ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-23955-2009 Size-resolved aerosol water-soluble ionic compositions in the summer of Beijing: implication of regional secondary formation Guo S. 1 Hu M. 1 Wang Z. B. 1 Slanina J. 1 Zhao Y. L. 2 State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, 100871 Beijing, China Department of Environmental Science, Policy and Management, University of California, 94720 Berkeley, CA, USA 11 11 2009 9 6 23955 23986 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/9/23955/2009/acpd-9-23955-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/23955/2009/acpd-9-23955-2009.pdf

To characterize aerosol pollution in Beijing, size-resolved aerosols were collected by MOUDIs during CAREBEIJING-2006 field campaign at Peking University (urban site) and Yufa (upwind rural site). Fine particle concentrations (PM<sub>1.8</sub> by MOUDI) were 99.8&plusmn;77.4 &mu;g/m<sup>3</sup> and 78.2&plusmn;58.4 &mu;g/m<sup>3</sup>, with PM<sub>1.8</sub>/PM<sub>10</sub> ratios of 0.64&plusmn;0.08 and 0.76&plusmn;0.08 at PKU and Yufa, respectively, and secondary compounds accounted for more than 50% in fine particles. PMF model was used to resolve the particle modes. Three modes were resolved at Yufa, representing condensation, droplet and coarse mode. However, one more droplet mode with bigger size was resolved, which was considered probably from regional transport. Condensation mode accounted for 10%–60% of the total mass at both sites, indicating it must be taken into account in summer. The formation of sulfate was mainly attributed to in-cloud or aerosol droplet process (PKU 80%, Yufa 70%) and gas condensation process (PKU 14%, Yufa 22%). According to the thermodynamic instability of NH<sub>4</sub>NO<sub>3</sub>, size distributions of nitrate were classified as three categories by RH. The existence of Ca(NO<sub>3</sub>)<sub>2</sub> in droplet mode indicated the reaction of HNO<sub>3</sub> with crustal particles was also important in fine particles. Linear regression gave a rough estimation that 69% of the PM<sub>10</sub> and 87% of the PM<sub>1.8</sub> at PKU were regional contributions. Sulfate, ammonium and oxalate were formed regionally, with the regional contributions of 90%, 87% and 95% to PM<sub>1.8</sub>. Nitrate formation was local dominant. In summary regional secondary formation led to aerosol pollution in the summer of Beijing.

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