Scientists May Have Finally Figured Out Why So Much Smog Forms in New Delhi

The night skies are rarely clear in New Delhi, the capital of India. For years, residents have looked up to find smog-infested air on winter nights—contrary to the usual way smog forms, in which air pollutants react with sunlight. Now, science may finally understand why.

A new paper published in the journal Nature Geoscience has revealed a connection between the contaminated air and residents’ practice of burning wood and crops. Researchers at the Laboratory for Atmospheric Chemistry at Paul Scherrer Institute (PSI) as well as the Indian Institute of Technology Kanpur suggest fumes from wood used for heating and cooking, and airborne particulates from burning the remains of the previous season’s crops, are the main contributors to the pollution, which can exceed 500 micrograms per cubic meter of air. (In Beijing, which also struggles with air quality, 70 micrograms is typical.)

Normally, these gas molecules wouldn’t be visible to the naked eye. But the temperature in New Delhi can drop quickly at night, causing the gasses to condense and clump together to form visible particles.

“The chemical processes that take place in the air at night are unique to the Indian capital and have not been observed anywhere else in the world,” Imad El-Haddad, a PSI atmospheric chemist, said in a statement.

Air quality has been a serious source of concern in New Delhi, which has seen its 20 million residents suffer from health issues. During particularly bad smog, the capital will order school and business closings; officials will dispatch water and “anti-smog” machines to try and tackle the problem.

[h/t Nature Geoscience]