Crop residue burning pollutes Delhi’s urban air, studies say

The spread of air pollution is not just one way, blowing from urban areas to rural ones. Rural activities can pollute urban air—with high economic costs, new studies find.

Studying the source of air pollution in New Delhi, a team of researchers from Stockholm University and the Indian Institute for Tropical Meteorology found that emissions from burning crop residues or other biomass in rural areas neighboring the city contribute nearly half of the airborne black-carbon particulate matter—soot—in New Delhi during the fall and winter (Nat. Sustainability 2019, DOI: 10.1038/s41893-019-0219-0). The rest of the black-carbon pollution comes from fossil-fuel burning, such as from cars and trucks. The biomass burning contribution drops to 20% in the summer.

“No other study has reported high amounts of black carbon from biomass burning in the middle of a megacity where the only source should be traffic,” says August Andersson, one of the researchers from Stockholm University. “The wintertime regional flux of black carbon into New Delhi suggests that to efficiently combat severe air pollution, it is necessary to not only mitigate the urban emissions, but also regional-scale biomass emissions, including agricultural crop residue burning.”

The findings mesh with a separate study led by Sagnik Dey of the Indian Institute of Technology Delhi. Dey and colleagues found that fall pollution-buildup in Delhi happens in two phases, with the first phase strongly linked to burning of crop residues, also called stubble (Atmos. Environ. 2019, DOI: 10.1016/j.atmosenv.2019.02.029).

A third, recent study estimates the cost of burning crop residues to Indian society in terms of respiratory infections. Stubble burning increases the risk of acute respiratory infections three-fold in India’s northern regions, according to research led by Samuel P. Scott of the International Food Policy Research Institute (Int. J. Epidemiol. 2019, DOI: 10.1093/ije/dyz022). Children younger than 5 years of age are particularly susceptible to such infections. Scott and colleagues estimate the related economic cost to be more than $30 billion annually.

Farmers typically burn stubble after rice harvesting to get fields ready for wheat cultivation. India’s National Green Tribunal banned stubble burning in 2015, with little effect in northern states. Last month, the Supreme Court asked the Punjab and Haryana state governments to consider granting some kind of subsidy to farmers to address the problem. “With better incentives, affordable technology, and proper implementation plan at the ground, stubble burning can be reduced,” Dey says.