Study On Temporal And Spatial Distribution Of CO₂ And CH₄ In Urban Atmosphere Based On Portable FTIR Spectroscopy

Carbon dioxide（CO₂）and methane（CH₄）in the atmosphere are the two greenhouse gases that have the most obvious impact on human activities,and cities are the main sources of anthropogenic emissions of greenhouse gases.Therefore,accurately mastering the temporal and spatial distribution of atmospheric greenhouse gases CO₂ and CH₄ in the urban atmosphere can provide basic scientific data for the research of urban greenhouse gas emission reduction.In this paper,portable Fourier transform infrared spectroscopy（FTIR）is used to study the long-term changes of CO₂ and CH₄ vertical column concentrations in urban atmosphere.Based on the inversion of near-infrared solar absorption spectra collected by portable FTIR spectrometer,the vertical column concentrations of CO₂ and CH₄ in the whole atmosphere of Hefei from September 2016 to may 2020 were obtained,and the seasonal,annual and variation laws of atmospheric CO₂ and CH₄ were analyzed;The portable FTIR observation results are compared with the observation results of high-resolution FTIR system and GOSAT satellite data to verify the accuracy of portable FTIR observation of greenhouse gases;The correlation between atmospheric greenhouse gases in Hefei is also analyzed based on the ground FTIR measurement results and surface in-situ measurement data,which is used to judge the source of greenhouse gases;and using the HYSPLIT backward trajectory model to analyze the source and transmission path of the long-distance air mass in Hefei area,and the influence of air mass transmission on the change of greenhouse gas concentration is studied.Finally,based on the column concentrations of CO₂ and CH₄ observed by FTIR in other cities（Shanghai and Linzhi,Tibet）,the differences of greenhouse gas concentrations between inland areas and background areas are analyzed.Firstly,based on the near-infrared solar absorption spectrum,surface atmospheric parameters,a priori gas profile and instrument line shape measured by portable FTIR observation system,the spectral modeling in atmospheric CO₂ and CH₄ column concentration inversion is studied.According to the spectral absorption characteristics of atmospheric CO₂ and CH₄ in the near-infrared band,the fitting spectral window and interference gas are determined,the inversion of atmospheric CO₂ and CH₄ column concentration is completed,and the detection accuracy of atmospheric CO₂ and CH₄ is analyzed.The detection accuracy of XCO₂ and XCH₄ is about 0.04%and 0.10%,which are equivalent to that reported by other ground-based remote sensing stations.Then,based on the long-term observation results of CO₂ and CH₄ column concentrations in Hefei from September 2016 to may 2020,it is found that CO₂ and CH₄ in the atmosphere of Hefei show obvious seasonal changes.In early spring and winter,the concentration of CO₂ is higher,and the concentration in summer is lower;in summer and early autumn,the concentration of CH₄ is higher,and lower in winter and early spring.The average annual growth rate of CO₂ in Hefei is about 2.71±0.66ppm yr^-1,and the average annual growth rate of CH₄ is about 10.95±3.82 ppb yr^-1.The comparison with the observation results of high-resolution FTIR spectroscopy system shows that the correlation coefficients R of daily average XCO₂ and XCH₄observed by the two spectrometers are 0.97 and 0.95 respectively,and the observation results have high consistency.The comparison results with satellite data show that the correlation coefficients R of daily mean values of CO₂ and CH₄ observed by portable FTIR spectrometer and GOSAT satellite are 0.86 and 0.69 respectively,and the results of ground-based observation and satellite observation show high consistency.Then,the correlation and concentration ratio of CO₂ and CO,CH₄ and CO₂ in Hefei atmosphere measured by ground-based FTIR and surface in-situ are analyzed.It is found that the correlation coefficient of XCO₂ and XCO measured by ground-based FTIR in autumn is 0.75,and the concentration ratio of XCO₂ and XCO is0.0795ppm/ppb.The strong correlation between them indicates that CO₂ and CO in Hefei in autumn are homologous,mainly from combustion;CO₂ and CO measured in situ on the surface have strong correlation in spring,autumn and winter,with correlation coefficients of 0.71,0.75 and 0.80 respectively.The concentration ratios of CO₂ and CO are 0.0699 ppm/ppb,0.0698 ppm/ppb and 0.0522 ppm/ppb respectively.There is a moderate correlation between XCH₄ and XCO₂ measured by ground FTIR in spring,autumn and winter;CH₄ and CO₂ measured in situ on the surface show moderate correlation in spring and autumn and strong correlation in winter.The correlation coefficient is 0.8 and the ratio of CH₄/CO₂ is 0.0036 ppm/ppm,indicating that the main source of surface CO₂ in Hefei in winter is man-made emission,and part of atmospheric CH₄ and CO₂ comes from biomass combustion.The backward trajectory analysis of air mass shows that the seasonal variation characteristics of air mass transmission in Hefei are obvious.The northwest air mass accounts for 47.78%in spring,mainly from the heavily polluted areas such as Inner Mongolia,Beijing Tianjin Hebei,Shandong,Shanxi and Jiangsu.The air mass may raise the CO₂ concentration;In summer,38.67%of the air mass comes from Guangdong Province,passing through Jiangxi and southern Anhui to Hefei,and the air mass comes from less polluted areas;The proportion of air masses affected by heavily polluted areas in autumn is 71.39%,mainly from the northwest and east of Hefei,which may bring high CO₂ concentration;In winter,89.04%of the air masses come from heavily polluted areas,mostly from the northwest and North.Finally,the results of atmospheric CO₂ and CH₄ column concentrations in Shanghai and Linzhi,Tibet were observed and compared with those in Hefei at the same time.It was found that the results of greenhouse gas column concentrations in Hefei were higher than those in Shanghai and Linzhi,Tibet at the same time.This reflects the difference of greenhouse gas concentration in the atmosphere between inland polluted cities,coastal polluted cities and background clean cities.