January 15, 2021 1:10:01 am
Delhi’s air quality slipped into the severe category on Thursday and is not expected to improve over the next three days in what is forecast as the first “extended extreme air pollution” episode of the new year, government agencies said.
Slow winds and unfavourable ventilation conditions were causing the dip in the air quality, coupled with cold weather as the capital recorded a severe cold wave on Thursday with minimum temperature dipping to 1.8 degrees Celsius at southwest Delhi’s Jafarpur.
The India Meteorological Department’s (IMD) Safdarjung observatory, which is representative of the city, recorded a minimum or night time temperature of 2 degrees Celsius.
VK Soni, head of the IMD’s Environment Monitoring and Research Centre, said, “Winds have slowed down and become calm and the ventilation is extremely poor… This may be a prolonged episode of severe air quality. Some improvement is expected January 18 onwards.”
The 24-hour average air quality index (AQI) of Delhi on Thursday was 429, higher than 354 on Wednesday when it was ‘very poor,’ as per the Central Pollution Control Board (CPCB). AQI of Ghaziabad, Noida, Faridabad was also ‘severe’ on Thursday while that of Gurgaon was ‘very poor.’
Soni added that low temperatures have reduced the mixing depth or the vertical height in which pollutants are suspended in the air.
Due to calm winds, with speed of around 3kmph, pollutants could not be dispersed and therefore the maximum ventilation index — the product of mixing depth and average wind speed — was around 2000 sqm/second on Thursday, as per the Centre’s Air Quality Early Warning System (EWS) for Delhi.
A ventilation index below 6000 sqm/second with average wind speed less than 10kmph is considered unfavourable for dispersion of pollutants.
A bulletin from the Ministry Earth Sciences’ air quality monitor SAFAR said on Thursday, “The AQI is forecast to deteriorate rapidly in the next three days. It will be in the higher end of the very poor to severe category tomorrow and then will enter in the severe category for 16-17 January. This is likely to be the first extended extreme air pollution event for 2021.”
The bulletin added, “The combination of dense fog formation leading to secondary particulate formation, under congenial conditions of high humidity, extremely low ventilation and shallow boundary layer height is a major reason for the predicted smog episode.”
Secondary particulate formation is when gaseous pollutants are converted into particles through chemical reaction in the atmosphere, aided by high humidity and low temperature.
Dense fog conditions were noted in Delhi on Thursday morning when visibility dropped to 100 metres at west Delhi’s Palam and 200 metres at Safdarjung, as per IMD officials.
Kuldeep Srivastava, head of the IMD’s regional weather forecasting centre, said, “The temperature is dipping due to cold northwesterly winds blowing from the western Himalayan region. We also have clear skies at night time which cools the surface further due to higher radiation from the earth.”
The maximum or day time temperature in the city settled at 19.2 degrees Celsius on Thursday. On Friday the mercury is expected to rise slightly before dipping again.
The IMD has forecast a cold wave on Saturday when the minimum temperature is expected to be 3 degrees Celsius and the maximum as 19 degrees Celsius.
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