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With temperature dropping -- and other meteorological factors such as wind speed slowing down, wind direction changing, haze setting in, pollution levels are again in the "very poor" and "hazardous" categories in most cities across the Indo-Gangetic plains.

The seasonal factors of firecrackers and stubble burning have, as usual, added to the problem, as the peak of crop residue burning incidents coincided with Diwali. Scientists said that it has been clearly established that the winter season favoures increase in pollution as cold air frequently settles over northern India and gets trapped under a layer of warm air.

In this phenomenon, the haze seen in the winter months is the result of a temperature inversion mostly. Since the cold air cannot rise above the warm air, pollution builds in the cold air as long as the inversion lasts. On other occasions, the 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 the number of colder days in the season, more would be the number of "poor" to "severe" air quality days ahead for the entire Indo-Gangetic Plains.

"With 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. In a nutshell, all else remaining constant, cooler conditions inhibit vertical mixing within the atmosphere. Therefore, possibilities are higher for poor air quality," Indian Institute of Technology-Bhubaneswar's Assistant Professor, School of Earth Ocean and Climate Sciences, Dr V. Vinoj said.

It is a vicious smog circle as consecutive days of bad air quality would persist.

Professor S.N. Tripathi, Head of Department, Civil Engineering at the IIT-Kanpur, said that the particulate matter (PM) changes its property after coming in contact with fog, paving way for more fog.

Tripathi, who is also a Steering Committee Member, National Clean Air Programme of the Ministry of Environment, Forests and Climate Change, said: "Definitely, intense winters would aggravate the situation. It would mean more amount of haze which would lead to increased trapping of pollutants available over the surface. This might lead to formation of smog, which would worsen the condition. All these conditions would result in a vicious smog circle, wherein we would not see clearance for days."

"Also more cold weather is associated with relatively high humidity, which increases the possibility of PM holding more water. After the fog disappears, water vapours or droplets evaporate leaving the PM behind. However, a very tiny chemistry takes place here and thus, the PM is not the same as before. It is more oxidised by that time.

"There is a strong relation between oxidised PM and fog condensation nuclei as compared to non-oxidised components. In fact, smaller droplets oxidise faster and oxidised PM are far more efficient and thus formation of fog would be much easier than the next day," he added.

Scientists have further warned of extra caution this season as weather and/or meteorological factors are beyond the control of humans and thus, focus still remains on curtailing local emissions.

"Whatever extra we would see, can be compensated only if we reduce the emissions, at least at the regional levels. But if we continue with the same amount of emissions along with unfavourable meteorological conditions, there could be some substantial increase in the pollution levels in the coming season. October had been fairly well in terms of pollution on account of extended monsoon rains and reduced stubble burning. Though pollution might be less, it was still above the permissible limits," Indian Institute of Technology-Delhi's Associate Professor, Centre for Atmospheric Sciences, Sagnik Dey, said.

"December and January are the core winter months when there is likely no cases of stubble burning. With record low temperatures likely, there would be no other option but to reduce emissions. Otherwise all clubbed together would multiply the impact and pollution would intensify manifolds," Dey, who is also the Coordinator, Centre of Excellence for Research in Climate Change and Air Pollution (CERCA), said.