Air was bad even before ‘severe’ AQI

Data analysed by HT shows 14 days of back-to-back “very poor” air days

Wednesday was the first “severe” air pollution day in Delhi this winter, making it the most delayed arrival for a “severe” day in a year. Does this mean that Delhi is enjoying relatively clean air this post-monsoon season?

HT has made a comparison for AQI only since 2018 because the number of air monitoring stations in Delhi prior to 2018 were less. (AFP photo)

Data analysed by HT, shows that this is not the case, thanks to 14 days of back-to-back “very poor” air days; the city has averaged an air quality index, or AQI, of 272 (classified as “poor”) in the first 44 days of the season, which is par for the course for early weeks of winter.

It is only in November that Delhi has shown a big departure from past trends of severe days…

HT has made a comparison for AQI only since 2018 because the number of air monitoring stations in Delhi prior to 2018 were less than half of the 39 that operate currently. Data since 2018 shows that Delhi generally records severe days (when AQI is 401 or higher) in only four months – October, November, December and January. In January this year, Delhi recorded three severe days, fewer than only the number of such days in 2019 (seven days) and 2021 (six days). There were no severe days in October this year, but no year had a severe day in October after 2019. Even in 2018 and 2019, only one and two severe days were recorded in October. The city, however, has clearly bucked historical trends this November, with only one severe day so far, compared to three to seven such days in the 2018-2023 period.

….and weather has played a big role in this

To be sure, warmer temperatures, which help air quality by preventing accumulation of air pollutants close to the ground, had a big role in Delhi avoiding severe days so far this year. At the Safdarjung station – considered representative of the city’s temperatures – minimum temperatures have averaged 17.6°C in November so far. This is 3.1°C warmer than the average minimum temperature in these days in the 2018-2023 period; and 1.5°C warmer than the warmest year (2019) in the 2018-2023 period. Minimum temperature is useful for tracking temperature’s impact on air quality because it is in the early hours of the day that the air quality is the worst on an hourly basis (See chart 1).

To be sure, winds have also helped air quality this November. For example, the bursting of firecrackers turns Delhi’s air severe often in the days following Diwali. This did not happen this year (Diwali was celebrated on October 31) because faster winds cleaned up the pollutants from firecrackers.

Even without severe days, the average AQI is still as bad as usual

The above trends must also be read with the fact that the threshold for severe days is arbitrary. For example, an AQI of 400 (classified as “very poor”) is not going to feel much different than an AQI of 401 (classified “severe”). Therefore, HT also checked the average AQI for the post-monsoon season. This shows that the average AQI for the first 44 days of the season this year is 272. Three years each have recorded an average AQI worse and better than 2024 since 2018.

This makes sense. While warmer temperatures have kept severe days at bay, a completely dry post-monsoon season has prevented even a single “good” or “satisfactory” day (categories assigned to AQI of up to 100) in this 44-day period. Such a dry post-monsoon season happened only in 2018 and 2020 in the seven-year period analysed here. They were much cooler years, however, and experienced somewhat worse AQI than 2024. In addition, as the accompanying chart shows, the average AQI for these 44-days has varied by only 62 points between the best and worst year from 2018 to 2024. Clearly, the swing in air quality in the post-monsoon period is never very drastic and largely the result of the weather. Or, broadly speaking, Delhi’s air is always bad at this time of the year -- and 2024 is no exception.