Characteristics and atmospheric processes of inorganic ions in PM2.5 and PM10 over heavily polluted industrial city in the Indo Gangetic Plain

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 in the Indo-Gangetic Plain. 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) for WSII measurements and Teflon filter substrates for mass measurements. Further, on-site meteorological parameters (including temperature and relative humidity) were recorded at both locations. The annual average PM2.5 and PM10 concentrations observed were ~150 µgm-3 and ~260 µgm-3 respectively. WSIIs constituted ~35% of PM2.5 mass concentration and ~23% of the PM10 mass concentration across both sites. The major ionic species were SO42-, NO3-, Cl- and NH4+ which contributed to ~90% to PM2.5 ionic mass and ~82% to PM10 ionic mass. Mann-Whitney U test revealed that both sites were similar with respect to WSII mass except for Ca2+ and Mg2+ indicating city's airshed average concentration with respect to the major ions. Pronounced seasonal variability was seen for the major ions with post-monsoon and winter concentrations twice-thrice to that during other seasons. Highest WSII concentration was experienced when relative humidity varied between 40-60% and temperature varied between 10-20°C. Sulphur Oxidation Ratio and Nitrogen Oxidation Ratio indicated high photochemical oxidation with greater extent of SO42- formation compared to NO3-. The atmosphere was found to be ammonia rich, however, annual average Ammonium Neutralization Ratio was 0.76±0.14 for PM2.5 and 0.62±0.14 for PM10 indicating that majority of the ammonia was present in the gaseous phase (NH3). Finally, aerosol acidity and liquid water content were estimated using ISORROPIA-II. It was found that PM2.5 pH varied from 2.9-6.2 (avg. 4.4±0.8) and the PM2.5 mass doubled at RH ~90% because of high particle bound water.