Expect ‘severe’ Delhi air today

EPCA flags uncertainty over wind direction; Friday AQI was season-worst  

By Shagun
Published: Saturday 24 October 2020

Delhi’s air quality may turn ‘severe’ for some time October 25, SAFAR (System of Air Quality and Weather Forecasting and Research) informed Environment Pollution (Prevention and Control) Authority (EPCA) October 24, 2020.

The Capital’s air quality index (AQI), in fact, inched towards ‘severe’ October 24, itself — at 366 it was the worst this season yet.

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 the ‘severe-plus or emergency’ category. 

Of Delhi’s 36 monitoring stations, at least 31 were in ‘very poor’ category, four in ‘severe’ and one in ‘poor’.  Faridabad, Gurgaon, Noida, Greater Noida and Ghaziabad were also in the ‘very poor’ category. 

“What is not certain is if the wind direction will change on October 25 for the better or worse,” the EPCA said and added:

“For the moment our recommendation is that we must ensure enforcement on ground to check all polluting sources. We are also reiterating the advice given to be safe from COVID-19 that we should only go out when absolutely necessary and at all times wear masks.”

Unfavourable meteorological conditions like highly calm surface winds and low ventilation coefficient, and increase in moisture coupled with local pollution sources and smoke from stubble burning in neighbouring regions has aggravated the pollution level. 

The concentration of toxic pollutants has increased sharply in Delhi’s air. While PM (particulate matter) 2.5 was at least five times more than its World Health Organisation (WHO) prescribed limit of 60 microgramme per cubic metre (μg/m3), PM10 was three times more than the prescribed standard of 100 μg/m3.

The wind speed has reduced to 5-10 kilometres per hour, while the ventilation index was around 10,000 metre squared per second. 

The higher the ventilation 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. 

Similarly, an increase in moisture due to dip in temperatures has also contributed to high pollution.

“If moisture  increases, the overall mass of pollution particles will increase even if number of particles remain the same,” explained Sachin Ghude, scientist at Indian Institute of Tropical Meteorology (IITM), which operates the SAFAR portal. 

Daytime moisture has increased to around 50 per cent from around 40 per cent the previous week; night levels have jumped to 65 per cent from 50 per cent.

“Due to all these conditions, pollution is getting circulated and trapped around Delhi and not getting dispersed either horizontally or vertically,” Ghude said. 

Smoke from stubble burning in Punjab and Haryana also contributed to an increase in PM2.5 particles. On Friday, the share of stubble burning in Delhi's PM2.5 concentration was around 16 per cent.

“Stubble burning will end by November 15. But  worse conditions will prevail mid-December and January. The overall situation will be like last year,” Ghude added. 


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