Enhanced aerosol particle growth sustained by high continental chlorine emission in India

Abstract

Many cities in India experience severe deterioration of air quality in winter. Particulate matter is a key atmospheric pollutant that impacts millions of people. In particular, high levels of particulate matter reduce visibility, which has severely damaged the economy and endangered human lives. But the underlying chemical mechanisms and physical processes responsible for initiating haze and fog formation remain poorly understood. Here we present the measurement results of chemical composition of particulate matter in Delhi and Chennai. We find persistently high chloride in Delhi, and episodically high chloride in Chennai. These measurements, combined with thermodynamic modeling, suggest that in the presence of excess ammonia in Delhi, high local emission of hydrochloric acid partition into aerosol water. The highly water-absorbing and soluble chloride in the aqueous phase substantially enhances aerosol water uptake through co-condensation, which sustains particle growth leading to haze and fog formation. We therefore suggest that high local concentration of gas-phase hydrochloric acid, possibly emitted from plastic-contained waste burning and industry causes some 50% of the reduced visibility. Our work implies that identification and regulation of gaseous hydrochloric acid emissions could be critical to improve visibility and human health in India.