Delhi Records Second Cleanest Post-Diwali Air Since 2015, Here’s Why

Delhi experienced its second cleanest post-Diwali air since 2015, with the Air Quality Index (AQI) remaining in the “very poor” range instead of escalating to “severe” levels. Strong wind ventilation at speeds up to 16 kilometers per hour played a key role in dispersing pollutants, offering a much-needed respite.

In spite of increased firecracker use, stubble burning, and emissions from local sources pushing AQI to “very poor” late Thursday, the city’s air showed significant improvement due to a natural ventilation effect. The 24-hour AQI gradually increased overnight, from 328 in the early evening to 362 by Friday morning at 9 a.m. However, strong, sustained winds dispersed smoke, reducing AQI to 339 by 4 p.m. and further down to 323 by 7 p.m.

Meteorologist Mahesh Palawat from Skymet explained that the city’s air quality was influenced by the interplay between temperature and pollution. “Higher temperature keeps mixing height high and allows pollutants to move and disperse freely. Low temperature slows down wind speed and also traps pollutants closer to the surface through a phenomena called inversion.” On Thursday, mixing heights reached 2,100 meters, which is notably higher than typical winter levels of 200-300 meters, helping pollutants to spread more effectively.

On Diwali day, while some areas in south Delhi reduced firecracker use, other areas saw an uptick in fireworks, coupled with an increase in stubble burning. TOI reported that stubble burning accounted for 27.61% of Delhi’s PM2.5 levels on Diwali, compared to 8.4% on Wednesday and 1.8% on Tuesday, illustrating how this practice contributed significantly to pollution levels.

Anumita Roychowdhury, executive director of Research and Advocacy at the Centre for Science and Environment (CSE), highlighted the unique conditions that helped mitigate the pollution impact this year. “Even though the air quality on Diwali was in the very poor category — hitting severe levels during midnight — these conditions dissipated quickly the next day to reach moderate levels by the afternoon. This is mainly because of relatively warm conditions with reasonable wind speed and natural ventilation. The deep calm winter inversion conditions are yet to set in.” She noted, however, that October saw more poor air days than in previous years, despite farm fires contributing only around 1-3% to pollution, underscoring the impact of local pollution sources.

The morning following Diwali was hazy, but as the wind picked up, conditions improved. According to weather reports, wind speeds ranged between 12-16 km/h on Diwali, decreasing around midnight but rising again Friday morning, which facilitated the clearance of pollutants. While the 2024 post-Diwali AQI marks an improvement, spikes in PM2.5 in certain areas remain a concern, with levels surpassing the national permissible limits by over 30 times in some instances.