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Abstract: Although laboratory studies suggest that aerosol acidity ([H+]) significantly enhances the production of secondary organic aerosol (SOA) through heterogeneous chemistry, field studies have provided limited evidence of such enhancement. In this study, correlation of strong aerosol acidity with WSOC was investigated using the 24-hr PM2.5 samples collected at sites near four major cities of China – Beijing (BJ), Shanghai (SH), Lanzhou (LZ), and Guangzhou (GZ) – during the summers of 2004–2006. PM2.5 samples were characterized by high atmospheric loadings of PM2.5, OC, EC, sulfate, nitrate, aerosol acidity, and aerosol-water, especially in Beijing and Shanghai. On average, OC and EC were distributed in the ratio of approximately 2:1 among carbonaceous aerosols (TC = OC + EC) in all four cities. However, the WSOC fraction in OC differed across the four cities (BJ ∼ 55% of OC; SH ∼ 35%; LZ ∼ 40%; GZ ∼ 32%). We found an increased WSOC content in organic carbon (OC) fraction in the samples with elevated aerosol acidity (H+) and the WSOC was thought to be influenced by aerosol acidity. The WSOC/OC ratio showed a strong positive correlation with the normalized strong acid concentration ([H+]/[OC]) in the four cities. The higher WSOC fraction in OC at higher strong acidity is postulated to be linked to the conversion of OC to WSOC via heterogeneous acid-catalyzed chemistry. [ABSTRACT FROM AUTHOR]