Crop burning intensifies the winter haze in north India. Credit: Subhra Priyadarshini

Four decades of satellite data and ground-based observations in northern India show that aerosol concentration in the lowest layer of atmosphere increased during the crop burning season in late-autumn and with winter haze, intensifying smog formation1.

The smog blocked sunlight and increased the number of days with poor visibility, affecting millions of people in the Indo-Gangetic Plain (IGP).Despite reports on worsening air quality in India, the link between smog build-up and aerosol pollution in the lower atmosphere is not well understood. To explore this, an international group of scientists, led by Ritesh Gautam, analysed satellite data and meteorological observations between 1980 and 2019.

They found that columns of tiny suspended particles that reflect or absorb sunlight (aerosol optical thickness) increased in the lower atmosphere from November to January in IGP.

The aerosol particles warmed the lower atmosphere while cooling the planet’s surface. Instead of decreasing, temperatures increased with height. This condition, known as ‘inversion’, made the earth’s surface cooler than the air above it.

The team, which included researchers at the Environmental Defense Fund in the United States of America and the University of Petroleum and Energy Studies in Dehradun, India, found that aerosol-induced warming rose by 70-80% between 2002 and 2019 and surface cooling increased by 10%.

This led to a stable lower atmosphere that prevents dispersal of aerosol particles, leading to greater accumulation of pollutants. Enhanced relative humidity then converted these particles to secondary aerosols, contributing to severe smog.