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

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 mu g 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 mu g 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 R-2 = 0.70 and an increase of [H+] by 100 neq m(-3) would increase 1.2 mu g m(-3) WSOC in PM2.5. When [H+] > 0, an increase of [H+] by 100 neq m(-)3 would increase 1.4 mu g 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.