Enhanced Aerosol Particle Growth Sustained by High Continental Chlorine Emission in India

Many cities in India experience severe air quality deterioration during the winter, affecting hundreds of millions of people. Particulate matter (PM), a key pollutant, adversely affects local air quality, regional climate, and human health. In particular, reduced visibility associcated with high levels of PM has severely damaged the economy and endangered human lives. Despite these consequences of increasing PM, the underlying chemical mechanisms and physical processes responsible for initiating haze and subsequent fog formation in India remain poorly understood. Here we report persistently high PM chloride in Delhi, and episodically high PM chloride in Chennai revealing novel insights for hygroscopic growth of particles with far reaching implications for visibility and atmospheric chemistry. Results from thermodynamic modeling suggest high local concentrations of gas-phase hydrochloric acid (HCl), which, in the presence of excess ammonia, partition into aerosol water. The highly hygroscopic and soluble chloride in aqueous phase then significantly enhances the water uptake through co-condensation, sustaining particle growth and ultimately haze and fog formation. The proposed mechanism explains the high light-extinction enhancement factor derived from visibility measurements in Delhi. Our work implies that identification and control of gaseous HCl emission sources may be critical to improve human health and visibility in India.