This year, the 24-hour average air quality in Delhi on Diwali and the day before saw worse pollution levels than in the past two years. However, on Friday morning, pollution levels dropped significantly, marking the cleanest air since 2015, aside from 2022. The post-Diwali Air Quality Index (AQI) was recorded at 339, an improvement from 358 the day after Diwali in 2023. In contrast, 2021 saw a staggering AQI of 462.
What Contributed to Cleaner Air?
While various factors influenced air quality, meteorological conditions had the most substantial impact.
Warmer Temperatures: This year, Diwali fell in late October, coinciding with unusually warm temperatures. The India Meteorological Department (IMD) reported that October was the warmest in 73 years, with average highs of 35.1 degrees Celsius and lows of 21.2 degrees Celsius. In contrast, Diwali last year was marked by cooler temperatures, with averages around 13 degrees Celsius at night.
Cooler weather typically leads to higher pollution levels due to reduced dispersion of pollutants. As temperatures drop, the inversion height—the altitude at which pollutants can disperse—lowers, resulting in higher concentrations of airborne contaminants.
Wind Patterns: Winds were also a crucial factor. Westerly winds started at 3 to 7 kmph on Friday morning and picked up to 10 kmph by 9 am, reaching 15 kmph by noon. Wind speeds around 10 kmph are considered effective for dispersing pollutants.
As a result of the stronger winds, levels of fine particulate matter like PM2.5 and PM10, which had surged on Diwali due to firecracker use and stubble burning in nearby states, fell dramatically the following day. The 24-hour average PM2.5 level spiked to 209.1 µg/m³ by Friday morning but dropped to 198.1 µg/m³ by the afternoon. Similarly, PM10 levels peaked at 322.6 µg/m³ early Friday before falling to 308.1 µg/m³ by noon.
Anumita Roychowdhury, Executive Director at the Centre for Science and Environment, noted that the warmer weather in October likely prevented the typical cooling conditions that contribute to pollution buildup. She emphasized that while nighttime pollution from Diwali caused a significant increase in PM2.5 levels, the favorable wind speeds helped to mitigate the effects by dispersing accumulated pollutants.
Overall, while Diwali festivities led to a temporary spike in air pollution, the combination of warmer temperatures and effective wind patterns contributed to a quicker return to cleaner air in Delhi this year.
