Linking Anthropogenic Chlorine Emissions to Regional Air Quality in India

India experiences severe air pollution driven by human activities. Role of anthropogenic chlorine is significant yet underexplored, with its mechanisms poorly understood and impacts largely unquantified, despite its importance in atmospheric oxidation and secondary pollutant formation. Using GEOS-Chem chemical transport model, we quantify the impact of human-derived chlorine emissions on particulate chloride (\ce{pCl^-}), particulate matter (\((\text{PM}_{2.5})\)), \ce{ClNO2}, and \ce{O3} in the boundary layer over India. Comprehensive model simulations reveal major chlorine hotspots affecting nearly \((\sim)\)700 million people across the Indo-Gangetic Plain (IGP). \((\text{PM}_{2.5})\) concentration increases due to \ce{pCl^-} formation (principally \ce{NH4Cl}). Annual mean \ce{pCl^-} and \ce{ClNO2} increase by 4-fold and 3-fold, respectively. Regionally and seasonally, enhancements range from 0.04-3.6 \si{\micro\gram\per\cubic\meter} for \ce{pCl^-}, 7-273 ppt for \ce{ClNO2}, and –0.47-0.44 ppb for \ce{O3} with strongest effects in autumn and winter. Compared to other polluted hotspots in the world, for example China, \ce{O3} showed a lower sensitivity to chlorine emissions over India. Anthropogenic chlorine significantly influences India’s air quality, underscoring the need to include chlorine -emission inventories and -chemistry in models.