The 21-day nationwide lockdown, coupled with favourable weather conditions, is not just giving Delhi its cleanest air possible, it is also allowing scientists to determine the city's baseline pollution levels -- for the very first time.
With pollution sources brought down to a bare minimum, within and outside the National Capital Region, researchers from the Delhi Pollution Control Committee (DPCC), IIT Delhi, IIT Kanpur and other private organisations, have started gathering data to determine Delhi's baseline pollution levels, or the lowest pollution level Delhi can ever hope to achieve.
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"Even though baseline data is important to fight air pollution, it is hard to get. There is always some form of pollution in a city that skews the data. If we can determine the lowest achievable pollution levels, it would help us to prepare a road map and strategy to cut down on sources," said D Saha, former head of the CPCB's air quality lab.
Prime Minister Narendra Modi announced the lockdown on March 25 to contain the spread of the coronavirus disease (Covid-19). Since then, factories, construction sites, and some other local sources that pollute Delhi have been totally shut, while vehicles on the roads have been at a bare minimum.
"The city's air quality is at its best at the moment, and is unlikely to get better than this. All man-made pollution has come down to near-zero levels. Nothing is operating within a range of 300km, barring a few essential vehicles, and service providers such as power plants and oil refineries. This is a never-before opportunity to collect the city's baseline pollution data," said MP George, a scientist with DPCC.
The 24-hour average Air Quality Index (AQI) of the last seven days has not crossed 100 on a single day. It dropped to 45 (good) on March 28, and has been hovering in the 50-100 (satisfactory) range for the rest of the past week. During the corresponding period in previous years, it remained much higher - often in the 200-250 range (poor). The AQI had crossed 300 (very poor) on March 25, 2016. In the winter, it frequently shoots up beyond 400 (severe).
"We hardly get such an opportunity when almost all major sources of pollution are stopped in one go. We plan to start work on preparing baseline pollution level for Delhi very soon with the data that is being collected during the lockdown period," said Mukesh Sharma, a professor of civil engineering from IIT Kanpur, whose team prepared the Capital's first source apportionment chart in 2016.
A 2018 study by The Energy Resources Institute (Teri) and Automotive Research Association of India (Arai) had shown that during winters 36% pollution is generated within Delhi. Around 34% comes from areas within the NCR and 18% comes from northwest India (beyond NCR). At least 13% of Delhi's pollution was coming from outside India. Real-time source apportionment of air pollution has been a problem in Delhi, and the state government is working on a project with Washington University to fill the gap.
"The shutdown is not limited to a particular sector like industry or vehicles. Neither is it a city-level shutdown. It is a pan-India shutdown, so chances of pollution coming from outside has also been minimised. We are gathering data as this an opportunity that Delhi will not get in years to come," said Sagnik Dey, associate professor at the Centre for Atmospheric Sciences, and coordinator of Cerca at IIT Delhi.
State pollution control board officials from Haryana and Punjab confirmed that summer stubble burning (when farmers clear their fields for the next crop) is not happening at present, which also contributes heavily to the city's pollution.
Now add to this good weather - strong winds, occasional sharp showers, and sunlight. The India Meteorological Department (IMD)'s Safdurjung observatory, which is taken to be a representative of Delhi, received 109.6mm rain in March 2020. This is the highest rainfall in the month of March since 1901. In 2015, Safdarjung received around 97.5mm rain. With moist soil there is hardly any dust in the air, say experts.
"Delhi, along with other parts of north-west India, received good rain because of more western disturbances this month. Over the past one week, north westerly winds with a speed of 15-20km per hour are blowing," said Kuldeep Srivastava, the head of IMD's regional weather forecasting centre in Delhi.
Though the yearly average PM2.5 level in Delhi usually remains two to three times higher than WHO permissible limits at all times, in the last one week, the levels of both PM2.5 and PM10 have remained well within the daily permissible limits of 60ug/m3 and 100ug/m3 respectively.
Such levels are usually seen only after heavy showers during the monsoon. In the winter, PM2.5 and PM10 breach their respective severe+ marks of 300ug/m3 and 500ug/m3.
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