Research On The Method And Application Of Accurately Obtaining HCHO Based On Ground-based MAX-DOAS

With the rapid development of cities and the continuous advancement of industrialization,the problems of atmospheric environment have become increasingly prominent,and the type of pollution has gradually changed from soot type to regional complex pollution.Ground-based multi-axis differential optical absorption spectroscopy（MAX-DOAS）can obtain multi-component pollutants in real-time and online,and has been widely used in tropospheric trace gas detection.The paper is based on the one-dimensional ground-based MAX-DOAS system to accurately obtain the tropospheric HCHO column density in Huaibei area from October 2019 to May2020,which mainly includes the following three aspects of research content.（1）A one-dimensional ground-based MAX-DOAS system was built,and the stepping motor was used to control the rotation of the telescope to achieve multi-elevation spectrum measurement.The performance of the spectrometer was also tested.The dark current linearity is 0.99421,the electronic noise value is 0.58696,and the resolution is 0.68 nm.（2）By optimizing the retrieval wave band and spectral de-noising process based on wavelet transform,the inversion accuracy of the tropospheric HCHO gas in ground MAX-DOAS was improved.By comparing the inversion results of different elevations angles and different bands,the optimal retrieval wave band of HCHO was obtained.So,when the retrieval wave band of 324-342 nm was selected,the average retrieval error is minimum(2.85×10¹⁵molecule·cm^-2),and the concentration of HCHO gas could be accurately obtained.The improved translation invariant threshold algorithm of wavelet transform was applied to the de-noising process of optical density spectrum,a particle swarm optimization algorithm was designed to screen the optimal adjustment factor,and filter out the noise structure in the spectrum and retain the gas absorption structure to be measured.Compared with the polynomial smoothing filter,the relative error is reduce to 0.66%under the condition of large signal-to-noise ratio,which is an increase of 0.21%;the relative error is reduce to6.03%under the condition of small signal-to-noise ratio,which is an increase of 1%.The comparison of the de-noising results of the polynomial smoothing filter and the improved translation invariant threshold algorithm show that the improved translation invariant threshold algorithm can improve the inversion accuracy better.（3）The results of the tropospheric HCHO measurements from June 2020 to December 2020 in Huaibei region were analyzed.The results show that compares with the before and after COVIN-9 epidemic,the concentration of HCHO in the middle of the COVIN-9 epidemic were reduced by 35%and 23%respectively.According to the wind field trajectory and the wind rose diagram,the northwest wind field accounts for 38%and 17%respectively under high value weather.The results show that the pollution in Dangshan,Fengxian and other areas were transported by the northwest wind field,which causes the HCHO concentration in Huaibei area to increase.Correlation analysis was conducted between the HCHO measurement results of MAX-DOAS system and the results of OMI satellite,and it is found that the two measurement methods has good consistency（R²=0.87）.