Characteristics of mass size distributions of water-soluble inorganic ions during summer and winter haze days of Beijing

Abstract

To investigate the size distribution characteristics of water soluble inorganic ions in haze days, the particle samples were collected by two Andersen cascade impactors in Beijing during summer and winter time and each sampling period lasted two weeks. On-line measurement of PM10 and PM2.5 using TEOM were also conducted at the same time. Sources and formation mechanism of water soluble inorganic ions were analyzed based on their size distributions. The results showed that average concentrations of PM10 and PM2.5 were (245.5±8.4)μg·m-3 and (120.2±2.0)μg·m-3 during summer haze days (SHD), and were (384.2±30.2) μg·m-3and (252.7±47.1) μg·m-3 during winter haze days (WHD), which suggested fine particles predominated haze pollution episode in both seasons. Total water-soluble inorganic ions concentrations were higher in haze days than those in non-haze days, especially in fine particles. Furthermore, concentrations of secondary inorganic ions (SO42-, NO3- and NH4+) increased quicker than other inorganic ions in fine particles during haze days, indicating secondary inorganic ions played an important role in the formation of haze pollution. Similar size distributions were found for all Sinorganic water soluble ions except for NO3-, during SHD and WHD. SO42- and NH4+ dominated in the fine mode (PM1.0) while Mg2+ and Ca2+ accumulated in coarse fraction, Na+, Cl- and K+ showed a bimodal distribution. For NO3-, however, it showed a bimodal distribution during SHD and a unimodal distribution dominated in the fine fraction was found during WHD. The average mass median aerodynamic diameter (MMAD) of SO42- was 0.64 μm in SHD, which suggested the formation of SO42- was mainly attributed to in-cloud processes. Furthermore, a higher apparent conversion rate of sulfur dioxide (SOR) was found in SHD, indicating more fine particles were produced by photochemical reaction in haze days than that in non-haze days. The MMAD of SO42- increased to 0.89 μm in WHD, local emission of SO2and the subsequently heterogeneous reaction became the main source of SO42- during winter time. The average MMADs of NO3- were 2.85 μm and 0.80 μm in SHD and WHD, respectively. Influenced by the seasonal temperature difference, NO3- mainly existed in the form of calcium nitrate in coarse mode during SHD while the fine mode nitrate was associated with ammonium during WHD.