Characterization of fine particulate matter water-soluble inorganic ions and estimation of aerosol acidity at three COALESCE network sites - Mysuru, Bhopal, and Mesra - in India.

The study was carried out to understand the chemical, spatiotemporal characteristics of water-soluble inorganic ions (WSIIs), their association with PM mass, and aerosol acidity in three COALESCE (carbonaceous aerosol emissions, source apportionment, and climate impacts) network sites of India (Mesra - Eastern India, Bhopal - Central India and Mysuru - Southern India). Alternate-day 24-h integrated bulk PM samples were collected during 2019 along with on-site meteorological parameters. Annual average PM concentrations were 67 ± 46 µg m, 54 ± 47 µg m, and 30 ± 24 µg m at Mesra, Bhopal, and Mysuru, respectively. PM concentrations exceeded the annual mean (40 µg m) recommended by the National Ambient Air Quality Standards (NAAQS) at Mesra and Bhopal. WSIIs existed in PM mass at Mesra (50.5%), Bhopal (39.6%), and Mysuru (29.2%). SO, NO, and NH (SNA) were major secondary inorganic ions in total WSIIs, with an annual average of 88.4% in Mesra and 82.0% in Bhopal 78.4% in Mysuru. Low NO/SO ratios annually at Mesra (0.41), Bhopal (0.44), and Mysuru (0.24) indicated that stationary sources dominated vehicular emissions (1.0). Aerosol acidity varied from region to region and season to season depending on the presence of NH, the dominant counter-ion to neutralize anions. Aerosols were near-neutral or alkaline at all three sites, except during the pre-monsoon season in Mysuru. An assessment of neutralization pathways for major anions [SO  + NO] suggests that they mainly existed as sulfate and nitrate salts such as ammonium sulfate ((NH)SO) and ammonium bisulfate (NHHSO) in conjunction with ammonium nitrate (NHNO).