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To characterize aerosol pollution in Beijing, size-resolved aerosols werecollected by MOUDIs during CAREBEIJING-2006 field campaign at PekingUniversity (urban site) and Yufa (upwind rural site). Fine particleconcentrations (PM1.8 by MOUDI) were 99.8±77.4 μg/m3 and78.2±58.4 μg/m3, with PM1.8/PM10 ratiosof 0.64±0.08 and 0.76±0.08 at PKU and Yufa, respectively, and secondarycompounds accounted for more than 50% in fine particles. PMF modelanalysis was used to resolve the particle modes. Three modes were resolvedat Yufa, representing condensation, droplet and coarse mode. However, onemore droplet mode with bigger size was resolved, which was consideredprobably from regional transport. Condensation mode accounted for10%–60% of the total mass at both sites, indicating that thegas-to-particle condensation process was important in summer. The formationof sulfate was mainly attributed to in-cloud or aerosol droplet process (PKU80%, Yufa 70%) and gas condensation process (PKU 14%, Yufa 22%).According to the thermodynamic instability of NH4NO3, sizedistributions of nitrate were classified as three categories by RH. Theexistence of Ca(NO3)2 in droplet mode indicated the reaction ofHNO3 with crustal particles was also important in fine particles. Arough estimation was given that 69% of the PM10 and 87% of thePM1.8 in Beijing urban were regional contributions. Sulfate, ammoniumand oxalate were formed regionally, with the regional contributions of90%, 87% and 95% to PM1.8. Nitrate formation was localdominant. In summary regional secondary formation led to aerosol pollutionin the summer of Beijing

Size-resolved aerosol water-soluble ionic compositions in the summer of Beijing: implication of regional secondary formation