In April, amidst the economic distress caused by the nationwide lockdown to curb the spread of the coronavirus, Delhi residents noticed a rare phenomenon: blue skies.

As econonomic activity ground to a halt, most factories stayed shut, construction projects were suspended and there were barely any vehicles on the streets. As a consequence, most of the sources that scientists had identified as causing atmospheric pollution in the Capital declined sharply.

This transformed the city – among the most polluted in the world – into a giant, real-life laboratory for researchers. Said MP George, a scientist at Delhi Pollution Control Committee: “It is the rarest set of data that policy makers and standard makers can use.”

So what did researchers learn from the low pollution levels during the summer lockdown? And as levels rise again, already hitting the “severe” mark on the weekend, what can this research tell us about developing strategies to control pollution and manage risk?

What the data shows

Over the years, there have been several studies about the poor air quality in Delhi. Sources of pollutants in the city range from “vehicles, re-suspended dust, industrial activities, burning of coal, dung and wood for cooking and heating...” noted a recent paper in Current Science.Crop burning and dust storms on seasonal basis also contribute significantly to extreme pollution events.”

In 2016, a study by the Indian Institute of Technology, Kanpur, showed that vehicular emissions mainly from trucks were one of the largest, most consistent sources of pollution in the city.

The study also noted that in the summer, coal and fly ash contributes 30% of the the city’s PM 10 pollutants – particulate matter of size up to 10 microns. These particles come from coal-fired power plants as well as from tandoors. Soil and road dust contri

Particulate matter smaller than 2.5 microns (or about a ten-thousandth of an inch) is particularly dangerous to human health. Such particles are small enough to travel deep into the respiratory system, potentially impariting lung function. To be considered safe, the National Ambient Air Quality Standards require PM 2.5 concentration be less than 60 micrograms per cubic metre of air in any given 24 hour period.

For this article, focussed on four air-quality monitoring stations spread around Delhi – RK Puram in South Delhi, Anand Vihar in East Delhi, Jawaharlal Nehru Stadium in Central Delhi and Jahangirpuri in North West Delhi. All four stations are monitored by the Delhi Pollution Control Committee, an autonomous body under the Delhi government.

The monthly averages of PM 2.5 and PM 10 for each month from January till May were considered.

The low pollution levels during this period were not just a result of the lockdown, experts said: a higher wind speed and other meteorological conditions also played a role.

The charts below, show that levels of particulate matter levels had actually started to decline from January before lockdown and then stayed at nearly consistent levels between March and May. For instance, all four stations recorded low levels in March and these decreased gradually through April. Three stations, except for RK Puram, recorded the lowest PM 2.5 levels in April with a slight increase in the beginning of May.

Similar trends were noticed for PM 10 levels. However, after touching their lowest levels in April, these started to increase gradually in May.

During lockdown, pollutants declined by 85% to 90%, according to the paper published in Current Science on October 10. The study was conducted after the lockdown was imposed in March, and continued through April and May. The 11 authors of the study were drawn from the Indian Institute of Tropical Meteorology in Pune, Delhi Pollution Control Committee, Indian Meteorological Department and Utkal University in Bhubaneshwar.

With virtually all major sources of pollutant emissions halted, the prolonged lockdown offered the researchers a unique opportunity to estimate the “baseline concentration” of various pollutants in the Capital, they wrote. This is the level of pollutants produced due to natural sources and stays steady in the absence of any additional sources of emissions.

Because the lockdown was nationwide, pollution from other places that floated into Delhi was minimum. This kind of emissions situation had not been available before for researchers to study.

The information helps researchers understand the lowest levels of pollution levels possible in Delhi and will help formulate policies to use these levels as the standard to target. This data is crucial as it helps define the national ambient air quality standards, said George, who is one of the authors of the paper.

Rising ozone levels

Curiously, the study found that while levels of primary pollutants were low, the levels of ozone in the atmosphere had increased.

Another study conducted from January 2019 till May 2020 by the Centre for Science and Environment found a similar trend in rising ozone levels even as it recorded low levels of pollution during the summer months.

“During February-May, at least on 23 days, ozone has emerged as the dominant pollutant,” the study said. “These are the early signs of a dangerous trend.” Doctors say that ozone is dangerous for the patients with respiratory ailments and could worsen heart disease.

Why did this happen?

“That is the complex chemistry,” said Anumita Roy Chowdhury, executive director of the Centre for Science and Environment.

Despite most industrial activities coming to a halt, gases were still being released in the air, particularly nitrogen dioxide and some volatile organic compounds, Chowdhury explained. “Once these gases are in the air then they react with each other under the influence of sunlight,” she said. “That forms ozone.”

George explained that the ozone levels increased with rising temperatures and a reduction in dust concentrations. “There was no dust, otherwise we have a blessing in disguise that the ultraviolet rays get filtered out because of it,” he said. “Here, what happened, because of lockdown, the dust was gone so we got more ozone.”

Finding solutions

For Delhi to lower the pollution levels, the problem must be considered at a regional level rather than at the state level, both experts said.

“We are in the disadvantaged location,” said George. “We are landlocked and located on the Indo-Gangetic plane which means high dust. We have to talk about airshed-based management. There is no other way.”

The airshed approach considers “the entire area over which the pollutants disperse due to meteorological and geographical factors”.

Chowdhury added that Delhi had to do a lot more to control the pollution from transport and waste management sectors. “Delhi has not yet been able to figure out how to reduce the volume of traffic and that is because of the gap in the public transport and infrastructure,” she said.

The air quality in Delhi gets worse during the winter. One of the major causes of this is the burning of crop stubble in neighbouring states such as Haryana and Punjab. This smoke coupled with falling temperatures and decreasing wind speeds create the conditions for Delhi to become a gas chamber.

Delhi’s Graded Response Action Plan formulated in 2017 requires specific measures to be taken in case pollution breaches certain levels. For instance, construction comes to a halt, diesel generators are not allowed to run and vehicular movement is restricted.

“Normally every winter we do the odd-even, intensify public transport services but this winter it will be tough to do that because of social distancing,” said Chowdhury, referring to the programme that allows vehicles with odd-numbered licence plates on the road on dates with odd numbers and those with even-numbered plates on others.