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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 512μgm⁻³ in Shanghai, 463μgm⁻³ in Suzhou, 354μgm⁻³ in Hangzhou, and 282μgm⁻³ in Nanjing. The hourly concentrations of PM_2.5 and PM₁₀ reached peak values of 466 and 744μgm⁻³, respectively, in Shanghai. With visibility <0.6km, the regional haze covered nearly the entire Yangtze River Delta. High ratios of PM_2.5/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 (<1ms⁻¹). The sulfur oxidation rate (SOR) reached 0.67 and the nitrogen oxidation rate (NOR) reached 0.61, signifying that formation …