A new study published in the journal Nature has identified chlorine produced by human activities as a significant yet underexplored factor contributing to toxic smog in Indian cities and towns. The study, titled 'Linking anthropogenic chlorine emissions to regional air quality in India', was conducted by researchers from IIT Madras, Harvard, Oxford, Georgia Tech, and the Max Planck Institute.
Lead researchers include Ankit Patel, Malasani Chakradhar Reddy, and Govindan Pandithurai from IIT Madras, along with Sachin S Gunthe from the Centre for Atmospheric and Climate Sciences and the Environmental Engineering Division. For the first time, they have mapped India's anthropogenic chlorine footprint across six locations: Ahmedabad, Delhi, Kanpur, Chennai, Munnar, and Mahabaleshwar.
Key Findings on Chlorine Emissions
The study reveals that India is the world's second-largest emitter of human-derived chlorine, behind only China. The dominant sources of chlorine emissions include burning of wood and dung for domestic cooking, which accounts for 55.9% of emissions. Open-waste burning contributes 19.2%, and industrial coal combustion adds 13%. Approximately 700 million people live in zones where chlorine measurably worsens PM2.5 particulate levels.
Ahmedabad: A Chemically Distinct Problem
Ahmedabad presents a chemically distinct issue due to its low humidity, often at or below 50% relative humidity. This prevents chlorine from condensing into fine solid particles. Annually, Ahmedabad's levels of particulate chloride hover near 0.5 μg/m³. The dry air keeps chlorine reactive and volatile, allowing it to combine overnight with nitrogen compounds to form nitryl chloride (ClNO2), 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, according to the study.
Delhi and Kanpur: High Winter Concentrations
Delhi and Kanpur sit in an ammonia-rich atmosphere, with 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.
Impact on Air Quality and Health
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, sunlight breaks nitryl chloride apart, releasing highly reactive chlorine radicals that accelerate the oxidation of volatile organic compounds (VOCs) from traffic, industry, and cooking, while recycling nitrogen dioxide (NO2) back into the atmosphere. This results in a sharp spike in ground-level ozone (O3) between 9am and 11am, precisely when millions are commuting. A shallow boundary layer in the morning traps pollutants close to the ground, maximizing human exposure to toxins.
The study underscores the need for targeted policies to reduce chlorine emissions from domestic cooking, waste burning, and industrial sources to improve air quality across India.
