Recent winters show a 20% improvement in winter air quality compared to the pre-pandemic years from 2015 to 2018, according to an analysis by the Centre for Science and Environment (CSE). However, Diwali this year may trigger yet another smog episode, the analysis shows.
At the ten oldest air quality monitoring stations in Delhi for which data is available, the average PM2.5 level in the 2015-16 winter was 193 µg/m3. At these ten stations, which include the ones at Anand Vihar, Mandir Marg, Punjabi Bagh, Dwarka and R K Puram, the average PM2.5 concentration in the 2021-22 winter was 151 µg/m3, while it was 162 µg/m3 in the 2020-21 winter, and 153 µg/m3 in the 2019-20 winter. The period from October 1 to February 28 is taken as winter.
For all 37 monitoring stations for which data is available from 2018-19 onwards, the average PM2.5 level last winter was 157 µg/m3, which is down from 179 µg/m3 in the winter of 2018-19.
CSE’s analysis notes that the winter PM2.5 levels “improved during the pandemic but stagnated post pandemic”.
This year, the average PM2.5 level of 37 µg/m3 in Delhi during the monsoon months of July, August and September was the second lowest in eight years after 36 µg/m3 recorded in the monsoon of 2020 which was preceded by a clean summer brought by lockdowns.
With rainfall extending into October, the city has also seen the “cleanest start” to the winter since 2018. The average PM2.5 level for the first two weeks of October this year was 43 µg/m3, lower than the 59 µg/m3 in 2021, 92 µg/m3 in 2020, and 72 µg/m3 in 2019.
However, with air quality now beginning to deteriorate and farm fires in Punjab and Haryana rising, “Diwali might yet again trigger a deadly smog episode,” according to the analysis. Though Diwali is falling earlier in the season when warmer conditions persist and winds might help dilute pollutants, Diwali night can add 300 to 600 µg/m3 of PM2.5 to Delhi’s air if it’s business as usual, CSE’s analysis shows. With fires from crop residue burning also set to increase, “conditions are ripe for a severe smog episode to start from Diwali night”.
Last year, the PM2.5 concentration on Diwali night was 747 µg/m3, the highest figure from 2017 onwards. This was higher than 613 µg/m3 in 2020, and 445 µg/m3 in 2019 on Diwali night.
The city’s first winter smog episode last year was around Diwali, from November 4 to 13. A ‘smog episode’ is when the PM2.5 levels remain in the ‘severe’ category for three consecutive days. Last winter recorded three smog episodes and 20 smog days, which is more than 14 smog days the previous winter and 19 smog days in the winter of 2019-20.