Numerical Study On Air Quality Attainment In Hangzhou

Air pollution issue has become increasingly prominent with China’s rapid urbanization.China has held various world-class major events in recent years,such as the World Internet Conference and the Group of Twenty（G20）Summit,in Hangzhou and adjacent regions.To improve the air quality during the events,this study used a Weather Research and Forecasting model coupled with Chemistry for air quality simulation.This modeling system was well evaluated by a substantial volume of observation data involving environmental monitoring stations,laser radars,and satellite remote sensing.Quantitative studies were then conducted on the spatial and temporal distribution of pollutants,formation mechanisms,and control strategies.The purpose of this study was to provide scientific and technological support for ensuring favorable air quality in Hangzhou.The characteristics of the air pollution in Hangzhou and adjacent regions over the years were analyzed.Results showed that the primary atmospheric contaminant for Hangzhou was PM_2.5.5 during the winter,and O₃ during the summer.The emissions of local pollutants and regional transport of pollutants were apparent,suggesting that to ensure favorable air quality in Hangzhou,the principle of joint prevention and control and coordinated reduction of multiple pollutants in the Yangtze River Delta is warranted.The simulation results confirmed the significance of the first strategic-level air pollution prevention and control measure—Action Plan of Energy Saving,Emission Reduction,Upgrading and Retrofitting of Coal-fired Power Plants—on ensuring favorable air quality.In addition,the results also clarified that coal-fired power plant emissions were not the primary source of PM_2.5.5 pollution.Accordingly,we separately proposed various air quality control measures involving multiple regions,industries,and intensities for ensuring favorable air quality during the two major events.The results during the World Internet Conference period（December 8–19,2015）showed that the control measures reduced the daily mean PM_2.5.5 concentration in Hangzhou by 7%–25%（Mean=15%）.The substantial presence of PM_2.5.5 pollutants(daily mean PM_2.5.5 concentration of 154μg m^-3)on the day before the event was due to the influence of the southward movement of the Siberian cold snap in winter and the long-range transport of air pollutants（contribution reaching 85%）.Concerning the G20 Summit period（August 24–September 6,2016）,the results showed that the control measures reduced the daily mean PM_2.5.5 concentration in Hangzhou by 15%–45%（Mean=34%）;the measures implemented in Hangzhou and adjacent regions accounted for 55%and45%of the decrease in PM_2.5.5 contribution,respectively.Overall,the measures displayed marked prevention effects and met the targets for ensuring favorable air quality.However,the effect of the measures on controlling O₃ emissions was limited.During the campaign period of G20 Summit,the reduction in O₃-8h concentration was up to 8%;however,during more than half of the total number of days,the level exceeded the Class 2 standards.Besides the anthropogenic emissions,the unfavorable typhoon weather conditions of the summer and the urban heat island effect were the primary causes for the increased reactions with atmospheric O₃ and increased O₃ pollution.In addition,the interaction effects of a tropical cyclone and the Westerly Jet Stream resulted in an extratropical cyclone that caused tropopause folding.This caused the high O₃ concentration in the stratosphere to considerably influence the surface layer in Southeastern China.This study enriched China’s experience in implementing short-term measures for maintaining favorable air quality,expanded applications of numerical simulation to world-class events,promoted the construction of long-term mechanisms for air pollution prevention and control in China,and instilled the concept of improving air quality among the public.