Number Theory: Better post-Diwali air in Delhi mostly coincidental

According to the Air Quality Index (AQI) bulletin published by the Central Pollution Control Board (CPCB), this year (AQI) saw the cleanest air since 2015, when the body started measuring air quality. But that may perhaps not be the best measure. This is because some air quality stations can stop functioning during the high pollution episodes of Diwali.

That Delhi woke up to a (relatively) cleaner morning after having celebrated Diwali was more than visible without looking at pollution data. What an HT analysis of the stats does tell us is that policy intervention has very little to do with post-Diwali morning pollution levels being better than what they have been in the recent past. Here are four charts which explain this in detail.

People burn firecrackers on Diwali at Mayur Vihar Phase2, in New Delhi, India, on Tuesday, October 25, 2022. (HT Photos)

How was the air this Diwali in the national capital?

According to the Air Quality Index (AQI) bulletin published by the Central Pollution Control Board (CPCB), this year (AQI) saw the cleanest air since 2015, when the body started measuring air quality. But that may perhaps not be the best measure. This is because some air quality stations can stop functioning during the high pollution episodes of Diwali.

Depending on which station stopped functioning in which year, CPCB’s average for the city can represent different parts of the city in different years. Moreover, CPCB’s bulletin also only tells us about the 24-hour average ending at 4pm. To solve for this problem, HT has checked which stations had data on PM2.5 (particles suspended in the air with diameter 2.5 micrometers or less) concentration for every hour from 6pm on Diwali day to 9am on the day after from 2019 to 2022. There are four such stations in Delhi, which are located in Aya Nagar, Narela, Punjabi Bagh, and Aurobindo Marg. Except the 6-8pm interval, the average PM2.5 concentration at these stations this year has been better than all Diwalis since at least 2019, up to 9am on the day after Diwali. In the 6-8pm interval, Diwali 2019’s air was cleaner than this year’s.

Diwali 2022 has been celebrated on October 24, much earlier than it was in 2021 (November 4) and 2020 (November 14), and slightly earlier than even in 2019 (October 27). Diwali being celebrated earlier means that temperatures are higher and pollutants do not settle near the surface due to cold air. An analysis of the India Meteorological Department’s (IMD) gridded database shows that both maximum and minimum temperatures were higher than what they were in 2021 and 2020 in the run up to Diwali. In the early hours of October 25 (when minimum temperatures are recorded), the minimum temperature was 1.5 degrees more than in 2020 and less than half a degree below the 2021 and 2018 minimum for the morning after Diwali.

In addition to higher temperatures, Delhi also experienced another favourable climatic factor this Diwali : wind speed. Data from three monitoring stations, which have consistent data on wind speed shows that the average wind speed was the highest on Diwali day since 2020 from 10am to 5pm. After that point, too, it hasn’t been as slow as it was in 2021 – when Delhi experienced one of its most polluted Diwalis in recent years – although it has been slower than in 2020. What has helped is wind speed picked up once again in the morning of October 25: from 3.3km/hr in the 8-9am interval to 6.6km/hr in the 12-1pm interval at the stations used in this analysis. As is expected, this air is likely to have aided in at least some dispersion of the smoke generated on Diwali.

That Diwali has been celebrated on days almost three weeks apart in the past four years also means that it has been celebrated at different points in the crop cycle. This in turn decides how many farm fires are burnt. Last year, when meteorological conditions were the least favourable for good air, farm fires were also at their highest level for any Diwali since 2019.

This year, when meteorological conditions have been more favourable, active fires in Punjab and Haryana were much lower than in 2019 and 2021 (courtesy delayed harvests and an earlier Diwali). To be sure, they also crossed the 1,000-mark for the first time this cropping season the day before Diwali, suggesting that air quality may take a turn for the worse in the coming days in the national capital if the weather gods do not do anything about it.