A double differential absorption lidar ozone inversion method

Abstract

This invention provides an inversion method for measuring ozone using a double differential absorption lidar. This method, which belongs to the field of ozone measurement, includes: preprocessing lidar signals of all wavelengths; performing geometric overlap factor correction on the lidar signals of all wavelengths, and performing distance square correction on the lidar signals of all wavelengths; selecting the wavelength of the lidar least affected by ozone absorption, and calculating the aerosol boundary layer using the gradient method for the echo signal of this wavelength; calculating the coefficient C of the double differential absorption lidar below the aerosol boundary layer based on theories such as Mie scattering and the Jung index; and calculating the coefficient C of the double differential absorption lidar above the aerosol boundary layer based on Rayleigh scattering theory; and inverting the ozone concentration to obtain a final ozone content profile. This invention can improve the accuracy of ozone measurement.

Description

Technical Field

[0001] The present invention belongs to the field of ozone measurement, and in particular relates to an inversion method for measuring ozone using a double differential absorption laser radar. Background Art

[0002] Ozone is a very important trace gas in the Earth's atmosphere, protecting life on Earth from harmful ultraviolet radiation from the sun. The stratosphere accounts for approximately 90% of Earth's ozone. Stratospheric ozone absorbs energy from solar radiation and is a significant energy source for the stratosphere and mesosphere. Small amounts of ozone also exist in the troposphere and mesosphere. Tropospheric ozone is a pollutant gas, and excessive tropospheric ozone is harmful to human and biological health. Differential Absorption LiDAR (DIAL) is a key method for measuring atmospheric ozone distribution, offering advantages such as high temporal and spatial resolution and measurement accuracy. DIAL uses two laser beams of similar wavelengths, one strongly absorbed by O3 and the other weakly absorbed by O3. The ozone distribution can be measured by differential absorption using the elastic scattering echo signals of the two wavelengths. The main measurement error of DIAL comes from the different effects of atmospheric aerosols and atmospheric molecules on beams of light of different wavelengths. At the same time, the absorption spectrum of ozone lacks narrow and deep structural changes, that is, a small change in wavelength will not cause a large difference in absorption intensity. This spectral feature determines that the wavelength difference used for differential absorption will be large, which will greatly increase the measurement error of ozone. To address this problem, researchers proposed a dual differential absorption lidar (Dual-DIAL) to measure ozone. The central idea is to select two differential absorption wavelength pairs. When these two differential absorption wavelength pairs are used to invert ozone, the inversion errors are the same or similar. Based on the two differentials, a further differential is performed to completely eliminate the error.

[0003] However, there is a problem when using double differential absorption lidar to measure ozone. When measuring the two wavelength pairs, the differences caused by aerosol and atmospheric molecular scattering are not completely equal. Therefore, when differencing, a coefficient needs to be formulated so that the inversion errors of the two differential wavelength pairs can be completely offset when subtracted. However, the value of the coefficient changes with the changes in atmospheric aerosol and atmospheric molecular scattering, so it is difficult to determine, which affects its application. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides an inversion method for measuring ozone using a double differential absorption lidar to improve the accuracy of ozone measurement.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A double differential absorption lidar ozone inversion method comprises the following steps:

[0007] Step 1: Remove background noise and smooth the lidar signals of all wavelengths to improve the signal-to-noise ratio;

[0008] Step 2: Perform geometric overlap factor correction on the lidar signals of all wavelengths, and then perform distance square correction on the lidar signals of all wavelengths;

[0009] Step 3: Select the wavelength least affected by ozone absorption in the lidar and use the gradient method to calculate the aerosol boundary layer for the echo signal of this wavelength;

[0010] Step 4: Based on Mie scattering and Jung index theory, calculate the coefficient C of the double differential absorption lidar below the aerosol boundary layer;

[0011] Step 5: Based on the Rayleigh scattering theory, calculate the coefficient C of the double differential absorption lidar above the aerosol boundary layer;

[0012] Step 6: Start inverting ozone concentration. For the area below the boundary layer, use the coefficient C calculated in the fourth step to calculate the ozone content; for the area above the boundary layer, use the coefficient C calculated in the fifth step to calculate the ozone content; finally, merge the data to obtain the final ozone content profile.

[0013] Beneficial effects:

[0014] The present invention first uses the raw laser radar signal to calculate the atmospheric aerosol boundary layer. Below the boundary layer, aerosols are abundant, and the main source of error is aerosol scattering. Ignoring molecular scattering, the coefficients of the double differential absorption laser radar are calculated based on the Mie scattering principle. Above the boundary layer, aerosols are scarce, and the main source of error is molecular scattering. The coefficients of the double differential absorption laser radar can be calculated based on the Rayleigh scattering principle. The coefficients determined in this way can effectively improve the accuracy of double differential absorption laser radar in ozone measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of dual-wavelength differential absorption.

[0016] Figure 2 This is a flow chart of an inversion method for measuring ozone using a double differential absorption lidar according to the present invention. DETAILED DESCRIPTION

[0017] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0018] like Figure 1 As shown in the figure, differential absorption lidar uses the principle that the laser wavelength is located at different positions on the gas absorption line, resulting in different absorption intensities of the gas to detect polluted gas. When using lidar to detect polluted gas, there are certain constraints on the laser wavelength generated by the laser. The laser wavelength emitted by the laser must form at least one set of wavelength absorption pairs, one of which must be located on the gas absorption line. , and the other wavelength is located on the side or outside of the absorption line, which is When the aerosol attenuates both laser beams equally, the intensity difference between the two laser beams can be attributed to gas absorption.

[0019] Therefore, the gas concentration can be inverted based on the difference in the echoes of the two wavelengths and combined with the differential absorption lidar equation. The equation can be expressed as follows:

[0020] ;

[0021] Where z is the measurement height, is the wavelength, is the received atmospheric backscatter intensity, Indicates the peak intensity of the pulse emitted by the lidar. is the geometric overlap factor, which can represent the overlap between the telescope and the laser emission system. is the speed of light, is the pulse width. is the area of ​​the receiving system. Surface air backscatter coefficient, represents the atmospheric extinction coefficient. is the number density of the gas to be measured, Its absorption cross section. Represents the exponential function.

[0022] The double differential absorption lidar can simultaneously transmit and receive laser beams of four wavelengths, so two wavelength absorption pairs can be obtained, namely 、 、 and , we can obtain the lidar equation as shown below:

[0023] ;

[0024] ;

[0025] ;

[0026] ;

[0027] By combining the above four formulas, we can get:

[0028] ;

[0029] ;

[0030] in,

[0031] ;

[0032] ;

[0033] By assuming a coefficient C, the above equations can be combined to obtain the ozone number density profile equation:

[0034] (1)

[0035] ;

[0036] ;

[0037] ;

[0038] in, is the error caused by the atmospheric backscatter coefficient, is the error caused by the atmospheric extinction coefficient, is the difference in ozone absorption cross section between wavelengths.

[0039] like Figure 2 As shown, the specific inversion steps are:

[0040] Step 1: Remove background noise and smooth the lidar signals of all wavelengths to improve the signal-to-noise ratio.

[0041] Step 2: Perform geometric overlap factor correction on the lidar signals of all wavelengths, and then perform distance square correction on the lidar signals of all wavelengths. The formula is as follows:

[0042] ;

[0043] Step 3: Select the wavelength least affected by the ozone absorption effect in the lidar, and use the gradient method to calculate the aerosol boundary layer for the echo signal of this wavelength; the gradient method uses the square of the lidar distance to correct the signal The change with height can directly reflect the change of aerosol particle concentration. The gradient method is to directly correct the signal according to the square of the lidar distance. The size of the attenuation rate with height is used as the basis for judging the height of the aerosol boundary layer.

[0044] gradient The definition is as follows:

[0045] ;

[0046] This gradient function reflects Attenuation changes at different heights z. The aerosol boundary layer height is The position where the fastest decay occurs, that is, the gradient Get the minimum height.

[0047] Step 4: Based on Mie scattering and Jung index theory, calculate the coefficient C of double differential absorption lidar below the aerosol boundary layer; assume that the aerosol satisfies the Jung index distribution. If the coefficient C is selected so that , then the measurement error caused by atmospheric aerosol scattering can be eliminated from the lidar formula. That is:

[0048] ;

[0049] Step 5: Based on the Rayleigh scattering theory, calculate the coefficient C of the double differential absorption lidar above the aerosol boundary layer. According to the Rayleigh scattering theory, the extinction coefficient caused by atmospheric molecular scattering is inversely proportional to the fourth power of the wavelength. Based on this, the following calculation formula for the coefficient C can be obtained:

[0050] ;

[0051] Step 6: Start inverting ozone concentration. For the area below the boundary layer, use the coefficient C calculated in step 4 and substitute it into equation (1) to calculate the ozone content. For the area above the boundary layer, use the coefficient C calculated in step 5 and substitute it into equation (1) to calculate the ozone content. Finally, merge the data to obtain the final ozone content profile.

Claims

1. A double differential absorption lidar ozone inversion method, characterized in that: The steps include: Step 1: Remove background noise and smooth the lidar signals of all wavelengths to improve the signal-to-noise ratio; Step 2: Perform geometric overlap factor correction on the lidar signals of all wavelengths, and then perform distance square correction on the lidar signals of all wavelengths; Step 3: Select the wavelength least affected by ozone absorption in the lidar and use the gradient method to calculate the aerosol boundary layer for the echo signal of this wavelength; Step 4: Based on Mie scattering and Jung index theory, calculate the coefficient C of the double differential absorption lidar below the aerosol boundary layer; assuming that the aerosol satisfies the Jung index distribution, if the coefficient C is selected so that , then the measurement error caused by atmospheric aerosol scattering can be eliminated from the lidar formula, namely: ； in, and 、 and Two wavelength absorption pairs formed by the four wavelength laser beams simultaneously emitted and received by the double differential absorption lidar; Step 5: Based on the Rayleigh scattering theory, calculate the coefficient C of the double differential absorption lidar above the aerosol boundary layer; According to the Rayleigh scattering theory, the extinction coefficient produced by atmospheric molecular scattering is inversely proportional to the fourth power of the wavelength. Based on this, the following formula for calculating the coefficient C is obtained: ； Step 6: Invert ozone concentration. For areas below the boundary layer, use the coefficient C calculated in step 4 to calculate the ozone content. For areas above the boundary layer, use the coefficient C calculated in step 5 to calculate the ozone content. Finally, merge the data to obtain the final ozone content profile.

2. The inversion method for measuring ozone using a double differential absorption lidar according to claim 1, characterized in that: The formula for the first step is as follows: ； Where z is the measurement height, is the wavelength, is the received atmospheric backscatter intensity, represents the peak intensity of the pulse emitted by the lidar, is the geometric overlap factor, which can represent the overlap between the telescope and the laser emission system. is the speed of light, is the pulse width, is the area of ​​the receiving system, Surface air backscatter coefficient, represents the atmospheric extinction coefficient, is the number density of the gas to be measured, Its absorption cross section, Represents the exponential function.

3. The inversion method for measuring ozone using a double differential absorption lidar according to claim 1, characterized in that: In the third step, the gradient method uses the square of the laser radar distance to correct the signal The change with height directly reflects the change in aerosol particle concentration; the gradient method directly corrects the signal according to the square of the lidar distance The magnitude of the attenuation rate with height is used as the basis for judging the height of the aerosol boundary layer; gradient The definition is as follows: ； The above gradient reflects Attenuation changes at different heights z; the aerosol boundary layer height is The position where the fastest decay occurs, that is, the gradient Get the minimum height.

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