CN106526614A - Method for optimizing laser radar detection atmospheric composition spectral line analysis - Google Patents

Abstract

A method to optimize the spectral line analysis of atmospheric composition by lidar detection, by establishing the IPDA lidar equation for hard target reflection, the calculation of the differential optical thickness of the two pulses and the weighted _CO2 air column mixing rate _XCO2 , a single spectral line Calculation of broadening, calculation of absorption cross section, calculation of CO ₂ absorption optical thickness, weight function profile, CO ₂ molecular number density profile, and weight function optimized spectral line. The weight ratio of spectral lines obtained by this method to the lower atmosphere is higher, and the detection is closer to the real value.

Description

A method for optimizing spectral line analysis of atmospheric components detected by lidar

technical field

The invention belongs to a method for analyzing and optimizing laser radar detection spectral lines, in particular to a spectral line optimization method for differential absorption detection of atmospheric components. .

Background technique

Before we can understand the contribution of greenhouse gases, clouds and aerosols to climate change, we first need to define radiative forcing. It is difficult to directly quantify the changes in surface temperature due to fluctuations in gas or particle concentrations, and introducing radiative forcing as an intermediate quantity is central to understanding either the greenhouse effect of _CO2 or the complex cooling produced by the interaction between clouds and aerosols The effects of radiation are all quantitatively analyzed by the change of radiative forcing. Radiative forcing is the net change in the energy balance in the Earth system due to some forcing perturbation. It is usually expressed in terms of power per square meter over a specified period of time, and thus the quantitative analysis comes from the imbalance in energy generation to the system when disturbance forcing occurs. Radiative forcing represents changes in the amount of radiation between two specified time periods, such as changes in the energy system between before the Industrial Revolution and the present. Radiative forcing can provide a basis for quantitative analysis of the potential climate responses caused by different forcing sources, especially global mean temperature changes. The change of radiative forcing over time provides more complete information for the study of climate change.

The main greenhouse gases are carbon dioxide (CO ₂ ), methane (CH ₄ ) and nitrous oxide (N ₂ O), and the large increase in their concentration is mainly directly caused by human activities. CO ₂ is the most important greenhouse gas. _CO2 is almost transparent to solar radiation. However, it is a strong absorber in the 15 μm band (about 12-18 μm) of the thermal infrared spectrum. An increase in the CO ₂ content in the atmosphere will cause the atmosphere to trap more thermal infrared radiation emitted from the surface and lower atmosphere, thereby enhancing the greenhouse effect and causing global warming.

In the "Kyoto Protocol", it is required to consider the determined distribution of carbon sinks on the earth in conjunction with national emission inventories. In most regions, the carbon fluxes specified by the protocol need to be 20% of the natural fluxes on the five-year time scale. Because the protocol stipulates that surface fluxes are considered only over defined regions, a top-down inspection method that only detects total fluxes cannot meet the detection requirements without additional information. If, in the future, the Kyoto Protocol can include the full distribution of Earth's fluxes in countries' carbon budgets, the use of a top-down approach to estimate fuel emission reductions from total fluxes will work well to address the issues listed above. out of the second purpose. This requires very high demands on precision.

Lidar (Light Detection and Ranging) is the abbreviation of laser detection and ranging system. It uses laser as the emission source to irradiate the target, and uses the effect of target emission, refraction, scattering and transmission on the echo radiation. Active remote sensing equipment for detection. Spaceborne lidar combines the advantages of active detection of lidar, and transmits and receives laser pulses from top to bottom in earth orbit to achieve the purpose of detecting the composition and properties of the global atmosphere. Space-borne lidar carries its own radiation source, which can conduct all-weather detection on the day and night side of the earth, and emit radiation energy into the earth's atmosphere in a small transmission volume. On top of this, the optical path of the active optical instrument is known, so inversion of the target gas concentration from the detection does not require solving complex radiative transfer equations. The laser source can be controlled to operate at a single frequency. Due to the different attenuation characteristics of different gases, the influence of other gases can be ignored, and active, continuous, and refined detection can be completed, which complements each other with passive remote sensing detection methods. Provide support with climate detection system, air pollution monitoring system, and ecological monitoring system.

Contents of the invention

The present invention does not solve the above-mentioned technical problems, and the present invention adopts the following technical solutions:

A method for optimizing laser radar detection atmospheric composition spectral line analysis is characterized in that the method comprises the following steps:

Step 1, establish the IPDA lidar equation for hard target reflection

The integrated path differential absorption (IPDA, Integrated Path Differential Absorption) lidar detection method is used to detect the backward reflection signal from the hard target. The echo signal received by the detector comes from the pulse echo signal reflected back from the hard target. Specifically, the IPDA lidar The equation is as follows:

Equations (1) and (2) are used to calculate the CO ₂ content in the entire atmosphere based on the optical thickness of each laser pulse wavelength absorbed by CO ₂ during atmospheric transmission. Receive the echo signal reflected by the ground surface through the detector,

Step 2, characterization of parameters in step 1

P _on (P _off ) is the energy of the callback signal received by the detector (mJ), which is the value to be detected; E _on (E _off ) is the pulse energy of the transmitter (mJ), A is the area of the receiver (m ² ), and ρ is Surface albedo, η _d is the detector quantum efficiency, η _r is the receiver efficiency, R _G is the height of the satellite from the surface (km), τ _g is the optical thickness caused by the extinction of atmospheric components other than the target gas, σ _on and σ _off is the absorption cross section of CO ₂ molecules at different heights (cm ² ), q _CO2 is the volume mixing rate of CO ₂ dry air, that is, the CO ₂ concentration, which is the value to be retrieved; n _air is the number density profile of air molecules (cm ^-3 ) , C is the lidar system constant.

Step 3. Calculation of the differential optical depth of the two pulses and the weighted CO ₂ air column mixing rate XCO ₂

The weighted CO ₂ dry air column mixing rate XCO ₂ is calculated by receiving the echo signal energy reflected by the ground surface through the IPDA laser radar, and using the differential absorption principle to eliminate the interference of absorbing components such as water vapor, and obtain the weighted CO ₂ Dry air column mixing rate inversion data XCO ₂ , the XCO ₂ is the secondary data product of IPDA lidar inversion, the specific calculation is as follows:

First, the spaceborne IPDA lidar receives the echo signals P _on (P _off ) of two laser pulses reflected by hard targets on the ground surface. According to (1) and (2), the differential optical thickness (τ _on -τ _off )

q _H2O is the water vapor volume mixing rate profile. Secondly, the relationship between the received echo signal and the CO ₂ mixing rate can be established according to the hydrostatic equation, the ideal gas state equation and the differential optical thickness obtained in the previous step. The weighted CO ₂ dry air column mixing ratio XCO ₂ is obtained by

Substitute into (3) to get,

where △σ is the difference in absorption cross-section between the strong and weak absorption lines, thus avoiding the influence on the _CO2 mixing rate profile , and the CO ₂ column content in the atmospheric column is retrieved by receiving the relative energy change of the pulse callback signal on the two wavelengths, where WF(r) is a weight function, representing the distribution of CO ₂ absorption capacity on the pulse path length , p _surf and P _top represent the pressure values of the lower atmosphere and the upper atmosphere, which are the upper and lower limits of the weight function calculation, where p _surf is 1013.25hPa, p _top is 0hPa, obtained from the hydrostatic equation:

Wherein, M _H2O and M _CO2 are water vapor and _CO Relative molecular mass, wherein M _H2O is 18g/mol, is 44 g/mol.

Step 4, calculate the differential optical thickness and the weighted CO ₂ air column mixing rate XCO ₂ of the two pulses in step 3 through the calculation of the data in the spectral database

The spectral parameter data selected according to the data in the existing HITRAN database include the position of the spectral wavenumber corresponding to the molecule or atom in vacuum, the line intensity of the spectral line, the temperature index, the frequency shift of the spectral line position caused by the air pressure, and Lorentz Spectral line self-broadening half-width and other parameters, using the frequency and line width of the laser pulse emitted by the laser radar, calculate the differential absorption cross section of _CO2 gas molecules from the absorption spectrum curves of CO2 gas molecules under different temperature and pressure conditions. The change in the vertical height is substituted into Step 3 for calculation, and the weighted CO ₂ dry air column mixing rate XCO ₂ is obtained;

Step 5, single spectral line broadening calculation

The broadening of the single spectral line is obtained by calculating and gradually integrating different temperature and air pressure conditions in a certain wavenumber range near the target wavelength using the Vogt line pattern. The Vogt profile used is the convolution of the Lorentz linetype and the Doppler linetype. The convolution integral on the infinite field cannot be evaluated under the closed condition, and the complex error function is used to solve it. f _V (x, y) is the broadened line shape of a single absorption line, which obeys the complex error function line shape,

in,

v ₀ (cm ^-1 ) is the position of the absorption peak in the wavenumber domain, and v(cm ^-1 ) is the integrated spectral line domain. γ _D is the Doppler half-width of the absorption line (cm ^-1 /atm), expressed as follows:

B is Boltzmann's constant (J·K ^-1 ), T is temperature (K), and m is relative molecular mass. The variation of Lorentz half-width γ _L with temperature and air pressure is:

γ _L (T,p)＝γ _L (296,1atm)p/p ₀ (296/T) ^φ (25)

φ is the molecular constant exponent obtained at a temperature of 296K.

Line strength S is affected by temperature:

S ₀ is the intensity of the spectral line at 296K, h is Planck's constant (J·s), and c is the speed of light (m·s ^-1 ). The half-width γ _L (296,1 atm), φ and E″ of the Lorentzian lineform in the standard state can be obtained from the HITRAN database.

For the spectral absorption line at the specified wavenumber when the temperature and pressure are constant, the Doppler broadening is known, that is, y is known; the broadening of the spectral line in the lower atmosphere is mainly Lorentzian line broadening, with Doppler broadening increases with height. Vogt lines are used, which can be obtained by convolution of Lorentz lines and Doppler lines. The lineshape calculated by Vogt broadening is the lineshape of single spectral line broadening.

Step 6, calculate the absorption cross section by line-by-line integration

The line-by-line integration calculation is to accumulate the discrete broadened absorption lines in the entire spectral range according to a certain wave number interval to calculate the absorption properties of the gas one by one. According to step 5, the absorption lines near the target wave number are calculated line by line. The broadening calculation, and then superimpose each broadening profile on the target wavenumber to obtain the absorption cross-section after line-by-line integration. The calculation formula of line-by-line integration within a given wavenumber range is:

Among them, α(v) is the absorption cross section at the target wavenumber v, S _i is the line intensity of the i-th spectral line in a given wavenumber domain, f(vv _{0, i} ) is the wavenumber after the i-th absorption spectral line is broadened The absorption line shape at v, the line-by-line integration meets the requirements of IPDA for high-precision spectral parameters.

Step 7, Atmospheric Model Establishment

The profile of conventional meteorological parameters (including temperature, air pressure and humidity, etc.) established by the United States in 1976 is used as the basis for studying IPDA lidar parameters. The establishment of the atmospheric temperature, pressure and density profile model in this model is shown in Table 1.

Table 4 The standard atmospheric temperature, pressure and density profiles of the United States in 1976

Where h is the altitude above sea level in meters, the sea level temperature T ₀ is 288.15K, the density ρ ₀ =1.225kg/m ³ , and the sea level standard air pressure P ₀ =1.1325N/m ² .

Step 8, _CO2 Absorption Optical Depth Calculation

The CO ₂ absorption optical thickness is the absorption capacity of CO ₂ in the whole atmosphere to the pulse energy, which reflects the total content of CO ₂ in the atmospheric column. The definition of the optical thickness is

Among them, τ is the optical thickness, L is the propagation distance of light energy on the path, σ is the absorption cross section, and N is the number of molecules of the gas to be measured on the path.

Step 9, Screening Optical Depth

After calculating the optical thickness of each point on the unit wavenumber interval in the target wavenumber domain, according to the result calculated in step 8, the optical thickness is screened for the wavenumber in the target wavenumber domain, and the optical thickness is selected to be greater than 0.02 and less than 4 as the standard for screening optical thickness , as shown in the following formula

v _step1 = where(0.02<τ _i <4) (29)

v _step1 is the wavenumber filtered by the optical depth, and _τi is the optical thickness obtained by step 8 at each wavenumber point in the target wavenumber domain.

Step 10, spectral line atmospheric sensitivity calculation

The temperature sensitivity equation of this spectral line is:

Δτ is the differential optical thickness. According to the hydrostatic equilibrium equation, the barometric sensitivity is calculated for the change in mixing rate obtained by increasing the proportionality constant ε in each layer of the atmosphere. The pressure sensitivity equation is:

As a strong absorbing gas in the atmosphere, water vapor changes the differential absorption optical depth, and the change of water vapor affects the integral weight function. Assuming that the water vapor profile in the entire atmosphere changes in an equal proportion (1+ε) for each layer, the water vapor sensitivity can be obtained by the following formula:

Step eleven, weight function profile

Divide equation (1) and equation (2) of IPDA lidar in step 1, and equation (4) in step 3 can be derived according to the hydrostatic equation and ideal gas state equation, and thus the differential optical thickness and XCO ₂ Using the relationship between the echo signal intensities at the two differential wavelengths directly measured by the IPDA lidar and other correction parameters, the differential absorption optical thickness of _CO2 gas can be obtained. The weight function is calculated by formula (6) in Step 3. The differential absorption cross sections at different heights are affected by air pressure and temperature. According to steps 5 and 7, the weight function profile can be obtained. According to the weight function profile, the vertical distribution of CO ₂ gas molecular number density or mixing ratio cannot be obtained in the differential absorption optical depth, and finally the column content of CO ₂ in the atmospheric column is obtained.

Step 12, _CO2 Molecular Number Density Profile

There are differences in the inversion between the profile with constant CO ₂ mixing rate and the profile with CO ₂ gradient in the vertical direction. The CO ₂ mixing rate profile used is shown in Table 2, where i represents the number of atmospheric layers, h and P are the altitude and pressure in the atmospheric model established in step seven. The number density profile of CO ₂ molecules has a layered structure, and its number density varies with height. The calculation formula is shown in the table below.

Table 5 CO ₂ mixing rate vertical stratification profile

layered CO ₂ mixing rate (ppm) 0～2 392.0713 3～24 (392.0713-10/22*(h(i)-2))*P(i) 25～71 (392.0713-10-2/47*(h(i)-24))*P(i)

Step 13, the weight function optimizes the spectral line

According to formula (4) in step 3, the weight function represents the distribution of differential optical thickness at each height in the vertical direction in the atmospheric column, and the absorption coefficient profile σ(r)N(r) in formula (12) is integrated in step 8 and the _CO2 mixing rate profile obtained in step 13, the absorption cross section obtained in step 6, and the density of each layer in step 7, to obtain the results related to the weight function. The spectral lines optimized by the weight function are shown in Table 3:

Table 6 The results obtained by weight function correlation

2. A method for optimizing laser radar detection of atmospheric component spectral line analysis as claimed in claim 1, wherein the system parameters of spaceborne IPDA laser radar in said step 1 are as described in Table 4 below:

Table 4

3. A method for optimizing spectral line analysis of atmospheric components detected by lidar as claimed in claim 1, characterized in that the hard target in the first step is the ground surface, water surface or thick clouds.

4. A method for optimizing the spectral line analysis of lidar detection atmospheric components as claimed in claim 1, characterized in that the data in the HITRAN database has been updated to the 2012 version

Beneficial effect:

_CO2 is one of the most important greenhouse gases and persists in the atmosphere for a long time. Rising levels of _CO2 in the atmosphere are the main cause of global warming. According to the fifth IPCC report, greenhouse gases such as _CO2 are mainly produced by fossil fuel combustion, land use and transformation. _CO2 concentration changes and climate effects require long-term observations of _CO2 flux changes and their spatial distribution. In order to meet the emission reduction requirements of the "Kyoto Protocol", help the government formulate emission reduction policies, and scientifically study the climate effects of CO ₂ , it is necessary to detect global and regional CO ₂ emissions and mixing rates with high precision and high reliability detection. However, the existing technology is difficult to meet this requirement.

Space-borne IPDA lidar, as a new generation of space-borne active detection means, can realize _CO2 detection with high precision and high temporal and spatial resolution. Its operating wavelength determines the response of the detection to the true situation of _CO2 in the whole atmosphere. After optimizing the IPDA lidar pulse spectrum according to the method discussed in this patent, it has the following beneficial effects:

The weight ratio of the obtained spectral lines to the lower atmosphere is higher, and the detection is closer to the real value. The relative molecular mass of CO ₂ is greater than that of air, and the air pressure decreases exponentially with altitude, so CO ₂ mainly exists in the lower atmosphere. The detection principle of IPDA lidar determines that the content of CO ₂ in the entire atmosphere can be obtained by spectral line detection. According to the weight optimization spectral line obtained by this method to detect CO ₂ , the detection result is closer to the atmospheric column CO ₂ content.

More sensitive to lower atmospheric _CO2 fluctuations. Sources such as fossil fuel combustion and forest fires produce large amounts of CO ₂ on a small scale, which is then diluted by atmospheric absorption and transport processes. Therefore, there will be large fluctuations in _CO2 content in the lower atmosphere in the local area, which requires the detection of these fluctuations while having high temporal and spatial resolution. The pulse spectral lines obtained after weight fitting optimization are used for detection, and the weights are more weighted towards the lower layers of the atmosphere, so the lower layer _CO2 fluctuations have a greater impact on the echo signal and are easier to be detected.

Improved time efficiency for spectral line optimization. IPDA lidar requires high-resolution spectroscopy to retrieve _CO2 content based on differences in absorption properties. In the process of instrument design, in addition to hardware parameters and existing technology as limiting conditions, it is still necessary to screen spectral points one by one in a considerable section of the spectral domain. Using traditional methods to optimize and screen spectral lines requires a lot of calculations, lacks optimization indicators, and makes more judgments due to human factors. By using the weight fitting optimization method, the optimization of spectral lines is given screening parameters in a reasonable way, which facilitates the screening of a large number of candidate parameters and greatly improves the efficiency of spectral line optimization.

After the weights are optimized, the detection error decreases. The spectral line obtained after weight fitting optimization has a higher specific gravity for the lower atmosphere and a higher sensitivity to CO ₂ fluctuations in the lower layer, making the detection closer to the CO ₂ column content in the real atmosphere, ensuring small detection errors and high detection accuracy high.

Improved source-sink distribution and flux accuracy. Rayner and O'Brien (2001) showed that the knowledge of CO ₂ sources and sinks can be effectively improved by linking the precision of CO ₂ column content with the precision of analyzing the distribution of surface sources and sinks. The pulse wavelength obtained by using the weight fitting optimization algorithm can improve the accuracy of _CO2 column content, improve the detection of _CO2 flux distribution, as well as source and sink distribution, and reduce the cost of model inversion of _CO2 source-sink distribution and surface flux Uncertainty can be quickly brought into the comprehensive data inversion model and data assimilation framework.

Provide a basis for understanding future climate change and provide a data basis for emission reduction policies. The IPDA lidar optimized by weight fitting has high-precision, high-sensitivity and high-spatial-temporal resolution detection capabilities, and can accurately detect the global _CO2 flux distribution, local fluctuations, and source-sink distribution. Its detection data is brought into the model for quantitative analysis, which helps to understand the relationship between the source and sink process of greenhouse gases and climate change, and is used to solve most of the scientific problems of the carbon cycle. Combining the distribution of sources and sinks with the national emission inventory can provide accurate and effective detection methods and data support for the country to formulate the emission inventory by using scientific means.

It is suitable for other gas optimizations in other bands. Although this method is aimed at the weight fitting optimization proposed for CO ₂ gas, it can be used as a selection and consideration method for selecting active working wavelength or passive detection wavelength for using IPDA and even other active and passive differential absorption remote sensing methods. It is a wide-ranging and effective technical means.

Description of drawings

Fig. 1 is a schematic diagram of satellite-borne IPDA detection of the present invention;

Fig. 2 is HITRAN12 spectral data parameter description of the present invention;

Fig. 3 is the HITRAN spectrum data on the 1572nm vibratory rotation belt of the present invention;

Fig. 4 is Vogt broadening line pattern of the present invention

Fig. 5 is _CO Molecular spectral absorption section of the present invention

Fig. 6 is temperature standard atmospheric model of the present invention

Fig. 7 is humidity standard atmospheric model of the present invention

Fig. 8 is air pressure standard atmospheric model of the present invention

Fig. 9 is the water vapor profile standard atmospheric model of the present invention

Fig. 10 is the _CO molecule number profile of the present invention

Fig. 11 is weight function profile of the present invention

Fig. 12 is CO ₂ and H ₂ O optical thickness line patterns under the standard atmospheric model of the present invention

Figure 13 is the CO2 and H2O optical thickness line pattern after the temperature change of the present invention is 10K

Figure 14 is the CO ₂ and H ₂ O optical thickness line pattern after the pressure change of 10hPa in the present invention

Figure 15 is the transmittance of the 1572nm vibration-rotation band of the present invention

Fig. 16 is _CO of the present invention Molecular number density profile (column content is 390ppm)

Fig. 17 is a flow chart of spectral line optimization in the present invention

detailed description

As shown in the figure, a method for optimizing laser radar detection atmospheric component spectral line analysis is characterized in that the method comprises the following steps:

Step 1, establish the IPDA lidar equation for hard target reflection

The integrated path differential absorption (IPDA, Integrated Path Differential Absorption) lidar detection method is used to detect the backward reflection signal from the hard target. The echo signal received by the detector comes from the pulse echo signal reflected by the hard target as shown in Figure 1.

, the specific IPDA lidar equation is as follows:

Equations (1) and (2) are used to calculate the CO ₂ content in the entire atmosphere based on the optical thickness of each laser pulse wavelength absorbed by CO ₂ during atmospheric transmission. Receive the echo signal reflected by the ground surface through the detector,

Step 2, characterization of parameters in step 1

P _on (P _off ) is the energy of the callback signal received by the detector (mJ), which is the value to be detected; E _on (E _off ) is the pulse energy of the transmitter (mJ), A is the area of the receiver (m ² ), and ρ is Surface albedo, η _d is the detector quantum efficiency, η _r is the receiver efficiency, R _G is the height of the satellite from the surface (km), τ _g is the optical thickness caused by the extinction of atmospheric components other than the target gas, σ _on and σ _off is the absorption cross section of CO ₂ molecules at different heights (cm ² ), q _CO2 is the volume mixing rate of CO ₂ dry air, that is, the CO ₂ concentration, which is the value to be retrieved; n _air is the number density profile of air molecules (cm ^-3 ) , C is the lidar system constant.

Step 3. Calculation of the differential optical depth of the two pulses and the weighted CO ₂ air column mixing rate XCO ₂

The weighted CO ₂ dry air column mixing rate XCO ₂ is calculated by receiving the echo signal energy reflected by the ground surface through the IPDA laser radar, and using the differential absorption principle to eliminate the interference of absorbing components such as water vapor, and obtain the weighted CO ₂ Dry air column mixing rate inversion data XCO ₂ , the XCO ₂ is the secondary data product of IPDA lidar inversion, the specific calculation is as follows:

First, the spaceborne IPDA lidar receives the echo signals P _on (P _off ) of two laser pulses reflected by hard targets on the ground surface. According to (1) and (2), the differential optical thickness (τ _on -τ _off )

q _H2O is the water vapor volume mixing rate profile. Secondly, the relationship between the received echo signal and the CO ₂ mixing rate can be established according to the hydrostatic equation, the ideal gas state equation and the differential optical thickness obtained in the previous step. The weighted CO ₂ dry air column mixing ratio XCO ₂ is obtained by

Substitute into (3) to get,

where △σ is the difference in absorption cross-section between the strong and weak absorption lines, thus avoiding the influence on the _CO2 mixing rate profile , and the CO ₂ column content in the atmospheric column is retrieved by receiving the relative energy change of the pulse callback signal on the two wavelengths, where WF(r) is a weight function, representing the distribution of CO ₂ absorption capacity on the pulse path length , p _surf and P _top represent the pressure values of the lower atmosphere and the upper atmosphere, which are the upper and lower limits of the weight function calculation, where p _surf is 1013.25hPa, p _top is 0hPa, obtained from the hydrostatic equation:

Wherein, M _H2O and M _CO2 are water vapor and _CO Relative molecular mass, wherein M _H2O is 18g/mol, It is 44g/mol, as shown in Figure 10.

Step 4, calculate the differential optical thickness and the weighted CO ₂ air column mixing rate XCO ₂ of the two pulses in step 3 through the calculation of the data in the spectral database

The spectral parameter data selected according to the data in the existing HITRAN database include the position of the spectral wavenumber corresponding to the molecule or atom in vacuum, the line intensity of the spectral line, the temperature index, the frequency shift of the spectral line position caused by the air pressure, and Lorentz Spectral line self-broadening half-width and other parameters are shown in Figure 1 and Figure 2. Using the frequency and line width of the laser pulse emitted by the laser radar, the CO ₂ is calculated from the absorption spectrum curve of CO2 gas molecules under different temperature and pressure conditions. The change of the gas molecule differential absorption cross-section in the vertical height is substituted into step 3 for calculation, and the weighted CO ₂ dry air column mixing rate XCO ₂ is obtained;

Step 5, single spectral line broadening calculation

The broadening of the single spectral line is obtained by calculating and gradually integrating different temperature and air pressure conditions in a certain wavenumber range near the target wavelength using the Vogt line pattern. The Vogt profile used is the convolution of the Lorentz linetype and the Doppler linetype. The convolution integral on the infinite field cannot be evaluated under the closed condition, and the complex error function is used to solve it. f _V (x, y) is the broadened line shape of a single absorption line, which obeys the complex error function line shape,

in,

v ₀ (cm ^-1 ) is the position of the absorption peak in the wavenumber domain, and v(cm ^-1 ) is the integrated spectral line domain. γ _D is the Doppler half-width of the absorption line (cm ^-1 /atm), expressed as follows:

B is Boltzmann's constant (J·K ^-1 ), T is temperature (K), and m is relative molecular mass.

The variation of Lorentz half-width γ _L with temperature and air pressure is:

γ _L (T,p)＝γ _L (296,1atm)p/p ₀ (296/T) ^φ (41)

φ is the molecular constant exponent obtained at a temperature of 296K.

Line strength S is affected by temperature:

S ₀ is the intensity of the spectral line at 296K, h is Planck's constant (J·s), and c is the speed of light (m·s ^-1 ). The half-width γ _L (296,1 atm), φ and E” of the Lorentzian lineform in the standard state can be obtained from the HITRAN database.

For the spectral absorption line at the specified wavenumber when the temperature and pressure are constant, the Doppler broadening is known, that is, y is known; the broadening of the spectral line in the lower atmosphere is mainly Lorentzian line broadening, with Doppler broadening increases with height. Vogt lines are used, which can be obtained by convolution of Lorentz lines and Doppler lines. The line shape calculated by Vogt broadening is a single spectral line broadening line shape as shown in Figure 3.

Step 6, calculate the absorption cross section by line-by-line integration

The line-by-line integration calculation is to accumulate the discrete broadened absorption lines in the entire spectral range according to a certain wave number interval to calculate the absorption properties of the gas one by one. According to step 5, the absorption lines near the target wave number are calculated line by line. The broadening calculation is shown in Figure 4, and then each broadening profile is superimposed on the target wavenumber, and the absorption cross section after line-by-line integration is obtained as shown in Figure 4. The calculation formula of line-by-line integration within a given wavenumber range is:

Among them, α(v) is the absorption cross-section at the target wavenumber v, S _i is the line intensity of the i-th spectral line in a given wavenumber domain, f(vv _0,i ) is the wavenumber The absorption line shape at v, the line-by-line integration meets the requirements of IPDA for high-precision spectral parameters.

Step 7, Atmospheric Model Establishment

The profile of conventional meteorological parameters (including temperature, air pressure and humidity, etc.) established by the United States in 1976 is used as the basis for studying IPDA lidar parameters, as shown in Figure 5, Figure 6, Figure 7, and Figure 8. The establishment of the atmospheric temperature, pressure and density profile model in this model is shown in Table 1.

Table 7 Standard atmospheric temperature, pressure and density profiles of the United States in 1976

Where h is the altitude above sea level in meters, the sea level temperature T ₀ is 288.15K, the density ρ ₀ =1.225kg/m ³ , and the sea level standard air pressure P ₀ =1.1325N/m ² .

Step 8, _CO2 Absorption Optical Depth Calculation

The CO ₂ absorption optical thickness is the absorption capacity of CO ₂ in the whole atmosphere to the pulse energy, which reflects the total content of CO ₂ in the atmospheric column. The definition of the optical thickness is

Among them, τ is the optical thickness, L is the propagation distance of light energy on the path, σ is the absorption cross section, and N is the number of molecules of the gas to be measured on the path as shown in Figure 11.

Step 9, Screening Optical Depth

After calculating the optical thickness of each point on the unit wavenumber interval in the target wavenumber domain, according to the result calculated in step 8, the optical thickness screening of the wavenumber in the target wavenumber domain is shown in Figure 14, and the optical thickness is greater than 0.02 and less than 4 as the screening optical thickness standard,

as shown below

v _step1 = where(0.02<τ _i <4) (45)

v _step1 is the wavenumber filtered by the optical depth, and _τi is the optical thickness obtained by step 8 at each wavenumber point in the target wavenumber domain.

Step 10, spectral line atmospheric sensitivity calculation

The temperature sensitivity equation of this spectral line is, as shown in Figure 12 and Figure 13:

Δτ is the differential optical thickness. According to the hydrostatic equilibrium equation, the barometric sensitivity is calculated for the change in mixing rate obtained by increasing the proportionality constant ε in each layer of the atmosphere. The pressure sensitivity equation is:

As a strong absorbing gas in the atmosphere, water vapor changes the differential absorption optical depth, and the change of water vapor affects the integral weight function. Assuming that the water vapor profile in the entire atmosphere changes in an equal proportion (1+ε) for each layer, the water vapor sensitivity can be obtained by the following formula:

Step eleven, weight function profile

Divide equation (1) and equation (2) of IPDA lidar in step 1, and equation (4) in step 3 can be derived according to the hydrostatic equation and ideal gas state equation, and thus the differential optical thickness and XCO ₂ Using the relationship between the echo signal intensities at the two differential wavelengths directly measured by the IPDA lidar and other correction parameters, the differential absorption optical thickness of _CO2 gas can be obtained. The weight function is calculated by formula (6) in Step 3. The differential absorption cross sections at different heights are affected by air pressure and temperature. According to steps 5 and 7, the weight function profile can be obtained. According to the weight function profile, the vertical distribution of CO ₂ gas molecular number density or mixing ratio cannot be obtained in the differential absorption optical depth, and finally the column content of CO ₂ in the atmospheric column is obtained.

Step 12, _CO2 Molecular Number Density Profile

There are differences in the inversion between the profile with constant CO ₂ mixing rate and the profile with CO ₂ gradient in the vertical direction. The CO ₂ mixing rate profile used is shown in Table 2, as shown in Figure 15, where i represents the number of atmospheric layers, h and P are the altitude and pressure in the atmospheric model established in step seven. The number density profile of CO2 molecules has a layered structure, and its number density changes with height as shown in Figure 9, and the calculation formula is shown in the table below.

Table 8 CO ₂ mixing rate vertical stratification profile

layered CO ₂ mixing rate (ppm) 0～2 392.0713 3～24 (392.0713-10/22*(h(i)-2))*P(i) 25～71 (392.0713-10-2/47*(h(i)-24))*P(i)

Step 13, the weight function optimizes the spectral line

According to formula (4) in step 3, the weight function represents the distribution of differential optical thickness at each height in the vertical direction in the atmospheric column, and the absorption coefficient profile σ(r)N(r) in formula (12) is integrated in step 8 and the _CO2 mixing rate profile obtained in step 13, the absorption cross-section obtained in step 6, and the density of each layer in step 7, to obtain the results related to the weight function, the process flow is shown in Figure 16, and the spectrum optimized by the weight function is as Table 3 shows:

Table 9 The results obtained by weight function correlation

The various system parameters of the spaceborne IPDA laser radar in the step 1 are described in Table 4 below:

Table 4

E _on 50mJ E _off 50mJ A 0.785m ² η _d 60% _ηr 60% R _G 450km _τg ^10-2 C 0.72 ρ(λ _on ) 0.08 ρ(λ _off ) 0.08

The hard target described in step one is the ground surface, water surface or thick clouds. The data in the HITRAN database has been updated to the 2012 version. The present invention will be further described below in conjunction with specific examples.

Claims (4)

1. A method for optimizing lidar detection atmospheric component spectral line analysis is characterized in that the method comprises the following steps: Step 1, establish the IPDA lidar equation for hard target reflection The integrated path differential absorption (IPDA, Integrated Path Differential Absorption) lidar detection method is used to detect the backward reflection signal from the hard target. The echo signal received by the detector comes from the pulse echo signal reflected back from the hard target. Specifically, the IPDA lidar The equation is as follows: Equations (1) and (2) are used to calculate the CO ₂ content in the entire atmosphere based on the optical thickness of each laser pulse wavelength absorbed by CO ₂ during atmospheric transmission. Receive the echo signal reflected by the ground surface through the detector, Step 2, characterization of parameters in step 1 P _on (P _off ) is the energy of the callback signal received by the detector (mJ), which is the value to be detected; E _on (E _off ) is the pulse energy of the transmitter (mJ), A is the area of the receiver (m ² ), and ρ is Surface albedo, η _d is the detector quantum efficiency, η _r is the receiver efficiency, R _G is the height of the satellite from the surface (km), τ _g is the optical thickness caused by the extinction of atmospheric components other than the target gas, σ _on and σ _off is the absorption cross section of CO ₂ molecules at different heights (cm ² ), is the CO ₂ dry air volume mixing rate, that is, the CO ₂ concentration, is the value to be retrieved; n _air is the air molecule number density profile (cm ^-3 ), and C is the lidar system constant. Step 3. Calculation of the differential optical depth of the two pulses and the weighted CO ₂ air column mixing rate XCO ₂ The weighted CO ₂ dry air column mixing rate XCO ₂ is calculated by receiving the echo signal energy reflected by the ground surface through the IPDA laser radar, and using the differential absorption principle to eliminate the interference of absorbing components such as water vapor, and obtain the weighted CO ₂ Dry air column mixing rate inversion data XCO ₂ , the XCO ₂ is the secondary data product of IPDA lidar inversion, the specific calculation is as follows: First, the spaceborne IPDA lidar receives the echo signals P _on (P _off ) of two laser pulses reflected by hard targets on the ground surface. According to (1) and (2), the differential optical thickness (τ _on -τ _off ) is the water vapor volume mixing rate profile, and secondly, according to the hydrostatic equation, the ideal gas state equation and the differential optical thickness obtained in the previous step, the relationship between the received echo signal and the CO ₂ mixing rate can be established, according to the following formula Get weighted CO ₂ dry air column mixing ratio XCO ₂ Substitute into (3) to get, where △σ is the difference in absorption cross-section between the strong and weak absorption lines, thus avoiding the influence on the _CO2 mixing rate profile , and the CO ₂ column content in the atmospheric column is obtained by inversion by receiving the relative energy change of the pulse echo signals at two wavelengths, where WF(r) is a weight function, representing the distribution of CO ₂ absorption capacity on the pulse path length , p _surf and P _top represent the pressure values of the lower atmosphere and the upper atmosphere, which are the upper and lower limits of the weight function calculation, where p _surf is 1013.25hPa, p _top is 0hPa, obtained from the hydrostatic equation: in, and is the relative molecular mass of water vapor and CO ₂ , where is 18g/mol, is 44 g/mol. Step 4, calculate the differential optical thickness and the weighted CO ₂ air column mixing rate XCO ₂ of the two pulses in step 3 through the calculation of the data in the spectral database The spectral parameter data selected according to the data in the existing HITRAN database include the position of the spectral wavenumber corresponding to the molecule or atom in vacuum, the line intensity of the spectral line, the temperature index, the frequency shift of the spectral line position caused by the air pressure, and Lorentz Spectral line self-broadening half-width and other parameters, using the frequency and line width of the laser pulse emitted by the laser radar, calculate the differential absorption cross section of _CO2 gas molecules from the absorption spectrum curves of CO2 gas molecules under different temperature and pressure conditions. The change in the vertical height is substituted into Step 3 for calculation, and the weighted CO ₂ dry air column mixing rate XCO ₂ is obtained; Step 5, single spectral line broadening calculation The broadening of the single spectral line is obtained by calculating and gradually integrating different temperature and air pressure conditions in a certain wavenumber range near the target wavelength using the Vogt line pattern. The Vogt profile used is the convolution of the Lorentz linetype and the Doppler linetype. The convolution integral on the infinite field cannot be evaluated under the closed condition, and the complex error function is used to solve it. f _V (x, y) is the broadened line shape of a single absorption line, which obeys the complex error function line shape, in, v ₀ (cm ^-1 ) is the position of the absorption peak in the wavenumber domain, and v(cm ^-1 ) is the integrated spectral line domain. γ _D is the Doppler half-width of the absorption line (cm ^-1 /atm), expressed as follows: B is Boltzmann's constant (J·K ^-1 ), T is temperature (K), and m is relative molecular mass. The variation of Lorentz half-width γ _L with temperature and air pressure is: γ _L (T,p)＝γ _L (296,1atm)p/p ₀ (296/T) ^φ (9) φ is the molecular constant exponent obtained at a temperature of 296K. Line strength S is affected by temperature: S ₀ is the intensity of the spectral line at 296K, h is Planck's constant (J·s), and c is the speed of light (m·s ^-1 ). The half-width γ _L (296,1 atm), φ and E” of the Lorentzian lineform in the standard state can be obtained from the HITRAN database. For the spectral absorption line at the specified wavenumber when the temperature and pressure are constant, the Doppler broadening is known, that is, y is known; the broadening of the spectral line in the lower atmosphere is mainly Lorentzian line broadening, with Doppler broadening increases with height. Vogt lines are used, which can be obtained by convolution of Lorentz lines and Doppler lines. The lineshape calculated by Vogt broadening is the lineshape of single spectral line broadening. Step 6, calculate the absorption cross section by line-by-line integration The line-by-line integration calculation is to accumulate the discrete broadened absorption lines in the entire spectral range according to a certain wave number interval to calculate the absorption properties of the gas one by one. According to step 5, the absorption lines near the target wave number are calculated line by line. The broadening calculation, and then superimpose each broadening profile on the target wavenumber to obtain the absorption cross-section after line-by-line integration. The calculation formula of line-by-line integration within a given wavenumber range is: Among them, α(v) is the absorption cross-section at the target wavenumber v, S _i is the line intensity of the i-th spectral line in a given wavenumber domain, f(vv _0,i ) is the wavenumber The absorption line shape at v, the line-by-line integration meets the requirements of IPDA for high-precision spectral parameters. Step 7, Atmospheric Model Establishment The profile of conventional meteorological parameters (including temperature, air pressure and humidity, etc.) established by the United States in 1976 is used as the basis for studying IPDA lidar parameters. The establishment of the atmospheric temperature, pressure and density profile model in this model is shown in Table 1. Table 1 The standard atmospheric temperature, pressure and density profiles of the United States in 1976 Where h is the altitude above sea level in meters, the sea level temperature T ₀ is 288.15K, the density ρ ₀ =1.225kg/m ³ , and the sea level standard air pressure P ₀ =1.1325N/m ² . Step 8, _CO2 Absorption Optical Depth Calculation The CO ₂ absorption optical thickness is the absorption capacity of CO ₂ in the whole atmosphere to the pulse energy, which reflects the total content of CO ₂ in the atmospheric column. The definition of the optical thickness is Among them, τ is the optical thickness, L is the propagation distance of light energy on the path, σ is the absorption cross section, and N is the number of molecules of the gas to be measured on the path. Step 9, Screening Optical Depth After calculating the optical thickness of each point on the unit wavenumber interval in the target wavenumber domain, according to the result calculated in step 8, the optical thickness is screened for the wavenumber in the target wavenumber domain, and the optical thickness is selected to be greater than 0.02 and less than 4 as the standard for screening optical thickness , as shown in the following formula v _step1 = where(0.02<τ _i <4) (13) v _step1 is the wavenumber filtered by the optical depth, and _τi is the optical thickness obtained by step 8 at each wavenumber point in the target wavenumber domain. Step 10, spectral line atmospheric sensitivity calculation The temperature sensitivity equation of this spectral line is: Δτ is the differential optical thickness. According to the hydrostatic equilibrium equation, the barometric sensitivity is calculated for the change in mixing rate obtained by increasing the proportionality constant ε in each layer of the atmosphere. The pressure sensitivity equation is: As a strong absorbing gas in the atmosphere, water vapor changes the differential absorption optical depth, and the change of water vapor affects the integral weight function. Assuming that the water vapor profile in the entire atmosphere changes in an equal proportion (1+ε) for each layer, the water vapor sensitivity can be obtained by the following formula: Step eleven, weight function profile Divide equation (1) and equation (2) of IPDA lidar in step 1, and equation (4) in step 3 can be derived according to the hydrostatic equation and ideal gas state equation, and thus the differential optical thickness and XCO ₂ Using the relationship between the echo signal intensities at the two differential wavelengths directly measured by the IPDA lidar and other correction parameters, the differential absorption optical thickness of _CO2 gas can be obtained. The weight function is calculated by formula (6) in Step 3. The differential absorption cross sections at different heights are affected by air pressure and temperature. According to steps 5 and 7, the weight function profile can be obtained. According to the weight function profile, the vertical distribution of CO ₂ gas molecular number density or mixing ratio cannot be obtained in the differential absorption optical depth, and finally the column content of CO ₂ in the atmospheric column is obtained. Step 12, _CO2 Molecular Number Density Profile There are differences in the inversion between the profile with constant CO ₂ mixing rate and the profile with CO ₂ gradient in the vertical direction. The CO ₂ mixing rate profile used is shown in Table 2, where i represents the number of atmospheric layers, h and P are the altitude and pressure in the atmospheric model established in step seven. The number density profile of CO ₂ molecules has a layered structure, and its number density varies with height. The calculation formula is shown in the table below. Table 2 CO ₂ mixing rate vertical stratification profile layered CO ₂ mixing rate (ppm) 0～2 392.0713 3～24 (392.0713-10/22*(h(i)-2))*P(i) 25～71 (392.0713-10-2/47*(h(i)-24))*P(i) Step 13, the weight function optimizes the spectral line According to formula (4) in step 3, the weight function represents the distribution of differential optical thickness at each height in the vertical direction in the atmospheric column, and the absorption coefficient profile σ(r)N(r) in formula (12) is integrated in step 8 and the _CO2 mixing rate profile obtained in step 13, the absorption cross section obtained in step 6, and the density of each layer in step 7, to obtain the results related to the weight function. The spectral lines optimized by the weight function are shown in Table 3: Table 3 The results obtained by weight function correlation 2. a kind of method for optimizing laser radar detection atmospheric composition spectral line analysis as claimed in claim 1, it is characterized in that in described step 1, each system parameter of spaceborne IPDA laser radar is as described in table 4 below: Table 4 E _on 50mJ E _off 50mJ A 0.785m ² η _d 60% η _r 60% R _G 450km _τg ^10-2 C 0.72 ρ(λ _on ) 0.08 ρ(λ _off ) 0.08 3. A kind of method for optimizing laser radar detection atmospheric component spectrum line analysis as claimed in claim 1, is characterized in that step one described hard target is ground surface, water surface or thick cloud. 4. a kind of method for optimizing laser radar detection atmospheric composition spectral line analysis as claimed in claim 1, is characterized in that the data in the described HITRAN database has been updated to 2012 version.