Farm fires behind Capital’s worst air days: Global study

NEW DELHI: Stubble burning in Punjab and Haryana may contribute up to 78% of Delhi’s pollution in the months of October and November, a new study by researchers from Harvard University and NASA claims.

The two months, which also coincide with the onset of winter in the northern plains, see the most burning of the agricultural residue.

“Depending on the year and emission inventory, our method attributes 7%-78% of the maximum observed PM2.5 enhancements in Delhi to fires,” said the study, which was published in the journal Environmental Research Letters.

Since the 1980s, many farmers in northwest India have switched to mechanised combine harvesting to boost efficiency.

This harvesting technique leaves abundant crop residue in the fields that farmers typically burn to prepare their fields for subsequent planting.

The researchers from Harvard University, NASA, Columbia University and other research institutes in the US gathered data for October and November, for the years 2012 to 2016, and fed them into the particle dispersion model — an algorithm that accounts for geography, wind patterns, and physics — to predict how far and in what direction smoke particles travel.

The study showed that the maximum effect of stubble burning on the levels of PM2.5 in Delhi was felt in the years 2012 and 2016. The effect was less in 2013, 2014, and 2015.

“Smoke from these fires consists of black carbon and organic particulate matter. The harvest season coincides with post-monsoon conditions that favour stagnation and weak surface northwesterly winds in the Indo-Gangetic plains. These conditions allow smoke to slowly permeate throughout the region, including Delhi,” the study says.

Previous studies have attempted to analyse the various sources of pollution in Delhi, but none have been conclusive.

A study, Socioeconomic and Environmental Implications of Agricultural Residue Burning published by Springer in 2015 , says that each tonne of crop residue on burning releases around 3kg of particulate matter, 60 kg of CO, 1,460 kg of CO2, 199kg of ash and 2kg of SO2. “Stubble burning also takes a heavy toll on the soil, resulting in loss of nutrients and micro-organisms that help to enrich the soil,” said Polash Mukerjee, senior research associate (air pollution), Centre for Science and Environment.

Though the National Green Tribunal banned crop burning in 2015, an estimated 35 million tonnes of stubble are still set on fire in Punjab and Haryana alone to make room for the winter crop.

The primary reasons behind farmers setting crop residue afire are cost concerns and the short time gap between summer and winter crops, besides lack of incentives and equipment to manually cut down the stubble.

To put an end to the menace a High Level Task Force (HLTF) led by the principal secretary to the Prime Minister has been constituted for management of air pollution in Delhi NCR in 2017.