AQI rebounds to severe zone as pollutants linger in Delhi's air

The AQI was at 411 when the Central Pollution Control Board released its 4pm pollution bulletin, and by 10pm, it had risen to 431 with average PM10 and PM2.5 pollutant levels nearing the emergency threshold.

Pollution levels rose sharply on Thursday as winds died down, pushing the air quality index (AQI) back into the severe category after a day of slight respite when it was still very poor.

The sun rises over buildings shrouded in smog in New Delhi. (Bloomberg)

Weather forecasters said such conditions could persist over the region for the next 24-48 hours.(HT)

The AQI was at 411 when the Central Pollution Control Board released its 4pm pollution bulletin, and by 10pm, it had risen to 431 with average PM10 and PM2.5 pollutant levels nearing the emergency threshold.

The emergency threshold is when PM10 concentrations cross 500ug/m3 and PM2.5 300ug/m3.

Weather experts attributed the deterioration to a drop in wind speed, which was caused by unusual circulation patterns over the country: a depression over the Bay of Bengal stretching into southern India and over parts of the eastern coast has blocked winds that sweep in from the northwest, flow over the Indo-Gangetic plains.

The north-westerly winds are what bring in the smoke from farm fires in Punjab and Haryana, and as they flow further over the Indo-Gangetic plans, they eventually disperse these pollutants away as well.

“South-easterly winds (due to the depression) have reached central India, up to Madhya Pradesh, and are stopping the stubble smoke-carrying north-westerly winds from passing freely. Since the winds are getting trapped over Delhi and adjoining areas, pollution is also concentrated over this region leading to high AQI recordings,” said Mahesh Palawat, vice-president (meteorology and climate change), Skymet weather services.

Such formations are unusual but they occur this time of the year every few years.

Weather forecasters said such conditions could persist over the region for the next 24-48 hours.

Thursday’s was the sharpest fall in air quality since the day after Diwali, when the AQI shot up to 462 from 382 a day before.

The AQI remained in the severe zone for two days after that, relenting only on November 8 briefly when winds picked up.

Now, with wind speeds in single digits, local pollutants are being trapped and mixed with emissions from farm fires. The Union ministry of earth science’ air quality early warning system said that the surface wind speed in the city on Thursday was only 4-8kmph, and this further dropped after sundown.

The mixing depth—the height above the surface throughout which pollutants can be dispersed—in Delhi was 1,400 metres on Thursday. According to Indian Institute of Tropical Meteorology (IITM), Pune, the ventilation index—a measure of air pollution based on the speed of the wind and the height of the column of air in which smoke or other pollutants mix—was also as low as 6,000m2/s.

Experts say a ventilation index lower than 6,000 m2/s, along with an average wind speed less than 10 kmph, is unfavourable for the dispersion of pollutants.

“Slow winds during daytime and calm winds during evening/night are extremely unfavourable for dispersion of pollutants. The air quality is likely to remain in the upper end of the very poor category on Friday and further deteriorate from November 15, and may reach the severe category. The predominant pollutant is expected to be PM2.5 (ultrafine particulate matter with diameter less than 2.5 micrometres),” a senior official of the India Meteorological Department (IMD) said.

According to the System of Air Quality and Weather Forecasting And Research (SAFAR), a central air quality monitoring centre, on Thursday 3,914 fires were reported from Punjab and Haryana, which contributed to 26% in Delhi’s PM 2.5 levels. This was only a notch down from the last two days when the share of stubble smoke in the air was 27%.

“Fire pollution plume (carbon monoxide from the last two days of heavy stubble burning in northwest India is now spreading across the Indo-Gangetic Plains and parts of Madhya Pradesh...,” said Sachin Ghude, scientist and project lead at the IITM.

This year, farm fires are taking place later than usual due to a delayed withdrawal of the monsoon rains.