Sensitivity of Phase Partitioning of Inorganic Aerosols to pH and ALWC In Northwestern Indo-Gangetic Plain

Keywords:   Inorganic ions, ISOROPIIA-II, Sensitivity regime, ALWC, pH

Abstract

The Indo-Gangetic Plain (IGP) is one of the world’s most critical aerosol pollution hotspots, experiencing severe air quality degradation during the transition from summer to cooler months. Enhanced aerosol loading arises from a complex interplay of meteorological conditions, anthropogenic activities, and the region’s unique topography. Fine-mode aerosols, particularly those containing inorganic nitrate, chloride, and ammonium, significantly impact atmospheric chemistry and air quality over this region. The interaction between aerosol, liquid water content (ALWC), and pH is a key determinant of gas-particle partitioning for these species, influencing their atmospheric residence times and depositional velocities. This study presents real-time measurements of inorganic ions (NH₄⁺, SO₄^2--, Cl^-, Na⁺, Mg²⁺, Ca²⁺, K⁺, NO₃^-) and major gases (SO₂, HCl, HONO, HNO₃, NH₃) in ambient air by deploying the MARGA-R (Monitor for AeRosols and Gases in Ambient Air, Metrohm) instrument during the post-monsoon to winter transition in the Northwestern IGP region, with a focus on the role of temperature, ALWC, and pH in gas-particle partitioning. Using the thermodynamic model ISORROPIA-II, aerosol pH and ALWC were determined and applied in a mathematical framework to elucidate the interactions between inorganic aerosols and major gaseous species and to identify the chemical domains (sensitivity regimes) where aerosol particulate matter is sensitive to NH₃ and HNO₃. Results illustrate that pH and ALWC conditions during the study period predominantly favoured the partitioning of nitric acid (HNO₃) into particulate nitrate (NO₃⁻). Conversely, ammonium (NH₄⁺) remained mainly in its gaseous form as ammonia (NH₃) over the study site. This distinct partitioning behaviour implies that NH₃ tend to stay localized near their source regions, whereas NO₃⁻ has a greater potential for long-range transport, depending on environmental parameters controlling its mobility and deposition. These findings underscore the critical role of region-specific meteorological processes and shifting source profiles from post-monsoon to winter in influencing secondary inorganic aerosol dynamics. This knowledge offers valuable insights for designing effective pollution mitigation strategies tailored to the unique characteristics of the IGP, emphasizing the importance of considering not just the mass concentration of species but also their sensitivities.