As La Nina makes an appearance for the second time in a row and meteorologists expect an intense winter, North India can expect to see prolonged spells of pollution over the coming months.

While fewer crop residue burning incidents and widespread rain and snow in October kept pollution under check in the region until recently, the situation is already changing, according to Climate Trends, a strategic communications initiative on climate ambition and low carbon development pathways.

Pollution levels worsen

With temperature dropping - and other meteorological factors like wind speed slowing down, wind direction, haze setting in - pollution levels are again in the ‘very poor’ and ‘hazardous’ categories in most cities across the Indo-Gangetic plains (IGP), Climate Trends quotes experts as saying.

The seasonal factors of firecrackers and stubble burning have, as usual, added to the problem, as the peak of crop residue burning incidents coincide with Diwali.

Meteorologists have been forecasting record low temperatures across IGP this year, with November and December expected to be colder than usual. GP Sharma, President, Meteorology and Climate Change at leading private forecaster, Skymet Weather, says that a second back-to-back La Nina is a large probability that may result in extreme cold from December 2021-February 2022.

ENSO gets colder

Although there is no rule book, with El Niño-Southern Oscillation (ENSO) getting colder than earlier estimates, the confidence in this forecast has grown off late. But it should be noted that the winter forecast is affected by many other factors from other parts of the globe, Sharma added.

It has been clearly established that the winter season is the favourable time for an increase in pollution. Winter-time temperature inversions contribute to the buildup of haze.

Inversions occur when cold air gets trapped under a layer of warm air. Since the cold air cannot rise above the warm air, pollution builds in the cold air as long as the temperature inversion lasts. The haze seen in the winter months is the result of a temperature inversion mostly.

Temperature inversion

Usually, air high in the atmosphere is cooler than air near Earth’s surface. Warmer air near the surface rises, allowing pollutants from the surface to disperse in the atmosphere.

More number of colder days likely in the season will invariably lead to more 'poor’ to ‘severe’ air quality days ahead for the entire IGP, especially Delhi NCR. Experts explain that wintertime is already conducive for pollution and a further drop in mercury would worsen the situation.

Potential stagnant air

Dr V Vinoj, Assistant Professor, School of Earth Ocean and Climate Sciences, Indian Institute of Technology Bhubaneswar, notes that with the drop in temperatures, there is potential for more stagnant conditions.

“However, this is assuming that winds do not change. If winds slow due to any reason and stubble or biomass burning increases during this period, the overall air quality situation may worsen in the Northern Plains, including New Delhi. In a nutshell, all else remaining constant, cooler conditions inhibit vertical mixing within the atmosphere. Therefore, possibilities are higher for poor air quality,” he added.

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