Aerosol acidity and its neutralization in the eastern Indo-Gangetic Plain: implications for water-soluble organic carbon

Aerosol acidity plays an important role in influencing precipitation pH, which has impacts on the environment as well as human health. It also has significance in shaping aerosol chemistry, including the catalytic formation of water-soluble organic carbon (WSOC), which in turn affects the hygroscopicity of aerosols. Past studies on aerosol acidity in the Indian subcontinent, mostly conducted in biomass burning (BB) source regions in the northwestern and central Indo-Gangetic Plain (IGP) and in western India, have identified Ca²⁺ and Mg²⁺ sourced from desert dust to be the predominant neutralizing agents. However, the prevalence of desert dust decreases progressively along the IGP corridor and is potentially rendered insignificant in the eastern IGP (eIGP). As such, there exists a critical weakness in our understanding of the processes governing aerosol acidity and its neutralization in the eIGP. To address this, the present study reports the seasonal variability of ionic species, WSOC and associated aerosol acidity in ambient PM_2.5 from a rural receptor site in the eIGP. To this end, a total of 88 PM_2.5 samples collected during the summer, post-monsoon and winter seasons of 2018 were analyzed for SO₄^2-, NO₃^-, Cl^-, Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺, F^-, PO₄^3- and WSOC, followed by estimation of strong acidity. Across all seasons, the aerosol phase was dominated by SO₄^2-, NH₄⁺ and NO₃^-, with values increasing by factors of 1.8-1.9, 1.4-2.9 and 1.8-11, respectively, for the regional BB-dominated post-monsoon and winter seasons as compared to summer. Significant positive Cl^- depletion in summer pointed towards the influx of marine air while negative depletion in post-monsoon and winter suggested a BB source, which was further supported by concentration-weighted trajectory analysis. The averaged pH of the aerosol extract decreased progressively from summer (5.5±0.4) to winter (4.5±0.2). NH₄⁺ was observed to be the major acid-neutralizing agent across all seasons, with dust-derived Ca²⁺ and Mg²⁺ playing only minor roles. In general, WSOC formation is known to be catalyzed by the presence of excess acidity; however, during winter, it appeared that the regional transport of organic acids in the BB plume contributed to aerosol acidity at this receptor site (r=0.92; p<0.01 for WSOC and H⁺). BB-derived K⁺ appeared to perform a dual function of neutralizing acidity as well as producing it via reactions with WSOC during atmospheric transport. The wintertime acidity was also strongly governed by aerosol NO₃^- sourced from BB emissions and possibly accentuated via nighttime atmospheric chemistry at lower ambient temperatures, resulting in the formation of haze. These observations of the NO₃^- and WSOC-driven wintertime acidity, the dual function of K⁺ and the dominant role of NH₄⁺ in neutralization points to complex atmospheric processing of the IGP outflow during its transport to the eastern end of the corridor, which warrants further investigation.