To study the characteristics of water soluble inorganic ions (WSIIs) of PM2.5 and aerosol acidity in coastal city, 352 samples were collected at four sites representing four functional zones (FJ: living town; XY: industrial area; TZ: scenery areas; HR: harbor) in the coastal city, Xiamen in China during 2015-2016. Mass concentrations of PM2.5, nine WSIIs, total acidity and in-situ acidity of aerosol/non-sea-salt aerosol were analyzed. Overall, the mean mass concentrations of PM2.5 in the study area in spring, summer, autumn and winter were 57.45±22.27, 28.05±12.60, 46.82±18.34 and 62.43±22.10 µg·m-3, respectively. WSIIs accounted for 36%-56% of PM2.5 at four sites. Secondary ions (SO42-, NO3- and NH4+) contributed more than 74% to total WSIIs. Neutralization degrees distributions indicated that 88.3% of the samples were acidic. Total acidity and in-situ acidity which both showed obvious seasonal variations, exhibited the peak values of 217.20 and 145.18 nmol·m-3 at HR sites in summer. Non-sea-salt acidity contributed more than 80%. The normalized ammonium concentration ([NH4+]/[SO42-]) and normalized nitrate concentration ([NO3-]/[SO42-]) showed greater correlation coefficients after eliminating the ammonium poor data points (greater than 0.66), especially at HR site. Thus, NH4HSO4 was the major form of WSIIs in PM2.5 and formation of aqueous HNO3 could enhance aerosol acidity. Liquid water contents and pH values of acidic samples showed similar distributions at four sites indicating that larger water surface could uptake more gas precursors to accelerate the oxidation and further enhance aerosol acidity. 

DOI: 10.4209/aaqr.2016.11.0513