Air pollution in Delhi increased overnight due to calm winds and high moisture, pushing the city’s air quality into the severe category on Saturday.
Scientists from the India Meteorological Department (IMD) said easterly winds blowing over the city had high moisture content, which led to secondary formation of PM 2.5 — fine particles suspended in the air.
Secondary formation of PM 2.5 is a process by which gaseous pollutants are converted into particulates through a chemical reaction in the atmosphere, which multiplies PM 2.5 levels.
VK Soni, head of the IMD’s Environment Monitoring and Research Centre (EMRC), said, “Air quality started deteriorating after midnight, as high moisture content in easterly winds led to secondary formation… the situation is expected to improve slightly on Sunday and Monday, when the wind speed will pick up.”
He said the PM 2.5 level was around 230-220 μg/m3 on Saturday, nearly four times higher than the 24-hour exposure limit of 60 μg/m3.
Delhi’s 24 hour average air quality index (AQI) Saturday was 404 (severe), as per the Central Pollution Control Board. This was further deterioration from the very poor range over the last five days.
Average AQI of Ghaziabad and Noida on Saturday was also severe while that of Gurgaon and Faridabad was very poor.
Meteorological factors such as calm winds and low temperatures reduce the mixing depth, or the vertical height available in the atmosphere for pollutants to disperse.
A bulletin from the Ministry of Earth Sciences’ (MoES) air quality Early Warning System (EWS) said the ventilation index in Delhi — the product of mixing depth and wind speed — was approximately 2,000 sqm/second on Saturday with wind speed up to 8kmph. A ventilation index lower than 6,000 sqm/second with average wind speed less than 10 kmph is unfavourable for dispersion of pollutants, as per the EWS.
The index is forecast to reduce to 1,000 sqm/second on Sunday, but wind speed is forecast to pick up to 10 km/h, which is likely to improve the air quality to the very poor category, as per the EWS. “Air quality is likely to improve further and reach the lower end of the very poor to poor category on December 7,” the EWS bulletin said.
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