STUDY OF THE VERTICAL WIND FIELD STRUCTURE AND ITS RELATIONSHIP WITH PM2.5 AIR POLLUTION OVER THE PEARL RIVER DELTA IN AUTUMN

Meteorological conditions, particularly the vertical wind field structure, have a direct influence on the PM_(2.5) concentrations over the Pearl River Delta(PRD). In October 2012, an exceptional air pollution event occurred in the PRD, and a high concentration of PM_(2.5) was registered at some stations. During days with PM_(2.5) air pollution, the wind speed was less than 3 m s~(-1) at the surface, and the vertical wind field featured a weak wind layer(WWL) with a thickness of approximately 1000 m. The mean atmospheric boundary layer height was less than 500 m during pollution days, but it was greater than 1400 m during non-pollution days. A strong negative correlation was detected between the PM_(2.5) concentration and the ventilation index(VI). The VI was less than 2000 m~2 s~(-1) during PM_(2.5) air pollution days. Because of the weak wind, sea-land breezes occurred frequently, the recirculation factor(RF) values were small at a height of 800 m during pollution days, and the zones with the lowest RF values always occurred between the heights of 300 and 600 m.The RF values during PM_(2.5) pollution days were approximately 0.4 to 0.6 below a height of 800 m, reducing the transportation capacity of the wind field to only 40% to 60%. The RF and wind profile characteristics indicated that sealand breezes were highly important in the accumulation of PM_(2.5) air pollution in the PRD. The sea breezes may transport pollutants back inland and may result in the peak PM_(2.5) concentrations at night.