A method for calculating the absorption coefficient of methane molecules based on line-by-line integration method

Abstract

The present invention discloses a method for calculating the absorption coefficient of methane molecules based on the line-by-line integration method, which belongs to the field of atmospheric science and technology. First, atmospheric temperature profiles and pressure profiles of different geographical regions in China are constructed, and linear factor functions of different height ranges are selected according to atmospheric height stratification information. Combined with the HITRAN2020 molecular spectrum database, the absorption coefficient of methane molecules corresponding to different height layers under standard atmospheric environmental conditions is obtained; based on the temperature and pressure compensation function, the absorption coefficient of methane molecules under standard atmospheric environmental conditions is corrected to obtain the absorption coefficient of methane molecules under actual atmospheric environment. The present invention establishes atmospheric temperature and pressure profiles suitable for China, which can better characterize environmental changes in different regions of China; improves the linear factor function selection method in the traditional line-by-line integration method, and uses the actual atmospheric temperature and pressure profiles to correct the final calculation results based on the temperature and pressure compensation function method.

Description

Technical Field

[0001] The invention belongs to the technical field of atmospheric science, and in particular relates to a method for calculating the absorption coefficient of methane molecules based on a line-by-line integration method. Background Art

[0002] Monitoring atmospheric methane concentration is an important part of accurately assessing methane emissions, and methane concentration inversion is a key method for obtaining high-precision methane concentration data. The accuracy of the inversion results is highly dependent on two core input parameters: simulated spectra and satellite measured spectra. Therefore, accurately obtaining high-quality simulated spectra is crucial for monitoring methane concentration. When using the atmospheric radiation transfer model for spectral simulation, the methane molecular absorption coefficient is a key input parameter and is one of the key factors that determine the accuracy of simulated spectrum calculation. Therefore, accurately calculating the methane molecular absorption coefficient is of great significance for high-precision methane monitoring.

[0003] At present, there are still some limitations and challenges in the calculation of gas molecule absorption coefficients: on the one hand, the current calculation methods are usually based on a large number of experimental and theoretical studies, and there are still differences and uncertainties between different methods; on the other hand, due to the complexity and variability of methane gas itself, the accuracy and applicability of the current calculation methods are also subject to certain limitations. In response to the current problems in the calculation of gas molecule absorption coefficients, relevant researchers have also proposed many methods. In "Modified Malkmus band model for clear-sky atmospheric transmission calculation", Wang and Zhang et al. proposed an improved Malkmus model, using the least squares method to fit the calculated absorption coefficient with the absorption coefficient of the MODTRAN software package, and obtained the fitting coefficient of the correction term under different atmospheric modes, which improved the calculation accuracy of the Malkmus model. In 2023, Zhu Mingwei proposed in "Research on Automatic Parameterization Method and Application of Atmospheric Radiation Absorption" that the relevant K distribution theory was used as the starting point, and an automatic atmospheric radiation absorption parameterization algorithm was developed based on the gradient descent method and quadratic sequence programming, which avoided the errors caused by manual operation, improved the accuracy and speed of gas absorption calculation, and realized its application in satellite radiation simulation. In 2023, IHou et al., in "Rapid calculation of atmospheric molecular absorption characteristics in the terahertz bandbased on wideband K-distribution method", based on the latest high-resolution transmission molecular absorption database HITRAN2020, combined with the line-by-line (LBL) spectral radiation calculation method, based on the Earth's atmospheric environmental parameters, using the LBL method with a resolution of 0.01cm-1, developed a non-correlated broadband K distribution model (WBK), and established an exponential kernel fitting coefficient table for calculating the water vapor molecular absorption coefficient, and calculated 24 broadband absorption coefficients within an altitude of 0-100km in the Earth's atmosphere.

[0004] The existing calculation methods of atmospheric molecular absorption coefficient lack research on the absorption characteristics of methane molecules. When using the line-by-line integration method to calculate the molecular absorption coefficient, on the one hand, the influence of the linear factor function at different altitudes is not considered, and on the other hand, the influence of the complex environmental changes in China on the calculation of the methane molecular absorption coefficient is not considered, resulting in low precision of the methane molecular absorption coefficient in China, which in turn leads to low accuracy when using satellite remote sensing to monitor methane in China. When applied to the forward model of atmospheric radiation transfer, the maximum MAE and RMSE of the calculated results are only 1.1×10 -3 and 1.2×10-3 Compared with the ERA5 reanalysis data and the U.S. standard atmospheric model, the atmospheric temperature and pressure profiles constructed by the present invention can better characterize the environmental changes in different regions of China. Summary of the invention

[0005] In view of the above problems existing in the prior art, the present invention proposes a method for calculating the absorption coefficient of methane molecules based on the line-by-line integration method, which has a reasonable design, solves the shortcomings of the prior art and has good effects.

[0006] A method for calculating the absorption coefficient of methane molecules based on a line-by-line integration method comprises the following steps:

[0007] S1. Construct atmospheric temperature and pressure profiles in different geographical regions of China;

[0008] S2. Select linear factor functions of different altitude ranges according to the atmospheric altitude stratification information provided by the Sentinel-5P satellite, and use the absorption line intensity information of different altitude layers in the HITRAN2020 molecular spectrum database to integrate each absorption line to obtain the methane molecular absorption coefficient corresponding to different altitude layers under standard atmospheric environment conditions;

[0009] S3. Using the atmospheric temperature profile and pressure profile in S1, the methane molecular absorption coefficient under standard atmospheric environment conditions is corrected based on the temperature and pressure compensation functions to obtain the methane molecular absorption coefficient under the actual atmospheric environment.

[0010] Furthermore, in S1, China is divided into multiple geographical regions according to climate and topographic characteristics, and the altitude, atmospheric temperature and pressure data in the UWYO sounding data are used to obtain the atmospheric temperature profile and pressure profile of each geographical region based on the least squares fitting method.

[0011] Furthermore, in S2, the altitude setting of each altitude layer is consistent with the altitude setting of the Sentinel-5P satellite, and each altitude layer corresponds to multiple absorption line intensity information; under standard atmospheric environment conditions, in the geographic region The absorption coefficient of methane molecules corresponding to the altitude layer The expression is:

[0012] ; (1)

[0013] in, Indicates Geographical area The altitude layer corresponds to The root absorption line intensity, represents the number of absorption lines, represents the linear factor function, represents the wavelength of the absorption line, Indicates The central wavelength of the root absorption line.

[0014] Furthermore, when the atmospheric altitude is less than 20 km, the linear factor function uses the Lorentz linear type, when the atmospheric altitude is between 20 km and 50 km, the linear factor function uses the Voigt linear type, and when the atmospheric altitude is greater than 50 km, the linear factor function uses the Doppler linear type.

[0015] Further, in said S3, the first The temperature corresponding to each altitude layer in different geographical areas is determined by fitting the absorption coefficient of methane molecules at different temperatures at the same altitude layer using a regularized multinomial fitting method. The functional relationship between the absorption coefficient of methane molecules and temperature at each altitude is:

[0016] ; (2)

[0017] in, Indicates temperature, , , , , Indicates The fitting coefficient corresponding to each altitude layer is: Indicates the absorption coefficient of methane molecules at different temperatures;

[0018] According to formula (2), the relationship between temperature and methane molecular absorption coefficient is nonlinear, so the initial temperature compensation function is in the form of:

[0019] ; (3)

[0020] in, The correction factor representing the absorption coefficient of methane molecules at different temperatures, is the absorption coefficient of methane molecules obtained under standard atmospheric conditions, is the standard atmospheric temperature;

[0021] The pressure compensation function is determined by the ideal gas equation, expressed as:

[0022] ; (4)

[0023] in, Indicates the actual atmospheric pressure, obtained through the pressure profile; represents standard atmospheric pressure, The correction factor representing the absorption coefficient of methane molecules at different pressures;

[0024] The absorption coefficient of methane molecules corrected by temperature and pressure compensation functions is:

[0025] ; (5)

[0026] in, Represents the absorption coefficient of methane molecules after correction by temperature and pressure compensation functions.

[0027] Beneficial technical effects brought by the present invention:

[0028] The present invention establishes atmospheric temperature and pressure profiles applicable to the Chinese region, which can better characterize the environmental changes in different regions of China; improves the linear factor function selection method in the traditional line-by-line integration method, and uses the actual atmospheric temperature and pressure profiles to correct the final calculation results based on the temperature and pressure compensation function method. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The figure is a flow chart of a method for calculating the absorption coefficient of methane molecules based on the line-by-line integration method in the present invention.

[0030] Figure 2 This is a flow chart for calculating the absorption coefficient of methane molecules at different altitudes under a standard atmospheric environment in the present invention.

[0031] Figure 3 This is a flow chart for the correction of the absorption coefficient of methane molecules under standard atmospheric conditions in the present invention.

[0032] Figure 4 This is an experimental comparison diagram between the method of the present invention and the traditional line-by-line integration method;

[0033] Among them, (a) is a comparison diagram of the absorption coefficient of methane molecules; (b) is a difference diagram of the absorption coefficient of methane molecules.

[0034] Figure 5 The comparison diagram of simulated spectra and measured spectra of different spectral channels of the method of the present invention and the traditional line-by-line integration method;

[0035] Among them, (a) is a comparison chart of the simulated spectrum and the measured spectrum of the 2300.5nm~2303nm spectral channel; (b) is a comparison chart of the simulated spectrum and the measured spectrum of the 2317.5nm~2320.5nm spectral channel.

[0036] Figure 6 It is a difference diagram of simulated spectra and actual spectra in different bands between the method of the present invention and the traditional line-by-line integration method;

[0037] Among them, (a) is the difference diagram between the simulated spectrum and the actual spectrum of the 2300.5nm~2303nm spectral channel; (b) is the difference diagram between the simulated spectrum and the actual spectrum of the 2317.5nm~2320.5nm spectral channel. DETAILED DESCRIPTION

[0038] The specific implementation of the present invention is further described below in conjunction with specific embodiments:

[0039] A method for calculating the absorption coefficient of methane molecules based on the line-by-line integration method, such as Figure 1-3 As shown, the following steps are included:

[0040] S1. Construct atmospheric temperature and pressure profiles in different geographical regions of China;

[0041] The Chinese region is divided into multiple geographical regions according to the climate and topographic characteristics. The altitude, atmospheric temperature and pressure data in the UWYO sounding data are used to fit the atmospheric temperature profile and pressure profile of each geographical region based on the least squares method. In this embodiment, the Chinese region is divided into seven geographical regions according to the climate and topographic characteristics: North China, East China, Northeast China, Central China, South China, Southwest China, and Northwest China.

[0042] S2. Select linear factor functions of different altitude ranges according to the atmospheric altitude stratification information provided by the Sentinel-5P satellite, and use the absorption line intensity information of different altitude layers in the HITRAN2020 molecular spectrum database to integrate each absorption line to obtain the methane molecular absorption coefficient corresponding to different altitude layers under standard atmospheric environment conditions;

[0043] When calculating the gas absorption coefficient, the traditional line-by-line integration method uses the Voigt linear factor function by default for all altitude layers in order to simplify the calculation process. Although the Voigt linear function takes into account the influence of certain atmospheric environments, using the same linear factor function at different altitudes will lead to low calculation results. In addition, the applicable altitude of the Voigt linear factor function is 20-50km, while methane is mainly distributed in the troposphere (0-16km). Therefore, using a unified Voigt linear factor function will have a certain error effect on the calculation results.

[0044] The height setting of each altitude layer is consistent with the altitude setting of the Sentinel-5P satellite, which is not necessarily evenly divided. Each altitude layer corresponds to multiple absorption spectrum intensity information. Under standard atmospheric environment conditions, the first in the geographic region The absorption coefficient of methane molecules corresponding to the altitude layer The expression is:

[0045] ; (1)

[0046] in, Indicates Geographical area The altitude layer corresponds to The root absorption line intensity, represents the number of absorption lines, represents the linear factor function, represents an arbitrary wavelength of an absorption line, Indicates The center position of the root absorption line;

[0047] The linear factor function is related to the altitude layer. Combined with the altitude stratification information provided by the satellite, different linear factor functions are selected to calculate the methane molecular absorption coefficient corresponding to different altitude layers of 0-100km under standard atmospheric conditions; when the atmospheric altitude is less than 20km, the linear factor function uses the Lorentz linear type; when the atmospheric altitude is between 20km and 50km, the linear factor function uses the Voigt linear type; when the atmospheric altitude is greater than 50km, the linear factor function uses the Doppler linear type.

[0048] The linear factor function is also related to the geographical area, because at the same altitude, the temperature and pressure in different geographical areas are different. For example, the expression of the Lorentz linear factor function is:

[0049] ; (2)

[0050] = (3).

[0051] S3. Using the atmospheric temperature profile and pressure profile in S1, the methane molecular absorption coefficient under standard atmospheric environment conditions is corrected based on the temperature and pressure compensation functions to obtain the methane molecular absorption coefficient under the actual atmospheric environment.

[0052] Since the molecular spectral parameters such as molecular spectral line intensity provided in the HITRAN2020 molecular spectral database are measured under standard atmospheric conditions, the influence of the actual atmospheric environment is not taken into account. In particular, China's climate environment is complex and changeable, and changes in temperature and pressure will affect the accuracy of the methane molecular absorption coefficient.

[0053] The temperature compensation function is used to correct the absorption coefficient of methane molecules in order to eliminate the influence of temperature change on the absorption coefficient. In order to determine the form of the temperature compensation function, the first The temperature corresponding to each altitude layer in different geographical areas is used to fit the methane molecular absorption coefficient using a regularized multinomial fitting method. The order of the polynomial is adjusted according to the goodness of fit and RMSE, and the functional relationship between the methane molecular absorption coefficient and temperature is finally determined as follows:

[0054] ; (4)

[0055] in, Indicates temperature, , , , , Indicates The fitting coefficient corresponding to each altitude layer is: Indicates the absorption coefficient of methane molecules at different temperatures;

[0056] According to formula (2), the relationship between temperature and methane molecular absorption coefficient is nonlinear, so the initial temperature compensation function is in the form of:

[0057] ; (5)

[0058] in, The correction factor representing the absorption coefficient of methane molecules at different temperatures, is the absorption coefficient of methane molecules obtained under standard atmospheric conditions, is the standard atmospheric temperature;

[0059] The pressure compensation function is determined by the ideal gas equation, expressed as:

[0060] ; (6)

[0061] in, Indicates the actual atmospheric pressure, obtained through the pressure profile; represents standard atmospheric pressure, The correction factor representing the absorption coefficient of methane molecules at different pressures;

[0062] The absorption coefficient of methane molecules corrected by temperature and pressure compensation functions is:

[0063] ; (7)

[0064] in, Represents the absorption coefficient of methane molecules after correction by temperature and pressure compensation functions.

[0065] Next, the method of the present invention is experimentally compared and verified;

[0066] (1) The methane molecular absorption coefficient in the troposphere 0-16 km is calculated using the method of the present invention and the traditional line-by-line integration method. Taking the 10 km altitude layer as an example, the methane molecular absorption coefficient calculated is as follows: Figure 4 As shown in (a), it can be seen from the figure that near the wavelength where the absorption characteristics of methane molecules are strong, the change pattern of the absorption coefficient of methane molecules obtained by the two methods is consistent. Further calculation of the difference in the absorption of methane molecules calculated by the method of the present invention and the traditional method is as follows: Figure 4 There is a big difference between the two methods shown in (b).

[0067] (2) The absorption coefficient of methane molecules in different spectral channels was calculated using the method of the present invention and the traditional line-by-line integration method, and the model spectrum was calculated using the SCIATRAN radiation transfer forward model. The model spectrum was then compared with the actual spectrum observed by the Sentinel-5P satellite. The comparison results are shown in Figure 2. Figure 5 As shown, the comparison between the simulated spectrum and the measured spectrum of the spectral channel 2300.5nm~2303nm is shown in Figure 5 As shown in (a), the comparison between the simulated spectrum and the measured spectrum of the spectral channel 2317.5nm~2320.5nm is shown in Figure 5 As shown in (b), it can be seen that the simulated spectrum calculated by the methane molecular absorption coefficient obtained by the method of the present invention is closer to the measured spectrum. Taking the measured spectrum observed by Sentinel-5P as the benchmark, the simulated spectrum obtained by the method of the present invention and the traditional method are subtracted from them. The difference comparison chart is shown in Figure 6 As shown, the spectral difference diagram of the spectral channel 2300.5nm~2303nm is as follows Figure 6 As shown in (a), the spectral difference diagram of the spectral channel 2317.5nm~2320.5nm is as follows Figure 6 As shown in (b), the maximum difference between the simulated spectrum calculated by the methane molecular absorption coefficient obtained by the method of the present invention and the measured spectrum is only 0.64×10 -3 The maximum difference between the simulated spectrum and the measured spectrum calculated by the traditional method for the absorption coefficient of methane molecules is 1.5×10 -3 , which further verifies the effectiveness of the method of the present invention.

[0068] (3) Finally, the MAE and RMSE of the simulated spectra obtained by the method of the present invention and the traditional line-by-line integration method and the measured spectra observed by the Sentinel-5P satellite are calculated. The calculation results are shown in Table 1.

[0069] Table 1 MAE and RMSE of simulated spectra before and after the improvement of the calculation method of methane molecular absorption coefficient;

[0070] ;

[0071] Through the analysis of the data in Table 1, it can be seen that the simulated spectrum calculated by the methane molecular absorption coefficient obtained by the method of the present invention is closer to the measured spectrum observed by the Sentinel-5P satellite, and its MAE and RMSE average values ​​are increased by 42.9% and 56.18%, respectively, which effectively improves the calculation accuracy of the SCIATRAN model, proving that the method of the present invention can improve the calculation accuracy of the methane molecular absorption coefficient, and further improve the accuracy of the atmospheric radiation transfer forward model, which is of great significance for high-precision monitoring of methane.

[0072] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for calculating the absorption coefficient of methane molecules based on line-by-line integration method, characterized in that: The following steps are involved: S1. Construct atmospheric temperature and pressure profiles in different geographical regions of China; S2. Select linear factor functions of different altitude ranges according to the atmospheric altitude stratification information provided by the Sentinel-5P satellite, and use the absorption line intensity information of different altitude layers in the HITRAN2020 molecular spectrum database to integrate each absorption line to obtain the methane molecular absorption coefficient corresponding to different altitude layers under standard atmospheric environment conditions; S3, using the atmospheric temperature profile and pressure profile in S1, based on the temperature and pressure compensation functions, correcting the methane molecular absorption coefficient under standard atmospheric environment conditions to obtain the methane molecular absorption coefficient under actual atmospheric environment; In S1, the Chinese region is divided into multiple geographical regions according to climate and terrain characteristics, and the altitude, atmospheric temperature and pressure data in the UWYO sounding data are used to fit the atmospheric temperature profile and pressure profile of each geographical region based on the least squares method; In S2, the height setting of each altitude layer is consistent with the height setting of the Sentinel-5P satellite, and each altitude layer corresponds to multiple absorption spectral line intensity information; The methane molecular absorption coefficient k corresponding to the i-th altitude layer in the j-th geographical area under standard atmospheric environment conditions j,i The expression is: Among them, S j,i,l represents the intensity of the lth absorption line corresponding to the ith altitude layer under the jth geographical area, line represents the number of absorption lines, and f j,i (vv 0,l ) represents the linear factor function, v represents the wavelength of the absorption line, v 0,l represents the central wavelength of the lth absorption line; In S3, the temperature corresponding to the i-th altitude layer in different geographical areas is obtained according to the atmospheric temperature profile, and the methane molecular absorption coefficients at different temperatures at the same altitude layer are fitted using a regularized multinomial fitting method to determine the functional relationship between the methane molecular absorption coefficient and temperature at the i-th altitude layer: k(T)=a 0,i +a 1,i T+a 2,i T 2 +a 3,i T 3 +a 4,i T 4 ;(2) Where T represents temperature, a 0,i 、a 1,i 、a 2,i 、a 3,i 、a 4,i represents the fitting coefficient corresponding to the i-th altitude layer, and k(T) represents the corresponding absorption coefficient of methane molecules at different temperatures; According to formula (2), the relationship between temperature and methane molecular absorption coefficient is nonlinear, so the initial temperature compensation function is in the form of: Wherein, C(T) represents the correction factor of the methane molecular absorption coefficient corresponding to different temperatures, k(T_std) is the methane molecular absorption coefficient obtained under the standard atmospheric environment, and T_std is the standard atmospheric temperature; The pressure compensation function is determined by the ideal gas equation, expressed as: Among them, P actual Indicates the actual atmospheric pressure, obtained through the pressure profile; P std represents standard atmospheric pressure, B(P) represents the correction factor of the absorption coefficient of methane molecules corresponding to different pressures; The absorption coefficient of methane molecules corrected by temperature and pressure compensation functions is: k v_actual =k j,i ×C(T)×B(P);(5) Among them, k v_actual Represents the absorption coefficient of methane molecules after correction by temperature and pressure compensation functions.

2. The method for calculating the absorption coefficient of methane molecules based on the line-by-line integration method according to claim 1, characterized in that: When the atmospheric altitude is less than 20 km, the linear factor function uses the Lorentz linear type. When the atmospheric altitude is between 20 km and 50 km, the linear factor function uses the Voigt linear type. When the atmospheric altitude is greater than 50 km, the linear factor function uses the Doppler linear type.

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