title: PM2.5 acidity at a background site in the Pearl River Delta region in fall-winter of 2007-2012.
abstract: 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 (SO4(2-)) 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 × [SO4(2-)]+[NO3(-)])>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.
authors.affiliation: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China; Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hong Kong, China; Shenzhen Research Institute, Hong Kong Polytechnic University, China.
authors.forename: Xiaoxin
authors.initials: X
authors.lastname: Fu
authors.affiliation: Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hong Kong, China; Shenzhen Research Institute, Hong Kong Polytechnic University, China. Electronic address: ceguohai@polyu.edu.hk.
authors.forename: Hai
authors.initials: H
authors.lastname: Guo
authors.affiliation: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China. Electronic address: wangxm@gig.ac.cn.
authors.forename: Xinming
authors.initials: X
authors.lastname: Wang
authors.affiliation: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China.
authors.forename: Xiang
authors.initials: X
authors.lastname: Ding
authors.affiliation: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China.
authors.forename: Quanfu
authors.initials: Q
authors.lastname: He
authors.affiliation: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China.
authors.forename: Tengyu
authors.initials: T
authors.lastname: Liu
authors.affiliation: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China.
authors.forename: Zhou
authors.initials: Z
authors.lastname: Zhang
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D002681:China; D004784:Environmental Monitoring; D006863:Hydrogen-Ion Concentration; D009566:Nitrates / Q000032:analysis; D052638:Particulate Matter / Q000032:analysis*; D045483:Rivers; D012621:Seasons
chemical_list: D000393:Air Pollutants; D009566:Nitrates; D052638:Particulate Matter
keywords: Haze; Nitrate; PM(2.5) acidity; Secondary organic carbon; Sulfate