment, ΔR is 300 m.

[0061] Reference Figure 5 , which shows the relationship between the absorption coefficient difference obtained according to the above equations (1) to (5) and the distance, where each data point corresponds to a set of absorption coefficient differences obtained by the detection laser. Figure 5 It can be seen that there are abnormal points with drastic changes in these data points, that is, the points where the background concentration of carbon dioxide changes abnormally, indicating that there are detailed sources and sinks of carbon dioxide at these points. When calculating the background concentration of atmospheric carbon dioxide, these abnormal points need to be eliminated.

[0062] When performing outlier removal, the difference in absorption coefficient at different distances is recorded as X1, X2, …, X n , calculate the median MAD of the absolute deviation of each data point to the median:

[0063] MAD=median(|X i -median(X)|) (6);

[0064] The MAD is converted to a quantity similar to the standard deviation using a scaling factor k, which is calculated by the following formula:

[0065]

[0066] Wherein, MAD is the median of the absolute deviation values ​​of each data point to the median; X is a data point, and its subscript represents the difference in absorption coefficient corresponding to the detection laser at different distances; k is the proportional factor, and erfcinv is the inverse function of the error function.

[0067] Afterwards, the outlier threshold can be obtained by formula (8):

[0068] Th = n × k × MAD (8);

[0069] Wherein, Th is the outlier threshold; n is a multiple related to the outlier threshold, and its value is 2.5-3.5; in this embodiment, the value of n is selected as 3. Therefore, in this embodiment, when performing outlier removal processing, if the outlier amplitude |X i -median(X)|>Th, that is, the outlier amplitude is 3 times MAD; then the absorption coefficient difference corresponding to the data point is eliminated.

[0070] Reference Figure 6 , which shows Figure 5 The absolute deviation value of each data point to MAD |X i -median(X)| and outlier threshold Th, we can see that the corresponding Figure 5 The two abnormal points in are higher than the outlier threshold, so these two abnormal points are removed, and the absorption coefficient difference corresponding to other data points below the outlier threshold is used to calculate the atmospheric carbon dioxide concentration.

[0071] After obtaining multiple sets of available absorption coefficient differences, these absorption coefficient differences are averaged and then the atmospheric carbon dioxide concentration is calculated according to the following formula:

[0072]

[0073] PV = nRT (11);

[0074]

[0075] Where N is the molecular number density in mol / cm 3 ; is the mean value of the absorption coefficient difference; σ is the absorption cross section, in cm 2 / mol; Δσ=σ on -σ off is the difference between the absorption cross sections of the first laser and the second laser; P is the atmospheric pressure, in Pa; V is the gas volume, in m3 ; n is the number of moles of gas; T is the ambient temperature, in K; R is the universal gas constant, which is 8.314 J·mol -1 ·K -1 ; N air is the number density of moist air molecules, in mol / m 3 ; N A is Avogadro's constant, which is 6.0225×10 23 mol -1 ;k b is the Boltzmann constant, which is 1.380649×10 -23 J.K. -1 ; is the background concentration of atmospheric carbon dioxide.

[0076] In this embodiment, the atmospheric carbon dioxide concentration is calculated to be 449.9475 ppm, which is consistent with the background concentration of carbon dioxide in the target atmosphere detected by this embodiment, indicating that the detection method provided by this embodiment can effectively and accurately detect the background concentration of carbon dioxide in the target atmosphere.

[0077] The description of the above specification and embodiments is used to explain the protection scope of the present invention, but does not constitute a limitation on the protection scope of the present invention. Through the enlightenment of the present invention or the above embodiments, ordinary technicians in this field can obtain modifications, equivalent substitutions or other improvements to the embodiments of the present invention or part of the technical features thereof through logical analysis, reasoning or limited experiments, which should be included in the protection scope of the present invention.

Claims

1. A method for detecting atmospheric carbon dioxide background concentration based on laser radar, characterized in that: include: Based on the laser radar, multiple groups of detection lasers with distance resolution are emitted into the target atmosphere, and corresponding echo signals are received; The detection laser includes a first laser and a second laser, and the absorption coefficient of the first laser to carbon dioxide is higher than the absorption coefficient of the second laser to carbon dioxide; Based on the echo signal, determining the difference in absorption coefficient of the same group of detection lasers at different distances; Determine a mean value of the absorption coefficient difference based on the absorption coefficient difference corresponding to each group of detection lasers; Based on the mean of the absorption coefficient differences, the atmospheric carbon dioxide background concentration is determined.

2. The method for detecting atmospheric carbon dioxide background concentration based on laser radar as claimed in claim 1, characterized in that Before calculating the mean value of the absorption coefficient difference, the absorption coefficient differences corresponding to each group of detection lasers are subjected to outlier elimination processing, and the absorption coefficient differences whose outlier amplitudes are greater than a preset outlier threshold are eliminated, and the absorption coefficient difference mean value is determined using the absorption coefficient differences corresponding to each group of detection lasers after the outlier elimination processing.

3. The method for detecting atmospheric carbon dioxide background concentration based on laser radar as claimed in claim 2, characterized in that: The outlier magnitude is calculated using the median absolute deviation method.

4. The method for detecting atmospheric carbon dioxide background concentration based on laser radar as claimed in claim 3, characterized in that: The outlier threshold is 2.5 to 3.5 times the median of the median absolute deviation value.

5. The method for detecting atmospheric carbon dioxide background concentration based on laser radar as claimed in claim 2, characterized in that: In the detection lasers, the first laser and the second laser have the same absorption coefficient for H2O in the target atmosphere.

6. The method for detecting atmospheric carbon dioxide background concentration based on laser radar as claimed in claim 5, characterized in that: The absorption coefficient difference is calculated by the following formula: Wherein, the subscript on represents the first laser, and the subscript off represents the second laser; N s is the number of returned photons; E is the pulse energy, in J; x i is the laser wave number, in cm -1 ; R is the distance in meters; h is the Planck constant; η0 is the optical efficiency of the transmitted signal; η q is the quantum efficiency; A t is the area of ​​the telescope in m 2 ; O(R) is the geometric overlap factor at distance R; c is the speed of light; τ is the pulse width, in seconds; β is the Mie volume backscattering coefficient, in meters -1 ; T r is the transmittance function; α a is the aerosol extinction coefficient, in m -1 ; α s is the molecular extinction coefficient, expressed as α s =α+α m , where α is the absorption coefficient of carbon dioxide, α m is the extinction coefficient of other molecules in the target atmosphere; i represents the distance i×ΔR from the emission point of the detection laser.

7. The method for detecting atmospheric carbon dioxide background concentration based on laser radar as claimed in claim 6, characterized in that: The outlier magnitude is calculated by the following formula: in, MAD is the median of the absolute deviation values ​​of each data point to the median; X is a data point, and its subscript represents the difference in absorption coefficient corresponding to the detection laser at different distances; k is a proportional factor, erfcinv is the inverse function of the error function; Th is the outlier threshold; n is a multiple related to the outlier threshold, and its value is 2.5-3.5; When performing outlier removal, if the outlier amplitude |X i -median(X)|>Th, the absorption coefficient difference corresponding to the data point will be eliminated.

8. The method for detecting atmospheric carbon dioxide background concentration based on laser radar as claimed in claim 7, characterized in that: The atmospheric carbon dioxide background concentration is calculated by the following formula: Where N is the molecular number density in mol / cm 3 ; is the mean value of the absorption coefficient difference; σ is the absorption cross section, in cm 2 / mol; Δσ=σ on -σ off is the difference between the absorption cross sections of the first laser and the second laser; P is the atmospheric pressure, in Pa; V is the gas volume, in m 3 ; n is the number of moles of gas; T is the ambient temperature, in K; R is the universal gas constant, which is 8.314 J·mol -1 ·K -1 ; N air is the number density of moist air molecules, in mol / m 3 ; N A is Avogadro's constant, which is 6.0225×10 23 mol -1 ;k b is the Boltzmann constant, which is 1.380649×10 -23 J.K. -1 ; is the background concentration of atmospheric carbon dioxide.

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