Mechanism of formation of the heaviest pollution episode ever recorded in the Yangtze River Delta, China

The heaviest aerosol pollution day in the historical record of the Yangtze River Delta occurred on 19 January 2007, in which the daily concentration of PM10 reached 512 μg m−3 in Shanghai, 463 μg m−3 in Suzhou, 354 μg m−3 in Hangzhou, and 282 μg m−3 in Nanjing. The hourly concentrations of PM2.5 and PM10 reached peak values of 466 and 744 μg m−3, respectively, in Shanghai. With visibility <0.6 km, the regional haze covered nearly the entire Yangtze River Delta. High ratios of PM2.5/PM10 (61%), SO2/PM10 (0.38) and NO2/PM10 (0.24) were observed. The unusual stagnant dispersion conditions before a cold front played the predominant role in this high pollution episode, with relative humidity (RH) of 88%, both surface and upper-level inversions, as well as low wind speed (<1 m s−1). The sulfur oxidation rate (SOR) reached 0.67 and the nitrogen oxidation rate (NOR) reached 0.61, signifying that formation of sulfate and nitrate contributed significantly to the high secondary aerosol concentration of PM2.5. The enrichment factors of As, Cd, and Pb reached 4058, 6971, and 3972, which were 4.45–6.68 times higher than those on good/moderate days. It was demonstrated that emissions from anthropogenic sources, especially stationary sources such as power plants, industrial boilers, and furnaces, were the major contributors to the heaviest aerosol pollution in the delta. More attention should be paid to the increases in emissions and the higher occurrence of haze days that have accompanied the rapid development of the economy in the Yangtze River Delta.