Delhi breathes in cleanest first half of December since AQI data collection started in 2015

The Capital has recorded its cleanest first half of December ever since the compilation of air quality index (AQI) started in 2015, recording just one “very poor” air day in the first half of the month and no “severe” air day, according to the Central Pollution Control Board (CPCB) data. Besides, there were six “moderate” air days, which is also the highest number for December, and eight “poor” air days, according to CPCB.

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Delhi’s average AQI between December 1 and December 15 this year was 238, the lowest ever for December. Over the past nine years, the average AQI in the first half of December has been over 300, making this a significantly cleaner first half of the month.

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Before this, the lowest average AQI in the first half of the month was 301, in 2022. The average AQI for the corresponding period last year was 327.

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To be sure, there has been no “good” or “satisfactory” air day recorded in December since data collection started. CPCB classifies AQI between 0 and 50 as “good”, between 51 and 100 as “satisfactory”, between 101 and 200 as “moderate”, between 201 and 300 as “poor”, between 301 and 400 as “very poor”, and over 400 as “severe”.

Meteorological experts said strong and consistent winds have been recorded across northwest India all through the month, with a lack of moisture and fog further helping improve the air quality. “Generally in December, we see moderate to dense fog, which makes dispersion of pollutants difficult. At the same time, winds are also slow. On average, winds have been (blowing at speeds of) over 8-10km/hr on most days, with this regularly touching 20km/hr on multiple days,” said Mahesh Palawat, vice president at Skymet meteorology.

He said that Delhi-NCR has also recorded clear skies on most days. “There has not been enough moisture in the plains, barring a few days. This has meant fog is not forming with the same intensity it was seen in mid-November,” he said.

The average AQIs between December 1 and 15 over the previous years were 322 in 2021, 335 in 2020, 321 in 2019, 330 in 2018, 304 in 2017, 367 in 2016 and 323 in 2015.

Before this year, the highest number of “moderate” air days recorded in December was three each in 2022, 2019 and 2015.

The Capital’s only “very poor” air day this month came on December 8, when the AQI touched 302. It has been in the “poor” on eight days, including on Sunday, when the 24-hour average AQI was 294, according to CPCB’s daily air quality bulletin. It was 193 (“moderate”) a day earlier.

Forecasts by the Centre’s Early Warning System for Delhi (EWS) show the AQI is likely to deteriorate to “very poor” by Monday due to a predicted drop in wind speeds.

“Delhi’s air quality is likely to be in the ‘very poor’ category from December 16 till December 18. In the subsequent six days, it should oscillate between ‘very poor’ and ‘poor’,” EWS said in its daily bulletin.

Sunil Dahiya, lead analyst at the think tank Envirocatalysts, said meteorological conditions have been playing a key role so far, stating even with favourable conditions, Delhi’s AQI did not go below “moderate”.

“This shows our background emissions are fairly high and the PM2.5 concentration did not dip below 110 micrograms per cubic metre during this period. So, while comparatively it may be somewhat cleaner, this is still over two times the permissible standard for PM2.5, which is 60 micrograms per cubic metre. If conditions become adverse again, all this pollution will again get trapped in Delhi,” he said.