Strong and consistent winds were observed in the first half of December, followed by record-breaking rainfall in the latter half of the month. This combination resulted in an average Air Quality Index (AQI) of 294, making it Delhi's cleanest December since the AQI was introduced in 2015.
However, an average AQI value of 294 is about three times worse than a "satisfactory" AQI, which is considered "poor" by CPCB criteria. Furthermore, the month had six days of "severe" air, defined by an AQI score of 400 or higher, resulting in strong construction restrictions, restricted car mobility, hybrid courses, and staggered office hours.
The CPCB ranks AQI levels as "good" between 0 and 50, "satisfactory" between 51 and 100, "moderate" between 101 and 200, "poor" between 201 and 300, "very poor" between 301 and 400, and "severe" above 400.
Despite the limitations, the average AQI of 294 was a significant reduction over the 348 reported last December, 319 in December 2022, and 336 in December 2021. The records show that the second-best AQI reading was 300, which was obtained in 2015.
Experts stated that, with the exception of a six-day period of "severe" air in the third week, conditions were mostly favourable for pollutant dispersion this month.
"Winds dominated the first half of December, with little moisture in the air. This meant that we weren't recording moderate or dense fog, and winds were allowing pollutants to spread. This kept the AQI under control until a western disturbance slowed winds in the third week of December," Hindustan Times quoted Skymet's vice president, Mahesh Palawat, as saying.
According to Palawat, slow winds caused additional moisture and stagnation. Still, the same spell was followed by a fairly active western disturbance around December 26, which improved Delhi's air quality again. “This latest western disturbance was much stronger. We saw rain help settle pollutants and strong surface winds post that, which have led to the AQI improving significantly,” he said.
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