Impacts of synoptic forcing and topography on aerosol pollution during winter in Shenyang, Northeast China

Northeast China frequently experiences aerosol pollution episodes in winter. In addition to the pollutant emissions, synoptic pattern and topography can impact the air quality in complex ways, which are still not well understood in Northeast China. Therefore, the impacts of synoptic forcing and topography on aerosol pollution in Shenyang were investigated combining surface observations, sounding measurements, and three-dimensional air quality simulations. The studied pollution episode occurred from January 1 to 5, 2020, along with poor meteorological dispersion conditions characterized by weak winds, strong thermal stabilities, and shallow planetary boundary layers (PBLs). During the formation of pollution, strong elevated thermal inversion layers were observed over Shenyang, induced by the large-scale synoptic pattern, which suppressed the PBL growth and the vertical dispersion of aerosols. Moreover, the blocking effect of mountains to the east of Shenyang further worsened the pollution when northwesterly/westerly flows prevailed in shallow PBLs. Numerical sensitive experiment was conducted to estimate the contribution of blocking effect of mountains to the near-surface PM2.5 concentration in Shenyang, and it was found that around one third of PM2.5 concentration during January 1–4 was relevant to the terrain effect. These findings can facilitate a comprehensive understanding of the physical formation of aerosol pollution in Northeast China and helpful for the pollution controls.