Mumbai topped in reduction of pollutants among six cities

Mumbai has seen the sharpest reduction in harmful air pollutants during the lockdown among major Indian cities, said the System of Air Quality Weather Forecasting and Research (SAFAR) on Tuesday.

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Researchers compared a three-week average pollutant concentration between March 1-21 and March 23 to April 14 for six cities (Delhi, Pune, Ahmedabad, Kolkata, Chennai and Mumbai), using data from multiple monitoring stations including those of the Central Pollution Control Board, the American Consulate, and SAFAR’s own monitoring stations. The results were presented by Gufran Beig, director, SAFAR, from a yet-to-be published research paper, during a the Ministry of Earth Sciences’ webinar on Tuesday.

Between March and April, Mumbai has witnessed maximum percentage reduction for two pollutants — nitrogen dioxide (NO2) and particulate matter (PM) 2.5 (breathable particulate matter which is 2.5 microns in size or smaller) — among the six cities. However, even without major industrial and automobile emissions, Mumbai’s lowest PM2.5 concentration, defined as the baseline or background PM2.5 levels by SAFAR, was highest among six cities.

Mumbai recorded a 76% decline in NO2 (gases mainly emitted from automobile fuel burning) followed by Kolkata (67%), Pune (56%), Delhi (39%), Ahmedabad and Chennai (25%). A 54% reduction was recorded for PM2.5 for Mumbai; 40% for both Kolkata and Ahmedabad; 30% for Delhi; 25% in Pune and 11% in Chennai.

“Geographical and weather factors are both responsible for maximum pollutant decline in Mumbai against other cities,” said Beig. “Being a coastal city, Mumbai got the advantage of sea breeze resulting in faster dispersion of pollutants. Though the same can be said for Chennai, baseline line levels for the city are already low, but the rate of dispersion is not as high as Mumbai as the financial capital is surrounded by the sea from three sides.”

At landlocked cities like Delhi and Ahmedabad, and low dispersion rate for Kolkata, human-induced bio-fuel and industrial emissions combined with sporadic incidents of mild dust storms during lockdown did not allow very high pollutant reduction, said Beig.

Meanwhile, background PM2.5 concentration for the study period in Mumbai was 33 micrograms per cubic metre (µg/m³) followed by Ahmedabad at 31 µg/m³, Pune 29 µg/m³, Delhi 22 µg/m³, Kolkata 18 µg/m³, and Chennai 6 µg/m³.

“Pollution levels can never be zero. Background levels are the permanent concentration in any city when anthropogenic emissions are at its lowest. This shows that even though Mumbai may have high pollutant dispersion, its background air already has higher PM2.5 than other cities. While the lockdown has presented us with an unique opportunity to gather such data, more investigation is needed to understand what causes this,” said Beig.

The national safe standard for PM2.5 is 40 µg/m³ for 24 hours while it is 25 µg/m³ for the same duration as per the World Health Organisation (WHO).

Independent experts said research was needed to understand sources from the larger airshed in the Mumbai region during lockdown to assess high background PM2.5. “The magnitude of sources like solid fuel usage, power plants or even minimal vehicle movement in the region’s airshed, and how it could be influencing background PM2.5 needs to be assessed,” said Anumita Roy Chowdhury, executive director (research and advocacy), Centre for Science and Environment.

PM2.5 LEVELS IN MUMBAI’S AIR IN 2019

SAFAR on Tuesday presented the emission inventory for PM2.5 pollutant and different sectors contributing to the pollutant in Mumbai’s air during 2019. While the transport sector was the most dominant source at 30.49%, sources titled ‘others’ including open fires, brick kilns, textile plants, dust storms, sea salt etc. all inside and outside the airshed accounted for 20.88%. The residential sector including biofuel or solid fuel emissions for domestic cooking and other activities accounted for 15.52%. Remaining sources 14.8% was from windblown dust and 13.08% from industrial emissions.