Bad air quality days continue to haunt the Indo-Gangetic Plains with the air quality Index (AQI) oscillating between 'poor' and 'severe' categories.
Even the brightest days in Delhi-NCR during the first half of December have kept air quality in the 'poor' category.
As stubble burning subsided across the northwestern states of Punjab and Haryana after the sowing of the Rabi crop, the burden of high air pollution levels rests on emissions from sources like industrial activities, transport, regional pollution corridors and prevailing weather conditions.
For dispersion of pollution, cities need to cut down on emissions across sectors and at source, say experts.
However, weather conditions in the form of rain would bring some immediate relief but with increasing climate change these systems have also become inconsistent.
According to meteorologists, there has been an absolute absence of winter rain across the plains. In the wake of this, a stable wind pattern can be seen over the region and speed is also very slow.
The minimum temperatures have been plunging continuously and settling in the range of 4-10 degrees Celsius. As the temperatures dip, cold north-westerly winds become heavier because of the increased moisture content.
This also increases the capacity of the winds to capture pollutants close to the earth's surface.
"With unabated cold north-westerly winds reaching the plains, minimum temperatures will now drop and settle in single digits. With this, dispersing pollutants from the atmosphere would be very difficult. The more the minimum temperatures dip, thicker will be the inversion layer. And the thicker the inversion layer, it would be more difficult for sun rays or winds to penetrate through this layer and disperse the pollution level," Mahesh Palawat, Vice-President, Meteorology and Climate Change, Skymet Weather, told IANS.
During winters, the air in the planetary boundary layer (the lowest part of the atmosphere) is thinner as the cooler air near the earth's surface is dense.
The cooler air is trapped under the warm air above that forms a kind of atmospheric 'lid'. This phenomenon is called winter inversion. Since the vertical mixing of air happens only within this layer, the pollutants released lack enough space to disperse in the atmosphere.
A significant correlation was also found between the elevated levels of air pollutants (PM10, PM2.5, sulphur dioxide, nitrogen dioxide, and carbon monoxide) and low ozone levels in winter months and meteorological parameters such as air temperature (low values), air humidity (high values), and wind speed (high values) at the same time.
Usually by this time of the year, the region witnesses at least one or two spells of winter rains and snowfall.
However, due to the absence of any strong Western Disturbance (WD) up in the Himalayas, rain has been evading the entire plains.
While on and off feeble Western Disturbances have been visiting, but they were not capable of triggering any significant weather activity.
"It is not only about the Indo-Gangetic Plains but most of the country saw air quality deteriorating. PM 10 was not the only contributor but carbon monoxide levels were also high. This shows construction activities were not alone but combustion was also high. Besides, large-scale meteorological phenomena like La Nina are also contributing by slowing down the circulations. We need more 'Early Warning Systems' to predict air quality," said S.N. Tripathi, Professor, Civil Engineering Department, IIT Kanpur.
Aarti Khosla, Director, Climate Trends, said given that meteorological and reduced emission factors are both at play, AQI levels across the Indo-Gangetic Basin should have been better had winter rains occurred as expected.
"With temperatures dipping across the plains in the last couple of days, pollution levels have directly and proportionately gone up. It is clear that while there is no substitute for reducing pollutants at source for better air quality, meteorology plays an intricate role in air quality levels. While it is heartening to see that air pollution in November 2022 might be relatively better in some cities across the Indo-Gangetic Plains, it still remains much higher than the CPCB's safety levels, which in any case are more relaxed in comparison to the WHO's stringent air quality guidelines," she added.