Characteristics of Water-Soluble Inorganic Components and Acidity of PM2.5 in a Coastal City of China

To investigate the characteristics of water-soluble inorganic ions (WSIIs) of PM_(2.5) and aerosol acidity in a 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 Xiamen, China in 2015-2016. Mass concentrations of PM_(2.5), nine WSIIs, total acidity and in-situ acidity of aerosol/non-sea-salt aerosol were measured. Overall, the mean mass concentrations of PM_(2.5) in the study area in spring, summer, autumn and winter were 57.5 ± 22.3, 28.1 ± 12.6, 46.8 ± 18.3 and 62.4 ± 22.1 μg m^(-3), respectively. WSIIs accounted for 36%-56% of PM_(2.5) at four sites. Secondary ions (non-sea-salt SO_4 ^(2-), NO_3 ^- and NH_4 ^+) contributed more than 74% to total WSIIs. Neutralization degree distributions indicated that 79.5% of PM_(2.5) was acidic. Total acidity and in-situ acidity both showed obvious seasonal variations, exhibited the peak values of 193.20 and 130.17 nmol m^(-3) at HR site in summer. Sea-salt contributed 2.58%-17.74% to acidity in four seasons. The normalized ammonium concentration ([NH_4 ^+]/[SO_4 ^(2-)]) and normalized nitrate concentration ([NO_3 ^-]/[SO_4 ^(2-)]) showed greater correlation coefficients after eliminating the ammonium-poor data points (greater than 0.66), especially at HR site. NH_4HSO_4 was the major form of WSIIs in PM_(2.5) and the formation of aqueous HNO_3 could enhance aerosol acidity.