Source and formation of secondary particulate matter in PM2.5 in Asian continental outflow

[1] Fifty-five 48-h PM2.5 samples were collected from March 2003 to January 2004 at Changdao, a resort island in Bohai Sea/Yellow Sea in Northern China. Sulfate, nitrate and ammonium accounted for 54 ± 9% of the PM2.5 mass concentration (annual average 47 μg m−3) while organic matter and K+ contributed to 27 ± 7% and 7 ± 7% of the total mass, respectively. The ratios of SO42− to NO3− mass concentrations could be divided into two regimes and demarcated at nitrate concentration of 5 μg m−3. In the low NO3− regime, NO3−, SO42− and EC were well correlated to K+, and the estimated contributions of NO3−, SO42− and EC from biomass burning were 50 ± 27%, 38 ± 24% and 47 ± 27%, respectively. These correlations substantially decreased in the high NO3− regime reflecting fossil fuel combustion and formation of ammonium nitrate and the estimated contributions of NO3−, SO42− and EC from biomass burning were 16 ± 12%, 28 ± 18% and 27 ± 16%, respectively. In most samples, the equivalent ratios of total anion to total cation concentrations were greater than unity, suggesting that the aerosols were acidic. When [H+] > 0, a moderately good linear correlation of the estimated aerosol acidity [H+] with the water-soluble organic carbon (WSOC) was observed with R2 = 0.70 and an increase of [H+] by 100 neq m−3 would increase 1.2 μg m−3 WSOC in PM2.5. When [H+] > 0, an increase of [H+] by 100 neq m−3 would increase 1.4 μg m−3 of secondary organic carbon (SOC) in PM2.5. Moreover, the correlation analysis result suggested that 60% of the estimated SOC (on average) in PM2.5 were water-soluble.