Ahmedabad: While the public debate circles endlessly around carbon dioxide and nitrogen oxides, a new study has found a “significant yet underexplored factor” in Indian cities and towns, toxic smog — chlorine produced by human activity.The study was published in the journal ‘Nature’ and was titled “Linking anthropogenic chlorine emissions to regional air quality in India”.Researchers included Ankit Patel, Malasani Chakradhar Reddy, and Govindan Pandithurai from IIT Madras. Others were Sachin S Gunthe from the Centre for Atmospheric and Climate Sciences and the Environmental Engineering Division, and scientists from Harvard, Oxford, Georgia Tech, and the Max Planck Institute.The researchers have for the first time mapped India’s anthropogenic chlorine footprint across six locations: Ahmedabad, Delhi, Kanpur, Chennai, Munnar, and Mahabaleshwar. The findings are concerning.“India is the world’s second-largest emitter of human-derived chlorine, behind only China,” the study states. The dominant sources are the burning of wood and dung for domestic cooking, which account for 55.9% of emissions.Open-waste burning accounts for 19.2%, and industrial coal combustion accounts for 13%. Nearly 700 million people live in zones where chlorine measurably worsens PM2.5 particulate levels.Ahmedabad presents a chemically distinct problem. Its “low humidity — often at or below 50% relative humidity” — prevents chlorine from condensing into fine solid particles.Measured annually, Ahmedabad’s levels of particulate chloride — “the dust-like solid form of chlorine — hover near 0.5 μg/m³ (micrograms per cubic metre).”The dry air keeps chlorine reactive and volatile, allowing it to combine overnight with nitrogen compounds to form nitryl chloride — a “nocturnal reservoir” that builds silently to concentrations exceeding 700 parts per trillion.The city accumulates gaseous hydrogen chloride (HCl), making it a “prominent hotspot”, the study says.Delhi and Kanpur sit in an “ammonia-rich atmosphere” — ammonia derived heavily from agricultural fertilizers and livestock.Winter chlorine concentrations in these cities reach a “staggering” 5.1 μg/m³ — ten times Ahmedabad’s levels — and contribute up to 40-50% of the aerosol liquid water content that thickens Delhi’s winter haze.Nationally, chlorine chemistry raises the annual mean PM2.5 by 1.18 μg/m³, “aggravating exposure to fine particulates associated with respiratory and cardiovascular diseases and premature mortality.”At sunrise, this reservoir detonates. Sunlight breaks nitryl chloride (ClNO2) apart, releasing highly reactive chlorine radicals that accelerate the oxidation of volatile organic compounds (VOCs) — carbon-containing gases from traffic, industry, and cooking — while simultaneously recycling nitrogen dioxide (NO2) back into the atmosphere.The result is a sharp spike in ground-level ozone (O3) between 9am and 11am, precisely when millions are commuting. A “shallow boundary layer” — the low ceiling of air in the morning that traps pollutants close to the ground — maximizes human exposure to toxins.
