Atmospheric Environment 37 (2003) 3001–3007 Size distributions and formation of ionic species in atmospheric particulate pollutants in Beijing, China: 2—dicarboxylic acids Xiaohong Yao a , Arthur P.S. Lau a , Ming Fang a , Chak K. Chan b, *, Min Hu c a Institute for Environment and Sustainable Development, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China b Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China c State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences, Peking University, Beijing 100871, China Received 8 January 2003; received in revised form 11 March 2003; accepted 19 March 2003 Abstract 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 SO 4 2 ,K + and Ca 2+ 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 0.770.1 mm, 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 0.870.2 mm 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 ¼ 0:86) and had an MMAD (0.4570.05 mm) similar to sulfate, but it sometimes dominated in the coarse mode, in association with Ca 2+ . The fine mode oxalate was attributed to heterogeneous formation but the coarse mode oxalate was probably related to biological formation. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Particle size; In-cloud processes; Biological processes; Oxalate; Malonate; Succinate 1. Introduction Dicarboxylic acids in atmospheric particles poten- tially play an important role as cloud condensation nuclei (CCN) in the global atmosphere (Cruz and Pandis, 1997; Facchini et al., 1999; Kerminen, 2001). They can reduce the surface tension and the hygroscopic property of CCN and can thus affect cloud formation and the earth’s radiation balance (Facchini et al., 1999). Kerminen (2001) suggested that organics are more important than sulfate in the initial growth of very small particles (o40–50 nm in diameter) because or- ganics can reduce surface tension, but sulfate becomes more important when the particles reach the sizes of the smallest CCN corresponding to high cloud supersatura- tions. Although the concentrations of dicarboxylic acids ARTICLE IN PRESS AE International – Asia *Corresponding author. Tel.: +852-2358-7124; fax: +852- 2358-0054. E-mail address: keckchan@ust.hk (C.K. Chan). 1352-2310/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1352-2310(03)00256-5