Cocktail of slow winds, stubble fires gives Delhi its worst November AQI
NEW DELHI: Air quality in the Capital this year in November was the worst since the Central Pollution Control Board (CPCB) started maintaining detailed records in 2015, with average Air Quality Index (AQI) at 376. Delhi last breathed air this bad in November 2016, when the average AQI was 374.
The national capital also recorded the maximum of 11 days in November when the index rose above 400, which is classified as “severe” by the pollution watchdog, as it affects healthy people and seriously impacts those with prior illnesses.
Besides human activities like construction and transportation that leads to harmful emissions, unfavourable meteorological conditions worsened the situation. Delhi usually sees at least two western disturbances in the month, which often bring rain, but more importantly, stronger winds that help in dispersing pollutants. This year, there were none, according to RK Jenamani, a scientist at the weather office.
“A western disturbance can bring rain and also increase wind speed a day before it is approaching, and when it is departing too,” Jenamani said. “This generally gives a two-three day window where air quality improves.”
November, however, was slightly cooler than usual, as average monthly maximum temperature of 27.8 degrees Celsius was the lowest since 2013, when it was 27.3 degrees. The normal monthly maximum for the month is 28.2 degrees.
The average minimum temperature was also slightly lower at 12.3 degrees compared with the normal 12.9 degrees.
All days except two saw AQI values exceeding 300, which means residents had no respite from the toxic blanket that smothered the Capital through the month.
Last year, Delhi had light showers on November 17, and AQI dropped to a more breathable 171. In 2019, November saw the influence of a western disturbance towards the end of the month that provided some relief.
“Meteorological conditions, including wind speeds and mixing height, became increasingly unfavourable for dispersion of pollution as November progressed. Unlike November 2020, Delhi did not experience any rainfall in November this year,” said Tanushree Ganguly, programme lead at the Council on Energy, Environment and Water, a Delhibased think tank.
“Further, this November was slightly cooler than last year, which suggests that emissions from local biomass burning for heating needs may have increased,”
This year, the peak pollution level for the month were recorded on November 12, when the AQI touched 471, while in 2016, the peak touched 497.
However, more air quality monitors could play a role in it, experts said. “The peak this year or that year may not be that different, since only nine stations were considered for the 2016 reading, while now, there are 36 to 38 stations in Delhi and the average reading gets impacted,” said Anumita Roy Chowdhury, executive director at the Centre for Science and Environment, an advocacy group. “Both can be equally dangerous,”
In contrast, October was the cleanest since 2015, with an average AQI of just 173, as a delayed monsoon kept pollution levels at bay.
Dipankar Saha, former head of CPCB’s air laboratory says while firecracker burning and stubble burning are known factors during November, meteorological conditions play a key role each year, with the mixing height also an important factor this year. “The emission sources for Delhi largely remain the same each year, but the concentration will vary. November is atransition phasefor
northern India as temperatures start to dip, the mixing height reduces and wind speeds drop,” says Saha, stating the mixing height dropped down to 200-300 metres this time around, trapping pollutants closer to the surface.
“Normally, this is around 500 metres and the closer it drops to the surface, the lesser room there is for it to disperse. We have seen numerous days with calm wind conditions, thereby trapping pollutants locally,” he added.