Delayed monsoon gives Delhi its cleanest October air since at least 2015

Delhi breathed its cleanest October air this month ever since the air quality index was launched in 2015, according to the Central Pollution Control Board’s (CPCB) data, thanks to the delayed withdrawal of monsoon holding back farm fires and two good spells of rain towards the end of the month.

The average air quality index for this October was 173, the data showed, compared to October 2019 -- the cleanest up till now -- when the average AQI was 234. According to the data, the city did not witness a single ‘very poor’ air day (AQI 200-300) or in severe category (AQI above 400). In October 2020, the city witnessed nine days of very poor air, and seven in 2019.

Experts said the capital has largely been helped by a delayed monsoon withdrawal, with two good spells of rain seen towards the end of the month, which caused October’s first ever ‘good’ air day -- the first in the last over 400 days. With several fields remaining wet after the late October rains, the usual spike in stubble fires stayed low this October. According to the Central Pollution Control Board, 9,001 active fire counts have been recorded in the northern plains till October 29 this year, as compared to 25,766 fire counts for the corresponding period last year.

The worst air quality day in October this year was recorded on October 17, when the AQI touched 298, while the lowest AQI recorded was just 46 (good) -- a day later, as Delhi recorded 87.9mm of rainfall in just 24 hours. In comparison, October 2020, despite a relatively clean first half of the year, saw the average monthly AQI of 265, while the highest AQI recorded that month was 395 (very poor).

In October 2019 – the cleanest October prior to this, the average AQI was recorded at 234, but readings touched as high as 419, with three consecutive ‘severe’ air days recorded towards the end of the month.

Average monthly AQI data shows, 2017 was the worst year with a monthly average AQI of 285, as 15 days in the month saw ‘very poor’ or ‘severe’ air. This was followed by 2016 (271) and 2019, which had an average AQI of 269. Experts say while Delhi’s air generally tends to reach ‘very poor’ or even ‘severe’ towards the end of October.

“Between October 7 and October 15, we see a sudden change in meteorological conditions each year and this creates stagnant conditions, which often leads to a spike in air quality. This year, a delayed monsoon meant this did not happen. We then saw two spells of rain in late October and it settled pollutants immediately,” said Dipankar Saha, former head of CPCB’s air laboratory.

Dipping temperatures, change in the wind direction and stubble fires in the agrarian states of Punjab and Haryana cause a winter air pollution emergency in the national capital each year. This, in addition to spike in instances of burning of garbage and dust pollution, pushes air quality in the national capital into dangerous zones, leading to a public health emergency.

Anumita Roy Chowdhury, executive director, research and advocacy at the Centre for Science and Environment, says the rains not only controlled local pollution, but also impacted farm fires. “Generally by this time, farm fire events would be nearing their peak. Late rains meant farmers could not burn their paddy and this could also mean a shorter window of intense farm fires, largely occurring in the next two weeks,” said Chowdhury.

Delhi ended October this year with 122.5mm of rainfall, nearly five times the normal monthly mark of 28mm. This is the fourth highest October rainfall recorded at Safdarjung, with the all-time record for October at 238.2mm (1954).

AIR QUALITY DETERIORATES, BUT MAY NOT TOUCH ‘VERY POOR’ YET

After a slight improvement in Delhi’s air quality on Saturday, the AQI dropped on Sunday touching 286 (poor), according to CPCB’s bulletin, compared to 268 on Saturday. On Friday, the average AQI was recorded at 283. Sunday’s spike came after wind speeds dropped, however, a weak western disturbance is set to influence the region in the next 48 hours, which will lead to a rise in temperatures, as well as a change in wind direction, scientists at the India Meteorological Department said.

Despite close to 1,800 farm fires recorded across the northern plains on Sunday, the contribution of stubble burning in Delhi’s overall PM 2.5 concentration was just 8%, four percentage points less than Saturday. This season the maximum farm fires contributed to Delhi’s particulate pollution was 20% on Friday.

According to the System of Air Quality and Weather Forecasting And Research (Safar), a government forecasting body, a slight improvement in air quality is expected in the next two days, owing to a change in wind direction to westerly/south-westerly.

“This will reduce the transport emissions from stubble burning. Isolated rainfall is also likely in the upwind region, which will improve air quality,” said Safar on Sunday.

Northwesterly winds, which were blowing towards Delhi for the past few days, were not only bringing farm fire emissions to Delhi from Punjab and Haryana, but also cooler winds from the hills.

IMD scientist RK Jenamani said a further rise in temperature is expected in the coming days, with local wind speeds to remain low. “Wind direction will switch to easterly, leading to a rise in temperatures. Minimum temperature may hover around 16-17 degrees in the next few days,” he said.

Delhi recorded a minimum of 16.5 degrees Celsius on Sunday – one degree above normal. On Saturday, it was 14.8 degrees, while it was 14 degrees on Friday.