Study On The Characteristics Of Atmospheric NO₂ And Aldehyde Pollution In Nanjing Based On Ground-based MAX-DOAS Observation

In this paper,the vertical column densities of NO₂,HCHO and CHOCHO,and the vertical profiles of NO₂ were retrieved using DOAS based on the observation data of the multi-axis differential absorption spectrometer（MAX-DOAS）from May 2018 to December 2020 in Nanjing.The spatial and temporal distributions and causes of pollutants were analyzed by combining the conventional meteorological data of Nanjing Weather Station.The second version of the TROPOMI NO₂ product was compared with ground-based MAX-DOAS data,and the surrounding NO₂ distribution was observed by using TROPOMI products.First,the vertical column densities and vertical profiles of tropospheric NO₂ were retrieved by parameterization method.Combined with the conventional meteorological data,we analyzed that the NO₂ in the northern suburbs of Nanjing was affected by temperature and solar radiation,showing the variation characteristics of low in summer and high in winter,low at noon and high in the morning and evening.NO₂ mainly originated from the pollution transport in the south-east direction,result in the average density in the southeast direction being 1-2times higher than that in the northwest direction.The pollution transport caused the high value area of NO₂ around 500m,and the peak density of NO₂ in summer was only half of that in winter.Secondly,a comparative analysis of the latest second version of TROPOMI NO₂ product（PAL）was performed to evaluate the PAL quality with the first version of TROPOMI NO₂product（OFFL）and MAX-DOAS.Compared with MAX-DOAS,the average relative error of PAL was-10.41%,which was a great improvement compared with OFFL（-28.72%）.In terms of correlation with MAX-DOAS,the difference between PAL and OFFL was very small,with a correlation coefficient of 0.738 and a decrease of 0.004.However,the correlation coefficient increased from 0.490 to 0.544,indicating the higher quality of PAL,The PAL product had high surface resolution and could effectively observe pollution hotspots,which were highly overlapping with urban population centers in the surrounding areas and were also more polluted along the Yangtze River than other areas.Nanjing Iron and Stell United Co.,Ltd was an import source of NO₂,and urban pollution on the south side and inland river shipping emissions on the east side could not be ignored.Finally,the tropospheric vertical column densities of formaldehyde and glyoxal were retrieved by geometric approximation,and the day-by-day correlation coefficient was 0.59.The daily variations all showed the characteristics of low in the morning and evening,and high at noon.The TROPOMI HCHO products also showed high background density of HCHO around Nanjing,with slightly pronounced pollution hotspots only in summer and a more even distribution in winter.These indicated that both were mainly produced by the photolysis of VOCs emitted by solar radiation.The oxidation capacity of Nanjing atmosphere was strong.Meanwhile,the correlation coefficient between concentration and relative humidity is around-0.5,and wet deposition played an important role in the sink.