The Effect Of The Variation Of Human Activities And Vegetation Water Content On Atmospheric Nitrogen Dioxide And Volatile Organic Compounds In China And Tropical South America

Atmospheric pollution,such as smog,ozone,is one of the most important problem of environment over the world.And smog and ozone are highly related to atmospheric VOCs(Volatile Organic Compounds)and NO2(Nitrogen Dioxide).So,investigating the source and variations of VOCs and NO2 is essential and important.In one hand,under the content of global warming,extreme weather events happen more frequently,which request more studies about the effects of vegetation water content on the biogenic VOCs emission.In another hand,human activities are crucial to the concentration of NO2,so how to relate the change of NO2 concentration with human activity intensity needs more investigations.The rain forest in Tropical America is the densest forest over the world with strong vegetation activities,and in contrast,the population in China is largest,with notable human activity intensity.In these two typical areas,using the correlation analysis,combining remote sensing and model simulation,we studied the effect of vegetation water content on the biogenic VOCs emission,and the effect of human activities on the NO2 in recent years.The main results and conclusion of the research are as follows:1.The controlling factors of atmospheric formaldehyde(HCHO)observed by OMI are investigated by combining the microwave emissivity difference vegetation index(EDVI),MODIS observed fire count,CERES surface radiation flux and ERA-Interim reanalysis temperature.In Tropical America,HCHO column density is mainly controlled by the seasonal wild fire where the fire events happen frequently,while in other places is controlled by radiation,temperature and vegetation.It is shown that the correlations between HCHO and other factors are uncertain when using monthly data.But after removing the fire samples using daily data,it is found that HCHO is correlated with temperature;besides,in rain forest,EDVI is relatively stable,HCHO is mainly controlled by radiation,and in savanna controlled by EDVI.In China,the seasonality of vegetation is significant,and is similar with temperature or radiation,thus the HCHO concentration there is controlled by EDVI,temperature and radiation together.2.In order to present the effect of vegetation water content on biogenic emission,we coupled EDVI into the biogenic emission model(MEGAN)to simulate isoprene emission from vegetation,and compared the simulated emission with OMI top-down retrieved emission.The results show that,in the northern and eastern savanna of Tropical America,using ED VI as the indicator of vegetation water content can improve the temporal consistence between simulation and retrieval.Using soil moisture factor to present the water stress,however,is likely to involve larger uncertainties to the simulation.In China,compared with NDVI and EVI,the simulation with EDVI-based water stress factor show highest correlation with top-down retrieval.Besides,the simulation using EDVI with daily variation show more consistent with top-down retrieval than that using EDVI with seasonal variation,indicating that the daily variation of ED VI can represent the variation of vegetation water content thus influence the biogenic emission.3.(3)In Tropical America,due to the low human activity intensity,the NO2 concentration observed by OMI is low.The seasonality of NO2 is mainly controlled by fire.In several megacities around coastline,NO2 is higher than surrounding areas which indicate the higher anthropogenic emission in those cities.In 2020,many countries were suffering the COVID-19 pandemic,and applied strong control strategies to movements.However,in these cities,NO2 shows little differences than before except Lima,Peru.In Lima,the NO2 column density declined in 2020 reflecting the decrease of human activities and can partly explain the change of the number of daily confirmed cases during the outbreak of COVID-19.4.In China,the OMI NO2 column is high due to the large anthropogenic emission.During the COVID-19 pandemic,NO2 decrease greatly due to the strict prohibition of human activities.By combining the ERA-5 meteorology data,it is found that during the pandemic,the weather condition didn't favor the decrease of NO2 in the eastern China.After considering the weather impact using a meteorological model,NO2 showed high correlation with Baidu Migration Indices(Immigration Index,Emigration Index,Intra-city Index).And the correlation between NO2 and Intra-city Index is highest,which demonstrates the contribution of intra-city movements to NOx emission.