Delhi “experiences one of the most severe air pollution in the world, linked with diverse anthropogenic and biomass burning emissions. First phase of COVID-19 lockdown in India, implemented during 25 March to 14 April 2020 resulted in a dramatic near-zeroing of various activities (e.g. traffic, industries, constructions), except the ‘essential services’. Here, we analysed variations in the fine particulate matter (PM) over the Delhi-National Capital Region.

Measurements revealed large reductions (by 40–70 per cent) in PM2.5 during the first week of lockdown (25–31 March 2020) as compared to the pre-lockdown conditions. However, O3 pollution remained high during the lockdown due to non-linear chemistry and dynamics under low aerosol loading. Notably, events of enhanced PM levels (300–400 µg m−3) were observed during night and early morning hours in the first week of April after air temperatures fell close to the dew-point (~ 15–17 °C).

A haze formation mechanism is suggested through uplifting of fine particles, which is reinforced by condensation of moisture following the sunrise. The study highlights a highly complex interplay between the baseline pollution and meteorology leading to counter intuitive enhancements in pollution, besides an overall improvement in air quality during the COVID-19 lockdown in this part of the world”, states the abstract of the paper.

The lead author is Surendra K. Dhaka from the Radio and Atmospheric Physics Lab, Rajdhani College, University of Delhi, whose associates in the same college in it were Vinay Kumar and Vivek Panwar. Others from India were A. P. Dimri of the School of Environmental Sciences, Jawaharlal Nehru University,Chetna of Department of Physics and Astrophysics, University of Delhi and Narendra Singh of Aryabhatta Research Institute of Observational Sciencess(ARIES).

The co-lead author from Japan was Prabir K. Patra, sennior scientist, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, and Visiting professor at CERES, Chiba University, Japan. Others from Japan were Masayuki Takigawa of JAMSTEC, Yutaka Matasumi of the Institute for Space-Earth Environmental Research, Nagoya University, Tomoki Nakayama of Faculty of Environmental Science, Nagasaki University, Kazuyo Yamaji of Graduate School of Maritime Sciences, Kobe University. Mizuo Kajino of Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Prakhar Mishra of Research Institute for Humanity and Nature, Kyoto, and Sachiko Hayashida of Research Institute for Humanity and Nature, Kyoto.

Air pollutants and COVID-19 are linked to a huge number of deaths in the NCR. But the intensity declined due to reduction in transport, construction works, industries and other commercial activities that spill pollutants or produce dust. ‘Unprecedented reductions in anthropogenic activities yielded to very low values of emissions resulting in significantly improved air quality over the Delhi-National Capital Region (NCR) [up to 50 per cent reduction in fine particle matter of aerodynamic diameter smaller than 2.5 µm (micrometer) Despite the gloomy and sad day-to-day life and situation in which millions of people are under distress, this period, nevertheless, allowed a unique opportunity for the scientific community to study the interplay between baseline air pollution and natural processes”, the paper states.

Northern Indian winters are visited by a severe widespread air pollution attributed to a variety of emissions being confined near the surface under stagnant metrological conditions Aerosol emissions in India is dominated by transportation, industrial, residential energy usage and biomass burning . The lockdown period was for the researchers an opportunity to ‘measure the baseline air pollutants and variability in a natural environment with the least anthropogenic effects.

Most of days of the lockdown witnessed clear sky conditions without any signature of visible contamination. These observations are therefore of paramount significance to study and understand the interactions of meteorology and baseline pollution in the tropical megacity. The difference in air quality before and during the lockdown can be used as an estimate of the regional pollution which accumulates on top of the background (levels during lockdown). Such studies are not possible in normal conditions, for example even during the very aggressive “odd–even scheme” the air quality over Delhi was only marginally improved which besides other factors highlighted a remarkable role played by the meteorology and lower atmospheric dynamics.’

‘Covid-19 lockdown’ , the team states, ‘ provided us a nearly natural laboratory to investigate the close association of haze and development of PM2.5 that amounted to be around 100–150 µg m−3 while rest of the day background PM2.5 remains as low as low as 15–20 µg m−3 (per cubic meter), which is unprecedented situation in Delhi-NCR, Northern India. This study opens up a microphysical regime to be investigated further in the context of subtle atmospheric processes operating in natural environment in conjunction with intensive measurements available on environmental issues’.

Patra in an email to this correspondent, wrote, “The aerosols have very different effect on the climate and weather. As far as the atmospheric scientists understand the aerosols overhead tend to reduce incoming solar radiation reaching the earth surface (the so called solar dimming). But what happened during the lockdown is a reduction of aerosols overhead, meaning we would expect heating of the air around us (calculations also can confirm the excess heating over Kanpur), contrary to what you/we have seen in the city areas. We have noted cooler Delhi in some months of 2020 as well. In light of the above scenarios, continuous measurements of air quality and meteorological parameters have been analyzed to unravel the impact of the lockdown on air quality and to elucidate the interactions between this baseline pollution and meteorology”. (IPA Service)