Most of Delhi-NCR's pollution in Oct-Nov locally created, shows study

Delhi-NCR's pollution during October-November is largely of local origin, with stubble burning in Punjab and Haryana contributing 14 per cent to the overall PM2.5 levels in 2022, a study has suggested.

Researchers, led by those at Japan's Research Institute for Humanity and Nature under the 'Aakash Project', said that changes in the national capital's air quality could be related more with the stepping up or down of the 'GRAP' anti-pollution measures.

The study, published in the journal 'npj Climate and Atmospheric Science', analysed fine particulate matter (PM2.5) data, recorded during the September-November months of 2022 and 2023. For the study, 30 sensors were installed across Punjab, Haryana and Delhi-NCR.

Stubble burning, a common practice for clearing land following rice harvest, is often blamed for the sharp and sustained rise in PM2.5 levels, seen in the national capital region during the October-November months.

However, the analysis showed that emissions over Punjab from burning stubbles "contributed only a meagre (nearly) 14 per cent to the overall PM2.5 over Delhi-NCR during October-November 2022".

Further, PM2.5 levels remained stable in Delhi, even as stubble fires in Punjab and Haryana (as recorded by satellites) declined by at least 50 per cent during 2015-2023, the authors said.

"This indicates that there exists only a very weak coupling between PM2.5 mass over Delhi-NCR and the (crop residue burning) over Punjab, highlighting the effectiveness of the Graded Response Action Plan (GRAP) in controlling air pollution in the region," the authors wrote.

GRAP refers to the set of anti-pollution measures, implemented in a staged manner in response to air quality levels. GRAP-4 is the most stringent, imposed when air quality index (AQI) crosses 450 and reaches 'severe plus' category.

The researchers observed that PM2.5 levels varied in line with the stepping up or down of the GRAP stages.

"The decrease of PM2.5 levels in Delhi-NCR was mainly attributed to GRAP IV when the major PM2.5 emissions from road traffic and construction activities, among other sources, were reduced. However, a rise in PM2.5 load was recorded after the revocation of GRAP IV for 2022 and 2023," they wrote.

"With the measurements at a network of about 30 sites covering Punjab, Haryana and Delhi NCR, we are able to separate the contributions of paddy straw burning to Delhi's PM2.5 variations on the basis of characteristic PM2.5 events and at week-monthly averages," said author Prabir Patra, the leader of Aakash Project and principal scientist at Japan Agency for Marine-Earth Science and Technology.