Air quality in the capital worsened to the ‘poor’ category for the first time this season, and is expected to fall further over the next three days.
Calm wind speed and lower temperatures are causing accumulation of pollutants from internal sources within Delhi and also external sources, including from increasing farm fires in Punjab and Haryana.
Forecasts show that the air quality index (AQI), which averaged at 215 or ‘poor’ on Wednesday as per the Central Pollution Control Board (CPCB), would further deteriorate.
A forecast from the Central government’s SAFAR air quality and weather forecasting system said, “The boundary layer wind direction and speed are favourable for slow transport from external emission sources and local conditions are conducive for accumulation of pollutants in Delhi.”
It added that the AQI would worsen within the ‘poor’ category until October 10.
The coronavirus lockdown and the slow resumption of economic activities had significantly improved the air quality of Delhi after March, but unfavourable weather conditions that begin every year after withdrawal of monsoon and crop residue burning in neighbouring states have begun influencing the air quality again.
The CPCB has also asked the Delhi government to take “immediate yet effective” actions to control air pollution.
Delhi environment department officials said contribution from stubble burning and traffic congestion from within the city have increased the concentration of particulate matter of 2.5 micrometres (PM2.5).
Officials added that dry winds from the northwest direction are also transporting dust, or particulate matter of 10 micrometres (PM10), from Haryana and Rajasthan to Delhi, along with emissions from construction activities within the city.
PM2.5 and PM10 are fine particles suspended in the air that can enter the bloodstream through breathing and cause health problems.
Taking the lead
The SAFAR forecast said, “PM2.5 is now becoming the lead pollutant instead of PM10 as characteristic of wintertime. Calm surface wind condition prevailed over nighttime yesterday and is likely for the next two days.”
Concentration of PM2.5, emitted through combustion activities, was 95.3 micrograms per cubic metre air (µg/m3) as of 5 pm on Wednesday, and that of PM10, comprising dust and smoke, was 235.8 µg/m3. Their average 24-hour safe levels are 60 and 100 µg/m3.
Fire counts have been increasing in the two main stubble burning states of Punjab and Haryana. In Punjab there were 1,523 fire counts between September 21 and October 6, as per data from the Punjab Remote Sensing Centre.
These are much higher than 258 fire counts recorded in the same period last year.
In Haryana, the state pollution control board recorded 526 fire counts between September 25 and October 6. State officials said there were about 150-200 fire counts in the same period last year.
“A reason behind this could be that paddy crop was sown a week early this year,” a Haryana pollution control board official said.
Meanwhile, low wind speed and temperatures are causing accumulation of pollutants, said Kuldeep Srivastava, head of the India Meteorological Department’s (IMD) regional weather forecasting centre in Delhi.
The minimum or night time temperature in the city has been falling this month and on Wednesday was recorded as 18.6 degrees Celsius, three degrees below normal.
The maximum or daytime temperature was 36 degrees Celsius, two degrees above normal.
Srivastava said wind speed between midnight and 7.30 am on Wednesday dropped to nearly zero, but picked up around 15 kmph after that.
“Calm winds and low temperature are leading to accumulation of pollutants. In the daytime, when the wind speed picks up and the temperature rises, the pollutants are dispersed,” he said.
This condition of low windspeed in the early hours and higher in the day time is expected to continue until Friday, after which wind direction would change to east and southeasterly from northwest at present.
“This may lead to further deterioration of air quality as the wind speed would be low and pollutants gathered in Delhi by that time may not be easily dispersed,” Srivastava said.
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