Meteorological conditions conducive to PM_2.5 pollution in winter 2016/2017 in the Western Yangtze River Delta, China

Cities in Anhui province in the western Yangtze River Delta (YRD), China experienced more PM_2.5 pollution days in the winter of 2016/2017 (Dec 2016 to Feb 2017) than in the previous two winters under conditions of emission deductions. By employing back-trajectory-clustering analysis together with daily air quality index (AQI) data from 2015 to 2017, routine and reanalysis meteorological data, and some climate indices, we investigated the transport paths, large-scale vertical motion and related climate background conducive to PM_2.5 pollution in Anhui province. We obtained 5 air-mass paths affecting Anhui province in winter; among them, the slow-moving air-masses from the northeast and northwest often led to PM_2.5 pollution. Thus, they belong to adverse transport paths, which accounted for approximately 52% in northern Anhui and 62% in central Anhui. Compared with winter 2015/2016, the proportions of adverse transport paths in winter 2016/2017 increased 13% in Hefei (central site), 3% in Suzhou (northern site), and 9% in Chizhou (southern site); correspondingly, east winds increased, and north winds weakened in the boundary layer, which favoured the accumulation of pollutants in Anhui. The processes of pollution and cleaning in Anhui were also closely related to vertical motion of the middle troposphere (500 hPa), and the sinking (ascending) corresponding to the aggravation (mitigation) of pollution. Compared with the winter of 2015/2016, the percentage of downward vertical velocity at 500 hPa exceeding 0.2 Pa/s increased evidently in the winter of 2016/2017. Thus, the vertical velocity at 500 hPa can be used as an important factor for air quality prediction in winter. The interannual changes in transport conditions are related to changes in the Asia zonal and meridional circulations and may further be ascribed to the thermal and dynamic conditions in the Tropical Ocean.