Study On The Spatiotemporal Distribution, Influencing Factors And Health Effects Of Atmospheric NO₂ In The Yangtze River Delta Based On Satellite Remote Sensin

Nitrogen dioxide（NO₂）is one of the main air pollutants.Long-term exposure to NO₂will have adverse effects on human health.With the development of social economy and the advancement of urbanization,the Yangtze River Delta region has become one of the most serious areas of NO₂ pollution in China.The study of the spatial and temporal distribution of NO₂ in the Yangtze River Delta region,the influencing factors and the environmental health effects of NO₂ is conducive to understanding the problem of atmospheric NO₂ pollution and providing theoretical support for regional environmental governance.Based on the NO₂remote sensing data of the multi-year OMI and TROPOMI,this paper analyzes the temporal and spatial changes of the tropospheric NO₂ column concentration in the Yangtze River Delta region from 2005 to 2020 and during the outbreak of the novel coronavirus epidemic.Combined with the Geodetector model,the effects of meteorology,geographical environment and anthropogenic emissions on the variation of tropospheric NO₂ concentration in the Yangtze River Delta were discussed.The HYSPLT model was used to simulate the main NO₂transport paths in the Yangtze River Delta region in winter,and the potential source areas of NO₂ pollution were analyzed.Based on the random forest model,the near-surface NO₂concentration in the Yangtze River Delta was estimated,and the health effect associated with NO₂ exposure in the Yangtze River Delta was evaluated in combination with the empirical equation.The main conclusions are as follows:（1）In terms of spatial distribution,the spatial distribution of tropospheric NO₂ in the Yangtze River Delta region from 2005 to 2020 is centered on the Taihu Lake urban agglomeration as a high concentration area,and the’northwest-southeast’direction from Lu’an to Taizhou is the dividing line,showing the distribution characteristics of high in the northeast and low in the southwest.The low concentration areas are concentrated in the southwestern cities such as Huangshan and Lishui.（2）In the long-term trend,the annual mean value of tropospheric NO₂ column concentration in the Yangtze River Delta from 2005 to 2020 showed a trend of increasing first and then decreasing,reaching a peak in 2011,and then showing a downward trend.In terms of seasonal variation,there is a strong seasonal variation cycle.The concentration of tropospheric NO₂ from large to small is:winter,autumn,spring,summer;in terms of monthly variation,the peak value mostly occurs from December to January of the following year,and the valley value occurs from July to August;the concentration of tropospheric NO₂ column in the Yangtze River Delta during the epidemic period was generally at a low concentration level,and the social production and economic activities in the Yangtze River Delta region were greatly impacted in the months after the new coronavirus epidemic.（3）In terms of influencing factors,temperature and precipitation were significantly negatively correlated with tropospheric NO₂ concentration on the monthly scale.The wind field condition is not the dominant condition affecting the long-term change of NO₂concentration in the Yangtze River Delta region,which only affects the pollution diffusion and transmission in the short term.The higher the elevation and the higher the vegetation index,the lower the tropospheric NO₂ column concentration.The topographic characteristics of the northeast high and southwest low in the Yangtze River Delta weaken the transmission and diffusion of NO₂ pollution from the high concentration area of the central plain to the southwest.The NO_x emissions of the industrial sector and the transportation sector are the largest.The NO_x emissions of the power sector declined sharply after 2011,and the NO_xemissions of the civil sector are much lower than those of the other three sectors.The long-term variation of tropospheric NO₂ is mainly affected by geographical environment factors and anthropogenic emission factors,while meteorological factors affect the seasonal and short-term changes of tropospheric NO₂.The simulation results of NO₂ transport show that the regional differences of NO₂ pollution in the Yangtze River Delta region in winter are obvious,and are affected by the superposition of pollution processes such as local emission sources and inter-regional pollution transport.（4）In terms of environmental health risks,this study accurately estimated the near-surface NO₂ concentration in the Yangtze River Delta by constructing a random forest model to fuse multi-source data.Although the current NO₂ pollution in the Yangtze River Delta region has improved,under the new NO₂ air quality standards,only 6.21 million people in the Yangtze River Delta region live below the WHO’s new 10μg/m³ NO₂ exposure level,accounting for 2.4%of the total population of the Yangtze River Delta,the vast majority of the population in the Yangtze River Delta region is still at a high level of NO₂ exposure,and the harm caused by NO₂ needs attention and vigilance.In large cities with concentrated population,human health is more threatened.It is necessary to formulate more targeted regional policies for NO₂ pollution in megacities in the Yangtze River Delta.