Evaluation Of The NO_x Emission Reductions From The Air Quality Assurance Actions For Typical Events In The Yangtze River Delta Region Based On Satellite Observation Constraint

Nitrogen oxide?NO_x?plays an important role in tropospheric atmospheric chemistry.As a key precursor for the formation of the complex air pollution in China,it poses direct or indirect adverse effects on human health.China was one of the countries with the largest amount and fastest growth in NO_xemissions in the world before 2010.A series of regular control policies have been implemented,leading to reduced NO_x emissions of China since2012.Besides regular policies stringent short-term air quality assurance plans have been implemented for various important events held in the Yangtze River Delta?YRD?region,eastern China since 2010,and those plans aimed to reduce emissions and to improve air quality during the event within a short period of time.After the event,most of the short-term controls were relieved.The implementation and cancelation of these measures provide a good opportunity to investigate the direct impact of short-term controls on NO_x emissions and to explore the maximum emission reduction potential at the city scale.The"bottom-up"emission inventories were subjectively developed,and spatio-temporal resolution of them were too low to identify the changes of NO_x emissions in a relatively short time.Therefore,this study improved the inversed method based on satellite observation and air quality modeling?AQM?,and realized the estimation of NO_x emissions at the city scale with relatively high spatial and temporal resolution.Through the analysis of the emission changes before,within and after the seven events from 2010 to 2016 in YRD including Shanghai World Expo in 2010?EXPO?,Nanjing Asian Youth Games in 2013?AYG?,Nanjing Youth Olympic Games in 2014?YOG?,National Memorial Ceremony?NMC?in 2014,2015 and2016,and Summit of G20 Finance Ministers and Central Bank Governors in Hangzhou in2016?G20?,the direct impact of different short-term air quality assurance plans on NO_xemissions was investigated.The inversed method of NO_x emission with high spatial and temporal resolution at the city scale based on satellite observation was improved and developed in this study.By improving the process of satellite-based NO₂ vertical column data,the uncertainty of satellite observation can be reduced.Combining the non-linear relationship between emissions and concentrations obtained by AQM and the a prior"bottom-up"emission inventory,satellite observation was used as the constraint to estimate the a posterior NO_x emission.The NO_xemissions were then estimated at a spatial resolution of 9km×9km and a temporal resolution of 3-days based on satellite observation.The reliability of this method was evaluated and verified by applying the available local emission inventory,ground-based observation and air quality model.It was found that the a posterior emissions were closer to the local scale emission inventory than the a prior emission,and that the model performance of the a posterior estiamte was better than the a prior.In this study,the stage of implementing the most stringent emission control measures for each event was considered as the core control period.The research period was divided into three stages:before,within and after the core control,and the non-core control period was before and after the core control.Based on the above division,the NO_x emission intensities of each host city and important surrounding cities before,within and after the core control were obtained.The NO_x emission rate decreased in most of the host cities during core control period compared to non-core control period.The implementation of the air quality assurance plan of EXPO effectively controlled the NO_x emissions in Shanghai,with the largest reduction of 41.5%by comparing the highest emission rate in non-core control period and the lowest emission rate in core control period.In 2013,the NO_x emission rate of Nanjing,the host city of AOG,was already at a low level before the core control,indicating that the actual emission control measures of AOG had been implemented before the opening,with the largest reduction of 24.7%in Nanjing.During the YOG in 2014,the reduction of NO_x emission rate in Nanjing was limited under the influence of short-term control,with a maximum reduction of 12.7%.This might result from the improved implementation of the regular emission control policy?e.g.,the"ultra-low"emission policy in power sector?and the relativley weak traffic source control during the period.The long-term controls were expected to limite the potential of short-term emission control measures.The NO_x emission rate in Hangzhou during the G20did not clearly decline under the short-term controls.This resulted from the joint effect of short-term control measures and urban economic activities,and it also suggested the uncertainties in the existing methods.Under the effect of NMC emission control,the NO_xemission intensitis of the host city Nanjing in 2014,2015 and 2016 decreased by 64.3%,35.6%and 14.0%,respectively.The gradually reduced emission control pressures and the improvement of the regular emission policy were expected to joinly resulted in the weakend benefits of NO_x emission control for the three NMCs.The short-term impact from the air quality assurance plans on NO_x emissions were gradually declining.Due to the continuously tightened and improved regular emission control policies from 2010-2015,China's NO_x emissions decreased significantly.For example,the implementation of the"ultra-low"emission policy significantly reduced the NO_x emissions of power and other industries,especially for large-scale point source emissions in developed regions.As the power industry was one of the main target sectors for the short-term control measures,the regular emission control policies limited the potential of the short-term air quality assurance plans to reduce NO_x emissions and weakened the control effect for the events in recent years.By comparing the overall reduction of the host city and surrounding areas,it is found that the reduction of Shanghai World Expo was the only event in which the NO_x emission reduction in host city was significantly greater than the surrounding areas.On one hand,EXPO has less experience in joint regional prevention and controls,and the mechanism of coordination and cooperation between surrounding cities was not well developed.On the other hand,the opening time of EXPO was as long as six months,and it was difficult for surrounding areas to maintain the same control stress as the host city.In addition,the reductions of NO_x emission rate in the surrounding areas of some other events were greater than those of the host cities.As the years went by,the time of reduced benefit of emission control of the host city was earlier than that of the surrounding areas.Compared with the host city with relatively improved regular controls,the effect of short-term control could be more significant with larger emission reduction potential for surrounding cities.With the progress of medium and long-term NO_x emission control,the management and control of small and medium-sized sources were recommended with improvement on the emission control technology and emission standards,and the continuous reduction of NO_x emissions can be expected in the future.