Dangerously high levels of air pollution in New Delhi during the fall and winter months are largely the result of post-harvest burning of crop residue, according to a new study in the journal Nature Sustainability. Pollution levels from crop burning are so high they rival fossil fuel emissions during peak summer months.

New Delhi has long struggled with the worst air quality of any major city in the world, according to World Health Organization data. Last month, the megacity experienced air pollution levels 12 times United States government-recommended levels. Air pollution in India kills an estimated 1.5 million people every year, and a recent study in the journal GeoHealth found that nearly half of these deaths occur in the Indo Gangetic Plain, the northernmost part of the country that includes New Delhi.

Crop residue, such as roots and stems, is often burned to help prepare the field for seeding the next season. Burning this agricultural waste, however, also releases black carbon, a type of fine particulate matter formed from incomplete combustion. These particles are then funneled by air currents from rural farms to New Delhi, traveling from as far as 125 miles.

To study fluctuations in black carbon, researchers collected air samples in New Delhi throughout 2011. Using isotope dating, the researchers could compare the amount of black carbon that came from different sources, such as from fossil fuels or from biomass like crop residue. From this data, the researchers found that there was a strong seasonal variation: rainy summers had a greater amount of black carbon from fossil fuels, mostly from vehicle traffic, and dry autumns and winters saw black carbon levels from biomass in equally high amounts.

“Our findings contradict the widespread notion that the emission flux between cities and the countryside is mainly one-way,” August Andersson, a climate scientist at Stockholm University and co-author of the new study, said in a statement. “No other study has conclusively reported such high amounts of black carbon from biomass burning [found] in the middle of a megacity, where the main source is expected to be traffic. The wintertime regional influx 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.”

To learn more, read “Unraveling the Myriad Causes of North India’s Pollution Pall.”

—Emma Johnson