This year, the day after Diwali has seen the cleanest air in Delhi since 2015. This, despite the AQI remaining in the ‘very poor’ category on both Diwali and the day after, with pollution levels recording a surge when firecrackers were set off in violation of Delhi government’s ban.
The AQI (air quality index) on Diwali day was 312, while it was 303 on Tuesday, according to data from the Central Pollution Control Board (CPCB). AQI between 301 and 400 is considered to be ‘very poor’. The AQI on Diwali day this year is the cleanest that the city has seen since 2019.
CPCB data available from 2015 onwards shows that the worst post-Diwali air quality was seen last year, when the day after Diwali recorded an AQI of 462, in the ‘severe’ category. The AQI on Diwali day last year was 382. Since 2015, there have been four years when the day after Diwali saw ‘severe’ air quality.
Meteorological conditions played a role in the improved air quality this time. Gufran Beig, founder project director, SAFAR, said that wind speed helped prevent the accumulation of pollutants and an early Diwali this year meant that the temperature is still relatively warm.
“Wind speed picked up around 2 am on Tuesday. In the early hours of the morning, pollutants would have normally accumulated, when the temperature is cooler and the boundary layer comes down and winds become slower. But the wind speed picked up, aiding dispersion. The highest AQI levels were recorded around midnight, after which it improved and settled at 323 in the morning,” Beig said.
“It appears that there may have been a reduction in firecracker emissions compared to last year. The air quality did not deteriorate as much as it could have. But the contribution of firecrackers to the emission load is still unknown and could take a few days to determine,” Beig said.
The contribution of crop residue burning in Punjab and Haryana has also been low so far this year. The wind direction, which has been from the westerly-southwesterly direction of Delhi since Monday, has not been favourable to transport stubble burning smoke from the northwest, Beig explained. According to an update issued by the SAFAR forecasting system, the share of stubble burning to PM2.5 levels in Delhi was around 5.6% on Tuesday. In contrast, the contribution of stubble burning to Delhi’s air on Diwali day (November 4) last year was 25%, and 36% on the day after Diwali, according to data from SAFAR.
“Some control measures may have worked. It may be that people opted for firecrackers which release fewer toxic fumes though they produce sound,” Beig added.
Sachchida Nand Tripathi, Professor at IIT Kanpur, also said that there may have been some contribution of meteorology. “Crop residue burning has also not intensified compared to previous years. Additionally, lower temperatures would make the boundary layer thinner and would not allow particulate matter to disperse quickly. But the strong wind has offset this,” he said.
Anumita Roychowdhury, executive director, research and advocacy, Centre for Science and Environment, pointed to a combination of factors. “Diwali has happened early, with warmer weather and much before the intense inversion conditions set in. Comparatively, there have been better wind speeds and crop fires have also not been as intense. It’s difficult to assess if there were reduced firecracker emissions,” she said.
According to data from the Indian Agricultural Research Institute, there have been 5,798 crop residue burning events recorded in Punjab from September 15 to October 25. This is lower than the figure of 6,134 recorded till October 25 last year.