Air We Breathe | Why does Delhi's AQI take a turn for the worse in October? | Latest News Delhi - Hindustan Times
close_game
close_game

Air We Breathe | Why does Delhi's AQI take a turn for the worse in October?

Oct 05, 2022 06:34 PM IST

What factors contribute to the dip in air quality every year in the Capital? HT explains.

New Delhi: From recording one of its cleanest spells of the year in September, Delhi’s air quality takes a turn for the worse in October, with the air quality index (AQI) quickly touching the ‘very poor’ or even the ‘severe’ category by the end of the month. While several external factors come into play as soon as the calendar switches over to October 1, a majority of these commence when the monsoon starts to withdraw from the region, putting in place conditions that are favourable for stubble burning. At the same time, north-westerly winds that transport these emissions generated from stubble burning to Delhi and a transition period where wind speeds drop considerably, along with a steady dip in temperature are factors working in tandem, gradually pushing Delhi’s AQI to the ‘poor’ category and beyond.

If the average AQI between any two consecutive months was compared, the sharpest swing in average AQI occurs between September and October each year. (Arvind Yadav/HT Photo/Representative Image) PREMIUM
If the average AQI between any two consecutive months was compared, the sharpest swing in average AQI occurs between September and October each year. (Arvind Yadav/HT Photo/Representative Image)

If the average AQI between any two consecutive months was compared, the sharpest swing in average AQI occurs between September and October each year. This year too, Delhi is already beginning to see the same, with the impact of the withdrawal of the monsoon already pushing Delhi’s AQI to the higher end of the ‘moderate’ category, Central Pollution Control Board (CPCB) data shows. The withdrawal of the southwest monsoon from Delhi, Punjab, Haryana, Uttarakhand, and Jammu & Kashmir was officially declared by the India Meteorological Department (IMD) on September 29, with the moisture-laden winds quickly switching to dry north-westerly winds, of which dust and PM 10 is a key component.

Delhi recorded its cleanest spell of the year between September 23 and 25, recording an average 24-hour AQI of 57 (satisfactory) on September 23, an AQI of 54 (satisfactory) on September 24 and this year’s lowest AQI of 52 (satisfactory) on September 25. However, the AQI is currently over three times that number, with it hovering around 190 (moderate), which is on the edge of the ‘poor’ category.

Monsoon withdrawal puts in place a ‘transition’ phase

While background emissions largely remain the same in September and the start of October, a key factor which changes is the transition of easterly winds that bring moisture — to dry north-westerly winds. However, this transition does not happen automatically, with a week-long or even a 10-day period, where a change in wind direction is simultaneously met with a period where local winds become extremely calm. This means pollutants being generated in the city are not able to disperse easily, creating a chamber for local emissions to gradually start to accumulate.

“Easterly winds are largely responsible for bringing rain, while north-westerly winds are dry, and the transition signifies the start of the winter season. Since these winds blow from northwest India, the impact of snowfall in the higher reaches is reflected through it and the plains start to see a drop in temperature too. During the withdrawal period, the transition phase, which the NCR region is currently seeing, sees local pollutants start to accumulate as wind speed is calm,” explains RK Jenamani, a scientist at IMD.

It is only towards the second half of October that wind speeds start to pick up pace, with the monsoon withdrawal process coming to an end. While this is good to disperse local pollutants and the emissions accumulating over Delhi, it also means long-range transport of emissions, most notably stubble-related emissions, starts reaching the capital.

Stubble emissions can be as high as half of all local emissions

While farmers in Punjab, Haryana, and Uttar Pradesh can start burning paddy stubble from September 15 onwards, a majority commence it at the start of October, with it gathering pace from October 15 onwards. This leads to an intense one-month period between October 15 and November 15, where the daily fire count can reach upwards of 5,000 fires.

As per data from the System of Air Quality and Weather Forecasting and Research (Safar), stubble emissions on average contributed to 14.6% of Delhi’s overall PM 2.5 concentration last winter, with the daily contribution peaking at 48% on November 7. The highest contribution of stubble burning to PM2.5 in Delhi for a single day was 46% in the year 2020, 43% in 2019 and as high as 58% in 2018, Safar data shows.

Gufran Beig, founder, project-director at Safar says while the impact of stubble burning on Delhi at present is negligible, owing to the ‘transition’ phase, however, wind speeds will pick up pace, with stubble burning expected to contribute in some form to Delhi’s AQI from mid-October.

“Long-range transport of stubble emissions may start from October 10 onwards and the fire count is now starting to gather pace. When the fire count is fairly high and the intensity of fires is strong, these emissions can travel a fairly long distance and end up reaching Delhi, as long as the upper-level winds are strong. Even winds of 10-15 km/hr can bring the impact of paddy burning to Delhi,” says Beig.

Satellite data shows stubble emissions can travel all the way across the Indo-Gangetic Plains. However, the pollutants start to disperse after a few hundred kilometres and generally lose their potency after crossing Delhi-NCR. The longer the distance they travel, the lesser the impact and emissions. With Delhi-NCR's vicinity to Punjab and Haryana, it is often impacted the most by these emissions.

Mercury begins to drop with meteorological conditions becoming adverse

Delhi’s pleasant weather is quick to give way to a chilly period, where nights become cool, and the daytime temperature also drops to around 32 degrees. By end of October, the night-time temperatures dip below the 20-degree mark. The lower the temperature, the ‘mixing height’ — an invisible boundary layer which traps pollutants — comes closer to the surface. The lower the mixing height, the less space there is for pollutants to disperse freely, which also gives Delhi a ‘haze’ during this period.

Low temperatures also slow down the dispersion process and can give periods where wind speed is almost ‘calm’, without any movement of local pollutants taking place.

Dipankar Saha, former head of CPCB’s air laboratory, says October stubble emissions can start reaching Delhi in October, with local conditions, particularly calm winds and low temperature not only trapping local pollutants over the capital, but they also allow for stubble emissions to accumulate.

“October is the period when we first start to see haze appear. This peaks in early October and Diwali and its date is also a key factor around it. As meteorological conditions become adverse and there are no strong winds to disperse pollutants, any form of emissions — be it local or through stubble burning, gets trapped in Delhi’s air,” he says.

An early Diwali may lead to a highly polluted October-end

This year, Diwali will be celebrated on October 24, a good 10 days earlier than last year’s November 4, and 20 days earlier than 2020's date of November 14. Despite a ban on firecrackers, Delhi flouts it on a large scale, with firecracker emissions getting trapped in Delhi’s air for a day or two. Last year, Delhi’s AQI on Diwali day was 382 (very poor), with it touching 462 (severe) the day after, on November 5 — Delhi's worst for the day-after Diwali, since 2016, when it was 445.

The last time Diwali was celebrated in October was in 2019. However, even then, the AQI was recorded in the ‘very poor’ category. Delhi’s AQI rose from a reading of 337 (very poor) on Diwali day, which was October 27, to a reading of 368 (very poor) the next day. Similarly, in 2017, when Diwali was celebrated on October 19, the AQI rose from 319 (very poor) on Diwali day to 403 (severe) the day after.

Anumita Roychowdhury, executive director, research and advocacy at the Centre for Science and Environment (CSE) says while meteorological conditions are relatively better in October, as compared to November, firecracker emissions can still lead to the AQI rising by over 100 points.

“This year, we will have to see how effective the ban on firecrackers is and the role an early Diwali plays in the overall pollution contribution for the winter period. The Graded Response Action Plan (Grap) is being implemented two weeks earlier than normal this year and is also being based on a forecasting system as compared to previous years, where action was taken after the AQI touched a particular level. This may also prove to be beneficial and if it appears that the AQI is expected to deteriorate considerably around Diwali, then preventive measures under Grap can be implemented in advance,” she says.

Unveiling 'Elections 2024: The Big Picture', a fresh segment in HT's talk show 'The Interview with Kumkum Chadha', where leaders across the political spectrum discuss the upcoming general elections. Watch now!

Continue reading with HT Premium Subscription

Daily E Paper I Premium Articles I Brunch E Magazine I Daily Infographics
freemium
SHARE THIS ARTICLE ON
Share this article
SHARE
Story Saved
Live Score
OPEN APP
Saved Articles
Following
My Reads
Sign out
New Delhi 0C
Friday, March 29, 2024
Start 14 Days Free Trial Subscribe Now
Follow Us On