PM2.5 acidity 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 PM2.5 acidity 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 PM2.5 significantly decreased (F-test, p<0.05) at a rate of -32 +/- 1.5 nmol m(-3) year(-1) and -9 +/- 1.7 nmol m(-3) year(-1), respectively. The variation of acidity was mainly caused by the change of the PM2.5 component, i.e., the decreasing rates of [H+](total) and [H+](in-situ) due to the decrease of sulfate (SO42-) exceeded the increasing rate caused by the growth of nitrate (NO3-). [H+](total), [H+](in-situ) and 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-situ) increased (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 x [SO42-] + [NO3-1]) > 0.6), NH4NO3 was most likely formed via homogenous reaction. When the acidity was high (R <= 0.6), the gas-phase formation of NH4NO3 was inhibited, and the proportion of NO3- produced via heterogeneous reaction of N2O5 became significant. (C) 2015 Elsevier B.V. All rights reserved.