Stubble-burning in Haryana, Punjab worsening Delhi air quality

A new study has conclusively proved that burning of crop residues in winter months in the neighbouring States of Haryana and Punjab contributes prominently to the toxic air in the Indian capital and surrounding areas and has in fact even quantified for the first time its share in Delhi air samples.

The study, by a team of Swedish and Indian scientists appeared in the journal Nature Sustainability early this week, showed up to 60 per cent of sooty carbon particles, the main constituent in the deadly particulate matter of sizes 2.5 micron (PM2.5), could be from biomass burning.

Studies published last year suggested that severely poor air quality in Delhi can on an average knock off about 10 years from lifespan of the city dwellers.

Analytical method

“High levels of particulate air pollution in Delhi raise serious concerns on both health and climate. Every winter and fall seasons, the city air pollution levels reach 10 times the safe limits recommended by the World Health Organisation,” said Srinivas Bikkina, first author of the study and a postdoc at the Bolin Centre for Climate Research at the Stockholm University. While many other studies in the past too implicated stubble burning that takes place in October-November months in the States like Haryana and Punjab to worsening of air quality, this was the first one to quantify black carbon particles from different sources. The team, using an analytical method perfected by Orjan Gustafsson, Stockholm University professor and main author of the study, estimated the precise contribution of crop residue burning and fossil fuels in the ambient air. The scientists accurately estimated the ratio of radio carbon isotope — C14 — and stable carbon particles from each of the sources.

While the contribution of black carbon particles from vehicles and coal combustion over Delhi remained more or less constant through the year, those from biomass burning spiked in winter and fall seasons.

In terms of percentage, the contribution of black carbon particles from burning was the highest — 42 plus or minus 17 per cent — in winter months and 36 per cent plus or minus 10 per cent in fall months, said Bikkina, who recently moved to Chubu University in Japan as a postdoc, after spending four years in Gustafsson’s lab.

According to Bikkina, this has been one of the best studies on understanding the composition of the foul air in Delhi. The measurement of radiocarbon/stable carbon isotopes of black carbon in ambient aerosols requires highly skilled training and tedious clean sample preparation procedures.

“For the study, we prepared 21 CO2 isolates from BC combustion in the PM2.5 sampled over Delhi, each costing approximately $350-$450. The analysis of microscale sample preparation of CO2 isolates from black carbon for the radiocarbon measurement, cannot be done by every lab,” said Bikkina.