Hydrochloric acid gas, possibly emitted from the burning of plastic-containing wastes and from industry, boosts the growth of aerosol particles1, helping to form intense haze and fog over Delhi during winter.
Measurements, combined with a modelling study by an international research team reveal that such haze and fog reduce visibility over the capital city considerably.
Identifying and controlling hydrochloric acid emissions could improve visibility and human health in India, the researchers say.
Aerosol particles, also known as particulate matter, have been known to help form more haze and fog in Delhi than in other major Asian cities. To find out why, scientists, including researchers from the Indian Institute of Technology Madras in Chennai measured the chemical composition of non-refractory particulate matter smaller than one micrometre from Delhi and Chennai during winter.
The researchers, led by Sachin S. Gunthe, found that the highest chloride fraction averaged over the entire study period occurs during the early morning hours. A plausible explanation, the researchers say, is that morning hours have the day's lowest temperatures and the highest relative humidity. These factors favour the partitioning of gas-phase hydrochloric acid into particulate chloride.
Such chlorides can then take up more water, enhancing haze and fog formation that contributes to a 50 per cent reduction in visibility over Delhi during winter.
In contrast, particulate chloride levels increase in Chennai only during Bhogi, a post-harvest festival; during Diwali in Ahmedabad; and during biomass-burning events in Kanpur.
1. Gunthe, S. S. et al. Enhanced aerosol particle growth sustained by high continental chlorine emission in India. Nat. Geosci. 14, 77-84 (2021)