Delhi’s PM2.5 levels fell 48% during 2020 lockdown, finds study

ByJayashree Nandi, Hindustan Times, New Delhi
Sep 04, 2021 02:26 AM IST

Among other cities in India, Chennai and Amritsar recorded even higher improvements in air quality -- with 62% and 51% reductions in PM 2.5 concentrations, respectively

Delhi recorded a 48% reduction in PM 2.5 concentrations during the nationwide hard lockdown from March 25 to May 31 last year compared to the same time span in the years between 2015 and 2019, a study has found. This improvement, however, was not visible before or after the lockdown period, underscoring the need for long-term measures to battle air pollution across cities.

Scientists also found that levels of nitrogen dioxide reduced by 55% during the lockdown period in Delhi (Arvind Yadav/HT PHOTO)
Scientists also found that levels of nitrogen dioxide reduced by 55% during the lockdown period in Delhi (Arvind Yadav/HT PHOTO)

Particulate Matter 2.5 -- or PM 2.5 -- are extremely tiny pollutants (2.5 microns in diameter or smaller) that can be inhaled and dissolve into the blood stream. High exposure to them is known to cause respiratory and heart issues.

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Indian and some Chinese cities recorded some of the highest improvements in PM 2.5 concentrations globally during the complete lockdown period, but not so much during pre- or post-lockdown periods, the study by the World Meteorological Organisation and the Global Atmosphere Watch said.

For example in contrast, during the pre-lockdown period Paris recorded 58% reduction but only 9.5% reduction during lockdown; Augsburg recorded a 70% reduction in PM 2.5 levels during pre-lockdown but only 6.8% during lockdown period. Munich recorded only 5.3% reduction during lockdown; New York 27%; Sao Paolo 17% and Madrid saw an increase by 17% which may be linked to meteorological factors the study said.

Among other cities in India, Chennai and Amritsar recorded even higher improvements in air quality -- with 62% and 51% reductions in PM 2.5 concentrations, respectively.

Scientists also found that levels of Nitrogen dioxide (NO2) -- another prominent air pollutant -- reduced by 55% during the lockdown period in Delhi, compared to only 4.3% in pre-lockdown and 32% reduction during partial relaxation of lockdown curbs.

“For India, a noticeable reduction was observed during the partial lockdown despite lesser restrictions on mobility…During the full lockdown, reductions in PM2.5 concentrations were observed in both northern and southern hemisphere cities, more notably in India, a few Chinese cities (Chengdu, Guangzhou, Wuhan), Tartu, Seville, Moscow, Montreal, Mexico City, Quito and Lima,” the study titled “A global observational analysis to understand changes in air quality during exceptionally low anthropogenic emission conditions” said.

India went into a 68-day lockdown from March 25, 2020 to stem the spread of Covid-19 pandemic. All economic activities and businesses came to a standstill, factories and shops shuttered down and offices shifted to a work-from-home mode. Since then, there have been several regional lockdowns depending on the severity of an outbreak in an area, but the country as a whole has not implemented nationwide restrictions to the same degree.

Lockdown periods varied from region to region. The specific dates were provided by the city data providers and validated with other data sources. From this information, lockdown stages were defined to ensure a consistent comparison for the same city, across cities and regions.

Still, New Delhi ranked the world’s most polluted capital for the third straight year in 2020, according to IQAir, a Swiss group that measures air quality levels based on PM2.5 concentrations.

Last year, Delhi’s 20 million residents breathed some of the cleanest air on record in the summer because of coronavirus lockdown curbs, but battled toxic air in the winter months largely due to a sharp increase in farm residue burning in neighbouring states.

“Covid-19 proved to be an unplanned air-quality experiment, and it did lead to temporary localised improvements. But a pandemic is not a substitute for sustained and systematic action to tackle major drivers of both pollution and climate change and so safeguard the health of both people and planet,” said WMO secretary-general Petteri Taalas said in a statement on Friday.

The study, published in the National Library of Medicine on August 20, looked at the data from in situ ground-based air-quality observations from over 540 traffic, background and rural stations spanning across 63 cities from 25 countries in the world.

Globally, it observed decreases of up to about 70% in mean nitrogen dioxide (NO2), another pollutant, and between 30% and 40% in mean PM2.5 concentrations over the 2020 full lockdown compared to the same period in 2015–2019.

There were also some “complex signals” retrieved from the data studied. For instance, Spanish cities Madrid and Valencia saw an increase in PM 2.5 levels during their lockdown period mainly due to transport of pollutants from Africa. Similarly Beijing, Shenyang and Xi’an also showed similar spikes because of secondary formation of particulate matter, the WMO study led by Centre for Atmospheric and Climate Physics (CACP) and Centre for Climate Change Research (C3R), University of Hertfordshire and other institutions said.

Scientists said these deviations in results prove that governments need long-term measures to battle air pollution, high exposure to which is linked to not just health issues but also lower life expectancy.

“This unplanned air-quality experiment can serve as a benchmark for policymakers to understand whether existing air-quality regulations would protect public health. While lockdowns had a clear impact on air quality in urban areas, the spatial and temporal extent of that impact, the specific role of meteorology and of episodic contributions (e.g. from dust, domestic and agricultural biomass burning and crop fertilisation), and the cascade responses from indirect and non-linear effects are far from being fully understood,” WMO said in its firstAir Quality and Climate Bulletin on Friday.

“I think the message is very clear. Overall emissions levels from local pollution sources had reduced but more importantly, there was a reduction in emissions from the entire airshed, so even transport of pollution was addressed. We need a similar strategy for the entire Indo-Gangetic plains now,” said Anumita Roychowdhury, executive director of the Centre for Science and Environment.

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