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 PM2.5 concentrations over the Pearl River Delta (PRD). In October 2012, an exceptional air pollution event occurred in the PRD, and a high concentration of PM2.5 was registered at some stations. During days with PM2.5 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 PM2.5 concentration and the ventilation index (VI). The VI was less than 2000 m2 s-1 during PM2.5 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 PM2.5 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 PM2.5 air pollution in the PRD. The sea breezes may transport pollutants back inland and may result in the peak PM2.5 concentrations at night. [ABSTRACT FROM AUTHOR]