PM_2.5acidity at a background site in the Pearl River Delta region in fall-winter of 2007-2012

Based on field observations and thermodynamic model simulation, the annual trend of PM2.5acidity and its characteristics on non-hazy and hazy days in fall-winter of 2007-2012 in the Pearl River Delta region were investigated. Total acidity ([H+]total) and in-situ acidity ([H+]in-situ) of PM2.5significantly decreased (F-test, p<0.05) at a rate of -32±1.5nmolm-3year-1and -9±1.7nmolm-3year-1, respectively. The variation of acidity was mainly caused by the change of the PM2.5component, i.e., the decreasing rates of [H+]totaland [H+]in-situdue to the decrease of sulfate (SO42-) exceeded the increasing rate caused by the growth of nitrate (NO3-). [H+]total, [H+]in-situand liquid water content on hazy days were 0.9-2.2, 1.2-3.5 and 2.0-3.0 times those on non-hazy days, respectively. On hazy days, the concentration of organic matter (OM) showed significant enhancement when [H+]in-situincreased (t-test, p<0.05), while this was not observed on non-hazy days. Moreover, when the acidity was low (i.e., R=[NH4+]/(2×[SO42-]+[NO3-])>0.6), NH4NO3was most likely formed via homogenous reaction. When the acidity was high (R≤0.6), the gas-phase formation of NH4NO3was inhibited, and the proportion of NO3-produced via heterogeneous reaction of N2O5became significant.