[Observation of a photochemical event in Jiaxing during summer 2013]

To investigate the variation characteristics and causes of a high-level ozone pollution event during Aug. 5-11, 2013 in Jiaxing, the pollution gases (O3, NO2, NO, CO, SO2), atmospheric particles (PM10, PM2.5), and meteorological elements were observed in this study. The results indicated that the O3 concentration on the pollution days was 2. 4 times higher than that on normal days, with an exceeding rate of over 29.0% and up to 45. 8% on Aug. 9. This event was induced by the combined influences of intensive photochemical reaction at high temperature and in static stability weather condition with low humidity and wind speed. The diurnal variation of O3 showed a unimodal distribution on pollution and normal days, with the highest value at around 14:00. In the generation stage, the growth rates of O3 were 50.3 and 21.6 microg.(m3. h)-1 on pollution and normal days, respectively; in the extinction stage, the decrease rates of O3 were 16.8 and 23.4 microg (m3.h)-1. The concentrations of NO, NO2, and CO on pollution days were 1. 1, 1.5, and 1.5 times higher than those on normal days, which provided favorable conditions for photochemical reaction. The concentrations of PM2.5, PM10, and the ratio of PM2.5 /PM10 on pollution days were 2.5, 2.3, and 1.1 times higher than those on normal days, indicating that the photochemical reaction is more conducive to the formation of fine particles.