Lockdown helps city breathe easy

Researchers have found that the average concentration of most pollutants in the city’s air fell during the first three phases of the lockdown (from March 25 to May

Researchers have found that the average concentration of most pollutants in the city’s air fell during the first three phases of the lockdown (from March 25 to May 18). An analysis of pollutant data available from the monitoring stations of the Central Pollution Control Board (CPCB), done by Urbanemissions.info, found that the levels of five out of six pollutants were lower in the last two months, as compared to pre-lockdown levels.

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Urbanemissions.info, an air pollution research group, studied the concentration of particulate matter (PM2.5, PM10), nitrogen dioxide (NO2), sulphur dioxide (SO2), ozone (O3), and carbon monoxide (CO) over the 54 days of lockdown, divided into three phases. The first phase of the lockdown was from March 25 to April 14; the second, from April 15 to May 3; and the third, from May 4 to May 17. Levels were compared with a 30-day average (February 20 to March 22) during the pre-lockdown period. While levels for most pollutants dipped, SO2 saw a spike during lockdown.

Average levels of PM2.5 — breathable particulate matter of 2.5 micron size or smaller — fell from 45 microgrammes per cubic metre (µg/m3) during pre-lockdown to 27.8 µg/m3 during the first phase and then to 23.3 µg/m3 and 22.6 µg/m3 in second and third phases respectively. A similar trend was observed for CO and O3. The most substantial decline was witnessed for NO2 — produced by burning of coal, oil, and emissions from vehicle — which fell from pre-lockdown levels of 37.5 µg/m3 to 9.8 µg/m3 during the first phase; 7.3 µg/m3 in the second; and rose to 9.8 µg/m3 in the third.

However, the levels of SO2 — a colourless gas with pungent odour and taste that causes respiratory diseases such as asthma, bronchitis and emphysema — spiked during the lockdown. While it fell from 15.4 µg/m3 pre-lockdown to 12.5 µg/m3 during the first phase, a surge was seen during the second (24.6 µg/m3) and third phases (36.2 µg/m3).

“Heavy industries such as power and refineries were operating at reduced capacity. Sulphur is sourced from diesel, coal or heavy fuel oil. Since NO2 did not increase, transport as a source can be excluded, which leaves us with coal-based consumption (like power plants and cooking) or heavy fuel oil (like idling ships) as possible sources,” said Guttikunda. He said more investigation was needed to address the source of higher SO2 in Mumbai’s air during April and May.

A marginal spike in PM10 (larger, coarser particles) was witnessed during the third phase of the lockdown. “Small fluctuations are expected because of weather, but do not reflect a discernible change that can be attributed to the phases. At the same time, the tropical cyclone towards the east led to a reduction in moisture over parts of the west coast,” said Guttikunda.

Meanwhile, Urbanemissions.info also studied five satellite maps (Sentinel 5P satellite data) for 14-day averages between March 9 and May 18 for NO2 concentration. “Between May 4 and May 18, an increase in emission is being witnessed as lockdown restrictions get loosened,” said Guttikunda. “After May 17, in phase four, a change is being witnessed due to reduced restrictions. We are already witnessing an increase in industrial and vehicular activity in most cities, deteriorating air quality. Cities need to opened up in phased manner to ensure the access to clean air is not lost.”