Background PM2.5 levels were highest in Mumbai among 6 cities between February and April: StudyUpdated: Oct 18, 2020, 00:59 IST
A study has found that Mumbai recorded the highest background PM2.5 concentration among six Indian cities, between February and April. Background levels are the permanent concentration in any city when anthropogenic (human-induced) emissions are at their lowest.
According to the System of Air Quality Weather Forecasting and Research (SAFAR) under the Indian Institute of Tropical Management (IITM) Pune, the level of permanent concentration of PM2.5 was highest for Mumbai at 33 microgrammes per cubic metre (μg/m3) and lowest for Chennai (6 μg/m3). Following Mumbai were Ahmedabad (32 μg/m3), Pune (29 μg/m3), Delhi (22 μg/m3), and Kolkata (18 μg/m3). Data for the six cities was collected from February 20 to April 14, allowing a comparison of levels before and during the lockdown. The data has been published in a research paper by IITM.
PM2.5 is breathable particulate matter which is 2.5 microns in size or smaller; and can easily enter the lungs and cause health ailments. The national safe standard for PM2.5 is 40 µg/m³ for 24 hours.
The findings were presented at a discussion organised by the non-profit organisation Natural Resources Defense Council on Thursday. SAFAR project director and scientist Gufran Beig told HT, “Even though Mumbai may have high pollutant dispersion due to advantages such as sea breeze, its background air already has higher PM2.5 concentration than other cities, even landlocked ones such as Delhi.”
The period of IITM’s study — February 20 to April 14 — covered both pre-lockdown conditions (when human-induced emission sources were present) and those during lockdown. The data suggests those exposed to higher baseline levels of particulate pollution are at greater risk of dying from Covid-19. During the first phase of the lockdown (March 24 to April 14), 909 Covid-19 related deaths and 27,251 cases were recorded in Mumbai. Ahmedabad recorded 645 deaths and 9,724 cases in the same period. The lowest number of deaths were reported from Chennai (70) while the fewest cases were from Kolkata (1,570).
“Our study results indicate that, in general, cities with a high PM2.5 baseline level are associated with a higher number of deaths due to Covid-19 during the study period, and those cities which have a smaller magnitude of PM2.5 baseline level have relatively lesser numbers of mortality,” said Beig. “However, considering all uncertainties involved (testing capability, health infrastructure, environmental and climatological conditions etc.), more studies are needed to determine what is the cause of high baseline levels for a few cities along with connection between PM2.5 rise and Covid-19 deaths,” he added.
Beig further explained that baseline pollution represents the lowest levels of ambient air pollution or a level that would have been naturally present and remained under photochemical equilibrium. “Lockdown created an unprecedented emission scenario with near negligible magnitude, which was virtually impossible under normal scenarios and paved the way to determine the baseline levels, but this is the level we are chronically exposed to naturally and it is more dangerous because these levels cannot be curbed through human intervention,” he said.
Independent experts said the high background PM2.5 level in Mumbai clearly indicates elevated pollution levels in the air-shed of the city. “This is influenced by the pollution sources across the region, long-range movement of pollutants, and pollutants that are precursor to the formation of secondary pollutants. It is therefore important to have national or regional level policies to reduce the background pollutant concentration otherwise the city will always be fighting hard to reduce levels especially below the background level,” said Anumita Roy Chowdhury, executive director (research and advocacy), Centre for Science and Environment, Delhi.
SAFAR’s analysis also revealed the baseline values of nitrogen dioxide (NO2), which were highest for Delhi (8 parts per billion) followed by Ahmedabad (6 ppb), Mumbai (5 ppb) and lowest for Kolkata and Chennai (1.7 ppb). The high magnitude of NO2 (8 ppb) in Delhi implies the dominant role of fossil fuel emissions from transport sources in inﬂuencing the base level of NO2, the study said.
Even though Mumbai recorded the second highest wind speed (11.4 km/hr), lowest average maximum temperature (32.5 degrees Celsius) and third lowest average humidity levels (69%) among the six cities during the first phase of the lockdown, it still recorded the highest PM2.5 baseline levels. “In general, meteorology and topography of a city plays a major role in dispersion of pollutants but in this case all six cities observed calm, steady and consistent weather immediately after the lockdown when pollutants sharply dropped and remained steady until some restrictions were relaxed. So in this case, the drop in pollutants or baseline levels is more due to human-induced sources dropping significantly,” said Beig.
Sources of high PM2.5 baseline levels in Mumbai’s air
SAFAR recorded the emission inventory for PM2.5 pollutants 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-spray etc. — all inside and outside the air shed — 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%) were from windblown dust and 13.08% from industrial emissions. “Overall, the lockdown saw about 85-90% reduction in human-induced sources. The baseline may factor in some amount of household biofuel emissions,” said Beig.