Spatial-Temporal Characteristics and Background Circulations of Local PM_2.5 Pollution Episodes in the Pearl River Delta

In this paper, the spatial-temporal characteristics and the background circulations of local PM_2.5 pollution (LPP) episodes in the Pearl River Delta are analyzed by using the PM_2.5 data from 56 air quality monitoring stations and the meteorological data from 89 National Basic Weather Stations and 2 wind profile radars. The conclusions are as follows. First, the LPP episodes recorded in Guangzhou, Foshan and Zhaoqing are more than those in other cities. The LPP episodes occur more frequently in winter than in summer, and most of them occur during the period from night to the next morning. Second, the LPP episodes are caused by different background circulations, namely the continental cold high bottom, transformed sea high ridge, subtropical high and northly downdraft outside the tropical cyclone. Third, most typical LPP episodes are affected by the local circulations. Under the influence of mountains, plains and ocean thermal conditions, the surface wind changes, and the weak convergence line is formed, preventing the pollution from diffusing horizontally. At the same time, weak northerly winds from northern mountains of Guangdong Province arrive at the Pearl River Delta, causing downdrafts within the boundary layer, inhibiting the pollution from spreading vertically, resulting in the ground pollution.