TheSpatialandTemporalVariabilityofTroposphericNO2during2005–14overChinaObservedbytheOMI

As improved and accumulated satellite records become available,it is significant to provide up-to-date perspectives on the spatiotemporal signatures of tropospheric nitrogen dioxide(NO2)over China,the knowledge of which is helpful for air pollution control.In this study,the Ozone Monitoring Instrument NO2 dataset for the last 10 years(2005–14)was retrieved to examine multiple aspects of NO2 columns,including distributions,trends,and seasonal cycle.The pattern of average NO2suggests five hotspots with column density higher than 20×1015 molec cm-2:Jing-Jin-Tang;combined southern Hebei and northern Henan;Jinan;the Yangtze River Delta;and the Pearl River Delta.Furthermore,substantial and widespread NO2 growths are distributed over the North China Plain.By contrast,downward trends in NO2 amounts prevail in the megacities of Beijing,Shanghai,and Guangzhou,despite generally high loading levels.Except for the Pearl River Delta,there appears to be temporally consistent behaviors across all regions considered,where NO2 had an abrupt decline during 2008 to 2009,then a drastic increase up to 2013,before beginning to reduce again after 2013.However,the NO2 over the Pearl River Delta is not coevolving with the rest,having experienced a moderate rise from 2005 to 2007,followed by a reduction thereafter.A marked seasonality is apparent,with a maximum in winter and a minimum in summer,regardless of the region.The annual amplitude of NO2 is less pronounced over the Pearl River Delta,whereas the largest range is observed over the combined Southern Hebei and Northern Henan region,induced by enhanced NO2emission in wintertime due to intense domestic heating.