Short-Term Weather Patterns Modulate Air Quality in Eastern China During 2015-2016 Winter

The roles of anthropogenic emissions and weather conditions in air pollution over eastern China have been widely discussed but still controversial. Here we focus on the impact of the intraseasonal variability of midtropospheric weather circulations on air quality during 2015-2016 winter. We use the European Center for Medium-Range Weather Forecasts reanalysis data to calculate westerly wind index (WI) and meridionality in the midtroposphere and also calculate the intensity of Siberian High at the surface. The results suggest that there were lower WI, higher meridionality, and strengthened Siberian High in January 2016 compared with the conditions in December 2015. Lower WI and higher meridionality imply for wavier midtropospheric weather circulations and stronger surface northerly winds, thereby causing more southward cold fresh air advection and resulting in the lower PM2.5 concentrations over eastern China in January 2016. Sensitivity experiments conducted by the Weather Research and Forecasting Chemical model show that the intraseasonal weather variability dominantly causes lower PM2.5 concentrations in January 2016. It not only counteracts increments in PM2.5 concentrations induced by the increased emission but also reduces PM2.5 concentrations by an extra 25-100g/m(3). We also simulate two extreme weather patterns and compare their effects on PM2.5 concentrations. Difference in these two weather patterns induces changes in PM2.5 concentrations by 50-200g/m(3) over eastern China and the maximal change exceeding 200g/m(3). These findings suggest that understanding the variability of the dominant midtropospheric circulation is the key to successfully forecasting short-term air quality over eastern China in winter. Plain Language Summary The subseasonal variation of PM2.5 concentrations is dominantly induced by the intraseasonal variability of weather circulations during 2015-2016 winter. In the early winter (December 2015), a straight westerly wind prevails at the midtroposphere, confining cold air within the high latitudes and causing the accumulation of PM2.5 concentrations in eastern China, while in the late winter (January 2016), a north-to-south oriented airflow at the midtroposphere allows cold air advection to eastern China, reducing PM2.5 concentrations. Consequently, the intraseasonal weather variability leads to the maximal reduction of PM2.5 concentrations by 50-200g/m(3). These findings suggest that understanding the variability of the midtroposphere circulation patterns is the key to successfully forecasting and improving air quality in winter.