Coronavirus update: Clear blue skies in Delhi, AQI at 45 bring some good news
The air quality index for Delhi, according to the Central Pollution Control Board’s 4pm bulletin was 45, which falls in the “good” zone -- rare in a city where a reading in the “poor ” category is otherwise considered good.
Delhi woke up to clear blue skies yet again on Saturday, Day 4 of the 21-day lockout announced by the government to halt the spread of Covid-19.
Clear blue skies have become the norm rather than the exception in the National Capital Region (NCR) around Delhi for the past week. Data, too, shows a sharp decline in pollution.
Morning and evening peaks in the level of PM 2.5 (particulate matter 2.5, or fine, respirable pollution particles) have flattened out in the four days since the lockdown came into force, according to a new analysis by the Centre for Science and Environment (CSE). The air quality index for the Capital, according to the Central Pollution Control Board’s 4pm bulletin was 45, which falls in the “good” zone -- rare in a city where a reading in the “poor ” category is otherwise considered good.
The reduction in nitrogen dioxide (NO2) concentrations is even more pronounced because the major source of NO2 emissions are vehicles , which have gone off the roads, and industries, which have been ordered to close to reduce the risk of infection.
The hourly pollution trend in Delhi and in the neighbouring cities of Gurugram, Faridabad, Noida and Ghaziabad analysed by CSE used to be influenced by traffic peaks in the mornings and evenings, a factor hasn’t been relevant in the past four days.
For example, in Delhi the daily peak PM 2.5 concentrations has declined on a lockdown day by about 57% compared to a regular day and the peak on the “Janta Curfew” day (Sunday, March 22) dropped by about 24% compared to a regular day. The peak concentrations of NO2 dropped by 66% on a lockdown day compared to a regular day and by 32% on the people’s curfew day.
The so-called Janta Curfew was called for by Prime Minister Narendra Modi as a sort of a trial in social distancing and self-isolation. The lockdown was announced two days later.
“This analysis brings out the pronounced effect of traffic on hourly pollution trend and on daily exposures to toxic vehicular pollution. With traffic minimised, hourly trends plummet. This is sharply evident in NO2 trends,” CSE said in its analysis.
The fact that the impact was comparatively lower on Janta Curfew may have had to do with its shorter duration of 14 hours, the study said. “Subsequent lockdown is more rigorous and pertains to 24 hours with curfew rules in place,” it said. There’s also the cumulative effect to be taken into account.
The main source of NO2 emissions is fuel combustion by vehicles and industries. NO2 is more reactive than PM 2.5, and being exposed to high NO2 emissions even for a short duration can aggravate respiratory disease particularly asthma, according to the US Environment Protection Agency.
Globally, four million new child asthma cases are attributable to NO2 pollution a year; 64% of these occur in urban centres, according to a study published in Lancet Planetary Health.
“This experience has helped us to understand the strong impact of traffic volume on hourly variability in pollution and exposure in cities. We are already making massive lifestyle adjustments to practice social distancing,” said Anumita Roy Chowdhury, executive director, Centre for Science and Environment. “Can this crisis-induced change throw up long-term solutions to traffic impact on air pollution? Collective community action during this pandemic can help us to reinvent longer term solutions to reduce our vulnerability to toxic risk.”
“Since every source of emissions is curtailed, air pollution levels have reduced drastically in Delhi-NCR. We are in agreement with CSE but we are also doing an analysis in-house,” said VK Shukla, head of the air quality management division at CPCB.
The Regional Weather Forecasting Centre (RCFW) in Delhi, meanwhile, released historical data for the maximum temperature recorded in April, which ranges from 39.6 degree Celsius in 1994 to 43.7 degrees Celsius in 2010.
The impact of heat and humidity on the Sars-CoV-2 virus that causes Covid-19 is not clear.
“Data analysis since 1992 shows maximum temperature over Delhi reaches 40 degree C during April 17 to 20. If temperature and humidity play a role in reduction of transmission of the virus , we will be able to see the trends in April. But things will be different if people use air conditioning indoors,” said Kuldeep Shrivastava, head, RWFC.
The average maximum temperature in March in Delhi is 29.6 degree C compared to 28 degree C recorded till now. But on most days in March this year, the maximum temperature has been 3 to 4 degree C below normal with only seven days when the maximum was 30 degree C or above. This is because of the unusually high number of western disturbances which affected the northwestern region this month. There have been six active western disturbances which brought rain, hail or snowfall to different parts of northwest India compared to an average of 2-3 typical for March.
“The westerlies are still impacting the Indian region. They are supposed to move northwards gradually in March. This is why we are seeing more western disturbances. Parts of Central India and south India are already recording 35 degree C. We can expect it to hit 35 degree C in north India in early April,” said DS Pai, a scientist at the India Meteorological Department (IMD), Pune.
Clear skies and less noise means that bird song can be heard clearly in many neighbourhoods of the capital. “’I have had a lot of people asking me if bird song has increased. No, it has always been there particularly in this season. This is also the courting and nesting season for many birds -- sunbirds, barbets and doves. So they sing or call for long periods. People now have the time to pause, listen and take in the song. The lack of planes and cars also helps as planes and car horns are sources of noise pollution,” said Neha Sinha, a Delhi-based conservation biologist.
For PM 2.5 and NO2 concentration analysis following days have been considered by CSE:
• Regular: Average of March 18 and 19
• Lockdown: Average of March 25 and 26
• Janata Curfew: Average of March 22 March