Comparison Of Methods For Retrieving Atmospheric Composition Using MAX-DOAS Technique

In this paper,the spectral data observed by a multi-axis differential absorption spectrometer(MAX-DOAS)was used.First,the spectral data was processed by QDOAS software to obtain O₄,NO₂,SO₂,HCHO tropospheric differential slant column concentration(dSCD_trop);then three different methods calculated air mass factor(AMF)of pollutants NO₂,SO₂ and HCHO in Qingpu District of Shanghai.Tropospheric differential slant column concentration was converted to tropospheric vertical column concentration(VCD_trop)by air mass factor.Finally,reasons and characteristics of differences between the three methods were comprehensively analyzed,and the best method for calculating tropospheric vertical column concentration of NO₂,SO₂ and HCHO was obtained.The three methods are geometric algorithm,look-up table method and pattern simplification method.Based on our research,conclusions are summarized as follow:The tropospheric vertical column concentration calculated by three methods shows that:the concentration of NO₂ is lower in summer and higher in autumn or winter;the concentration of HCHO is higher in summer and lower in autumn or winter.Although seasonal variation of SO₂ tropospheric vertical column concentration is not as obvious as that of NO₂ and HCHO,it also shows the characteristics of lower SO₂ concentration in summer and higher in autumn or winter.Moreover,height of NO₂ and SO₂ are analyzed,and results show that the average height of SO₂ is higher than NO₂.NO₂ is distributed below 500m,while SO₂ is mainly distributed blow 800m.The NO₂ and SO₂ height during afternoon are slightly higher than those in the morning.The height of NO₂ is lower in summer than in winter,while the height of SO₂ is higher in summer than in winter.Taking geometric mothod A as reference,the tropospheric vertical column comparison of geometric mothod A and simplified model mothod B,the tropospheric vertical column comparison of geometric mothod A and look-up table method C show:(1)A good correlation was obtained between the tropospheric vertical column concentration for one same atmospheric composition obtained by three methods.The highest correlation coefficient is NO₂:the correlation coefficient between A and C reached 0.93,the lowest correlation coefficient is HCHO:the correlation coefficient of A and C reached 0.76.(2)With the increase of NO₂ and SO₂ tropospheric vertical column concentration,relative difference of the tropospheric vertical column concentration increases.While relative difference of HCHO tropospheric vertical column concentration decreases with the increase of tropospheric vertical column concentration.(3)For the different atmospheric component,the higher the distribution of pollutants,the larger the uncertainty of tropospheric vertical column concentration.For the same atmospheric component,the relative difference in the tropospheric vertical column concentration decreases as height increases.(4)When aerosol optical depth(AOD)is low(AOD<1),distinction between tropospheric vertical column concentration obtained by three methods increases whatever gas.NO₂ and SO₂ tropospheric vertical column concentration increases with the increase of aerosol,howverer,seasonal variation of HCHO is different from NO₂ and SO₂.Concentration of HCHO decreases as aerosol increases;when aerosol optical depth is high(AOD>2),the sensitivity of tropospheric vertical column concentration calculated by MAX-DOAS using three methods will decrease.Finally,influence of aerosol on the accuracy of trace gas tropospheric vertical column concentration is much greater than effect of gas vertical distribution.The comparison of methods AB and AC on height show the relative difference in SO₂ height is greater than that in NO₂.If height increases,relative difference of height also increases.If AOD(AOD<1)increases,the distribution of NO₂ and SO₂ height will decrease.High aerosol suppresses the diffusion of pollutants vertically.The optimal method for retrieving NO₂,SO₂ and HCHO tropospheric column concentration is look-up table method.The advantage of this method is it provides more precise aerosol setting.Comparative analysis of NO₂,SO₂,HCHO tropospheric vertical column concentration between MAX-DOAS and OMI shows although retrieved results have a good correlation with satellite products:NO₂,SO₂,HCHO reached 0.92,0.55,0.78,MAX-DOAS retrieved results tend to larger than satellite products mostly.The ratios of MAX-DOAS results to satellite results are respectively 1.1,2.6 and 1.8 for NO₂,SO₂ and HCHO.Finally,it was confirmed by simulation experiment that ground-based MAX-DOAS results were larger than the true values,and satellite products were smaller than the true values.This is related to different observation methods of the instrument and distribution of aerosol and pollutant layer.