Calm surface winds and a decrease in the ability of the atmosphere to disperse pollutants have made conditions conducive for such an episode
Delhi may be in for another 'severe' air quality episode after the air quality index (AQI) in the national capital and its satellite towns worsened on December 5, 2019.
Many areas recorded ‘Severe’ air quality though the overall average for Delhi was in the upper end of the ‘Very Poor’ category. At 7 PM, it was at 399, just on the verge of becoming ‘Severe’.
However, in the neighbouring towns of Ghaziabad, Greater Noida, and Noida, the AQI was already in the ‘Severe’ category. In Gurugram, it was ‘Very Poor’.
An AQI between 0-50 is considered 'Good', 51-100 'Satisfactory', 101-200 'Moderate', 201-300 'Poor', 301-400 'Very Poor', and 401-500 'Severe'. Above 500 is considered 'Severe-plus' or 'Emergency'.
Out of 39 monitoring stations in Delhi, 18 recorded ‘Severe’ air quality, while 19 were in the ‘Very Poor’ category. The data for the remaining two was not available.
Meteorological factors like calm surface winds and a decrease in ventilation coefficient — which is the product of mixing depth and average wind speed — have made conditions conducive for pollutant accumulation.
Ventilation coefficient depicts the ability of the atmosphere to dilute and disperse the pollutants over a region. The higher the coefficient, the more efficiently the atmosphere is able to dispose the pollutants and better is the air quality. On the other hand, low ventilation coefficients lead to poor dispersal of pollutants causing stagnation and poor air quality.
According to the SAFAR (System of Air Quality and Weather Forecasting and Research) website under the Union Ministry of Earth Sciences, AQI is forecast to deteriorate to the higher-end of the 'Very Poor category' by the night of December 5. Further deterioration is forecast for December 6, when it will stay between ‘Very Poor’ and ‘Severe’ until December 7.
The effective stubble fire counts estimated according to SAFAR-multi-satellite product were 228 on December 4 and were showing a decreasing trend. The stubble transport level winds were forecast to shift from the northwesterly to the northerly direction and biomass fire PM2.5 contribution will be insignificant for the next two days, said an analysis by SAFAR.
The concentration of ultra fine particulate matter (PM) 2.5 in Delhi’s air was at 202, five times over the safe standard of 60, while PM10 was recorded at 313, around three times above its safe standard of 100.
The situation in other north Indian towns like Baghpat, Bulandshahr, Kanpur, Moradabad, and Meerut in Uttar Pradesh, Ballabhgarh, Karnal, Palwal, and Panipat in Haryana, and Muzaffarpur and Patna in Bihar was equally worrying as the AQI for all these was either ‘Severe’ or close to touching ‘Severe’.
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