Study On Characteristics Of Aerosol Particles Based On Ground-based And Remote Sensing During Autumn And Winter Seasons Over Nanjing

The Yangtze River Delta(YRD)is one of the fastest developed regionthrough its economic and industrial sectors and rapid urbanization due to the people's huge comfortness and luxury.As one of the most important urban and industrially developed region in the YRD,Nanjing area is affected by the high population density and a large number of pollutants generated by the human and varied anthropogenic activities.Extreme and severe pollution will appear with unfavorable weather conditions that may lead to serious haze episodes during autumn and winter seasons due to large persistence of large amount of fine particles.The present study is conducted with its objective to examine the extreme weather conditions typically pollution over Nanjing region that occurred during autumn and winter seasons.This was achieved using the meteorological data observed over the study region and studying it's impact on AQI and PM2.5.It is revealed that fine model particles dominate relative to the coarse-mode aerosols.The severe air pollution days usually appear during autumn and winter seasons of the year due to its stagnant meteorological conditions.It is observed that the wind always blow in the direction from the east and southeast during autumn and winter seasons over Nanjing.While in the autumn season,severe air pollution days were highly appeared when the direction of wind is from the southwest with lower wind speeds.However,during winter extreme air pollution days have no obvious correlation which represents independent on the direction of wind.Followed this,we used aerosol optical depth(AOD)and Angstrom exponent(AE)derived from the Cimel Sun Photometer and Collection 6 Level 3 MODIS satellite remote sensing datasets data to study the optical characteristics of aerosols over Nanjing.It is depicted that the severe air pollution days were observed over the region during autumn and winter seasons attributed to strong anthropogenic activities such as biomass burning,urban-industrial/vehicular emissions apart from the local produced dust particles from construction of buildings and roads.We studied the scatter plot between spectral curvature(Coefficient a2)obtained from the polynomial fit and AOD revealed large amount of negative a2 indicates dominace of fine-mode aerosols at Nanjing.It is also observed that a2 decreases with increase in AOD and vice-versa,with accumulation of more scatter points at lower AOD.From the analysis of the spatial trends,it was found that the AOD and AE showed negative trends over Nanjing in autumn andwinter seasons of 2014?2017.The present study analyzes the trajectory clusters obtained jfrom the HYSPLIT model over Nanjing during 2014 and 2017.The results revealed that the distinct air masses were mainly originated from distinct directions in autumn seasons with maximum contribution(39%)from the East China Sea traversing through the major urban city,Shanghai.While the other air mass trajectory cluster were transported from Shandong(36%)and Yellow Sea(25%)before reaching the site.Whereas in the winter season,majority of the trajectory clusters come from the North China(59%)and the rest from Anhui province(41%).This study also analyzes the sources for the typical air pollution days appeared in Nanjing.It is found that the severe air pollution appeared during December 2013 were mainly affected by regional pollutants originated from the local sources and nearby surrounding areas.During December 2014,the severe haze air pollution appeared in Nanjing was mainly related to the long-range transport of aerosols originated from the North China.Whereas,in October 2015 the extreme air pollution attributed by the trajectory clusters coming from different sectors of Mongolia(16%),Hebei,Anhui(33%),East China sea(42%),and Russia(10%)towards Nanjing.Also,to analyze the air pollution appeared during December 2015 and 2016 we conducted the PSCF and CWT analysis using the HYSPLIT model.It is evident that the source of these two air pollution events were same,both caused by the regional pollution from local sources and its surrounding areas close to the observation site.