Characteristics and atmospheric processes of water-soluble ions in PM2.5 and PM10 over an industrial city in the National Capital Region (NCR) of India

This study investigates water-soluble inorganic ions (WSIIs), their potential formation mechanism, particle bound water content and acidity (pH) in ambient aerosols in Ghaziabad, an industrial city located downwind of Delhi, the Indian National Capital. 24-hour integrated PM2.5 and PM10 samples were collected using collocated samplers at two receptor locations every third day throughout the year. The samples were collected on Nylon filter substrates (preceded by a denuder upstream for capturing acidic gases) and Teflon filter substrates for WSII and mass measurements, respectively. Further, on-site meteorological parameters were recorded at both locations. The annual average PM concentrations observed were: PM2.5: 144 ± 81 μgm−3 (Site A), 163 ± 95 μgm−3 (Site B); and PM10: 245 ± 116 μgm−3 (Site A), 279 ± 152 μgm−3 (Site B). On average, SO42−, NO3−, Cl− and NH4+ contributed to ∼90% to PM2.5WSII mass and ∼82% to PM10WSII mass, which in turn made up ∼35% of PM2.5 mass and ∼23% of the PM10 mass across both sites. SO42− concentration was found to be highest followed by NO3−, NH4+ and Cl− for both PM fractions. Mann-Whitney U test revealed that both sites were similar with respect to WSII mass indicating city's airshed average concentration with respect to the major ions. The highest WSII contribution to the total PM was found in winter season (37–40% for PM2.5 and 27–31% for PM10) followed by monsoon (36–38% for PM2.5 and 18–21% for PM10), post-monsoon (28–34% for PM2.5 and 21–23% for PM10) and pre-monsoon (21–27% for PM2.5 and 15% for PM10). High contribution during winter was because of higher emissions of precursor gases in combination with moderate relative humidity (36–59%) and minimal precipitation (average ∼0.2 mm). Sulphur Oxidation Ratio and Nitrogen Oxidation Ratio indicated high photochemical oxidation with a greater extent of SO42− formation compared to NO3−. Although high concentrations of NH3 gas were observed, there was insufficient NH4+ ions for neutralizing counter ions with Ammonium Neutralization Ratio values of 0.76±0.14 for PM2.5 and 0.62±0.14 for PM10. Finally, aerosol liquid water content (ALWC) and acidity were estimated using a thermodynamic equilibrium model, ISORROPIA-II. It was found that PM2.5 pH varied from 2.9 to 6.2 (avg. 4.4±0.8) and the PM2.5 mass doubled at RH ∼90% because of the high particle bound water. Consequently, a reduction in SO2 would result in instantaneous cuts of SO42−, PM2.5, and ALWC.