LAUSR

Abstract This study investigated the impacts of meteorological and geographical conditions, the roles of individual physical processes, and the contribution of emissions during severe PM2.5 (fine particulate matter) pollution events in Shenyang, one of the largest industrial cities in China. Simulations of six severe PM2.5 pollution events that occurred in 2015 and 2016 revealed that unfavorable meteorological conditions, including high relative humidity, low depth of the planetary boundary layer, low wind speed, changes in wind direction, and unfavorable geographical conditions underly severe PM2.5 pollution events in Shenyang. Regarding individual physical processes, emission and aerosol processes increased the concentration of PM2.5, while horizontal advection, vertical advection, and vertical diffusion processes dominated the decrease in PM2.5. Analysis of source apportionment found that residential (37%) and transportation (30%) dominated PM2.5 pollution as categories, and Shenbei New District contributed 75% of regional emissions. The main causes of the severe PM2.5 pollution events in Shenyang are complex, and include local emissions associated with transportation emissions from Shenbei New District; local emissions associated with agricultural emissions and low northwesterly winds; long-range transport with a southeasterly wind; and middle-range transport with a northerly wind.