Home Cities Worst Dec air spell only after mercury dips further: Experts

Worst Dec air spell only after mercury dips further: Experts

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Though Delhi’s pollution levels worsened into the “severe” zone for the first time in a month on Sunday, experts said the worst of the city’s December phase of bad air is still to come, and will kick in after a further dip in temperatures towards the latter half of the month.

The Capital on Sunday logged an AQI of 407, the worst since 447 on November 4.

An AQI between 301 and 400 is classified as “very poor”, and one above 400 is “severe”. The AQI readings max out at 500.

Experts said that Delhi usually sees two broad spells of hazardous air quality: The worst is between October and November, driven largely by smoke from farm fires in upwind Punjab and Haryana; the next is in the second half of December and early January, thanks to falling temperatures, calm winds and a low mixing height, all of which combine to keep pollutants trapped close to the surface. The latter spiral is largely on account of Delhi’s own pollutants, such as dust, vehicular emissions, open waste burning and industrial emissions.

Experts said that the deterioration in AQI over the past few days shows how fast the air quality can turn if weather conditions are unfavourable.

The Capital on Sunday logged an AQI of 407, the worst since 447 on November 4.

Anumita Roychowdhury, executive director of research and advocacy at the Centre for Science and Environment (CSE), said forecasting models used by the Commission for Air Quality Management (CAQM), were unable to forecast the “severe” air day. “This shows the weather can turn adverse quite quickly at this time of the year, and that wind speeds can drop suddenly, leading to local emissions gradually accumulating,” she said.

“Till the end of December and early January, temperatures are fairly low and this brings down the mixing height, which leads to local emissions getting trapped near the surface. Fog makes it difficult for pollutants to be washed,” Roychowdhury said.

In weather science the “mixing height” is an invisible layer within which particulate matter gets trapped. The lower it is, the worse the air quality will be.

A study by the Delhi Pollution Control Committee (DPCC) last year found that between 2016 and 2020, the Capital averaged a PM2.5 concentration of 285 micrograms per cubic metre between November 1-15, the highest all year.The second highest average PM2.5 concentration was from December 16 to 31, at 218 micrograms per cubic metre.

The Indian 24-hour safe standard for PM2.5 is 60 micrograms per cubic metre.

Last year, November logged 11 ‘severe’ air days, and December had 7 (six of which were in the second half of the month). In 2020, November had nine severe air days and December had four.

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