Size distribution and source analysis of ionic compositions of aerosols in polluted periods at Xinken in Pearl River Delta (PRD) of China

Size-resolved chemical composition of aerosol was investigated during the PRIDE intensive field campaign in October 2004 at Xinken, at Pearl River Delta (PRD), China. Xinken is a rural/coastal site located south of Guangzhou. Size-segregated particles were sampled using Micro-Orifice Uniform Deposit Impactors (MOUDI) in order to characterize regional aerosol pollution. The meteorological conditions during the measurements were stable with low wind speed, high temperature and high RH, leading to high concentrations of fine particles, especially due to secondary components. The average concentrations of PM1.8 and PM10 were 51±19 and 84±27 μg m−3, respectively, with a ratio of PM1.8/PM10 0.64±0.09, which indicates that fine particles dominated in PM10. Sulfate, nitrate and ammonium were major ionic components. Aerosol mass size distributions showed either two or three modes. Land–sea circulation brought back aged aerosols to the sampling site leading to an increase of the concentrations of sodium and chloride in aerosols. The “condensation mode” of sulfate, which was formed by gas-to-particle conversion, was in the size range of 0.32–0.56 μm, and the “droplet mode” of sulfate was observed in the size range of 0.56–1.0 or 1.0–1.8 μm. Nitrate was distributed evenly over fine and coarse particles. The analysis of measured and calculated equilibrium constant of NH4NO3 indicates that dissociation of NH4NO3 in daytime is favored, but that at night ammonium nitrate is stable and constitutes the fine mode nitrate, while the coarse mode nitrate may be formed by the reaction of HNO3 and sea-salt or soil components. Chemical mass closures showed reasonable agreement.