Size distributions and formation of ionic species in atmospheric particulate pollutants in Beijing, China: 2 - Dicarboxylic acids

Sparse data on the size distributions of ionic species, especially organic ions, in atmospheric particles are available in the literature. To study the size distributions and the formation of atmospheric ionic species, 14 sets of measurements of the concentrations of the gases and size-segregated atmospheric particles were made in Beijing in the summer of 2001 and the spring of 2002. Significant differences of the size distribution characteristics and the formation mechanism of the ions in these two periods were found. Using the size distributions of SO4 2-, K+ and Ca2+ as references, the size distributions of oxalic, malonic and succinic acids were examined to investigate the formation of these acids in the atmospheric particles in Beijing. In the summer, oxalate was present mostly in the fine mode with an MMAD of 0.7±0.1μm, coincident with the droplet mode of sulfate. The formation of fine mode oxalate, like sulfate, was attributed to in-cloud processes. Malonate was also present mostly in the fine mode with an MMAD of 0.8±0.2μm and its formation was also attributed to in-cloud processes. The MMAD of fine mode malonate sometimes shifted to values larger than that of sulfate, which can be explained by a minor evaporation of malonate during water evaporation from cloud droplets. The size distribution and the formation of succinate are similar to those of oxalate. In the spring, oxalate sometimes dominated in the fine mode, where oxalate was highly correlated with sulfate (r=0.86) and had an MMAD (0.45±0.05μm) similar to sulfate, but it sometimes dominated in the coarse mode, in association with Ca2+. The fine mode oxalate was attributed to heterogeneous formation but the coarse mode oxalate was probably related to biological formation. © 2003 Elsevier Science Ltd. All rights reserved.