Acidic gases, ammonia and water-soluble ions in PM2.5 at a coastal site in the Pearl River Delta, China

Real-time measurements of acidic trace gases (HCl, HNO3, HONO, and SO2), ammonia, and water-soluble ions in PM2.5 were conducted at Xinken, a coastal site downwind of Guangzhou, from 4 October to 4 November 2004, as part of the Pearl River Delta (PRD) intensive field campaign. The average concentrations of HCl, HONO, HNO3, SO2, and NH3 are 2.8, 2.9, 6.3, 55.4, and 7.3 μg m−3, respectively, and 2.4, 7.2, 24.1, and 9.2 μg m−3 for Cl−, NO3−, SO42−, and NH4+ in PM2.5. The diurnal variations of both HCl and HNO3 showed higher concentrations during daytime and lower concentrations at night, and aerosol Cl− and NO3− showed an opposite diurnal patterns to HCl and HNO3. The diurnal variation of NH3 showed the similar pattern to that of aerosol NH4+ with lower concentration during daytime and higher concentration at night. The average concentration of SO2 during daytime was higher than that at night. The transportation of urban plumes to the sampling site could explain the higher concentration of SO2 during daytime. HONO showed a clear diurnal variation with lower concentration during daytime and higher concentration at night. The HONO concentrations were positively correlated with the particle surface area concentrations, suggesting the formation of HONO through the heterogeneous conversion on particle surfaces could be significant. The ionic charge balance analysis included the cations derived from filter measurements indicates that the contribution of the cations in fine particle (PM1.8) to the charge balance is not pronounced. The theoretical equilibrium constant (Ke) of NH4NO3 is higher than the observed concentration product (Km=[NH3]×[HNO3]) during daytime, and lower than Km at night. This indicates that the atmospheric conditions during the sampling period did not favor the formation of NH4NO3 during daytime.