Air pollution is here to stay ….it’s not just limited to a particular time of the year, but persists all through the year now. According to the WHO, in 2016, India had 14 out of worlds 20 most polluted cities in terms of PM2.5 levels; Kanpur topping the list and New Delhi taking the sixth place.
Air quality particularly deteriorates during the winter season and drops to the “severe” or “hazardous” categories.
Pollution levels are increased during winters due to a combination of atmospheric (metereological) conditions and local pollution (emissions of air pollutants).
Generally, air is warmer near the earth surface and gets colder higher up as the altitude increases. During winters, this state is reversed i.e. the temperature is colder near the surface and the air is warmer higher in the atmosphere.
This phenomenon is called “winter inversion” and is the reason why pollution levels increase during winters, especially in areas like Delhi where air pollutant levels are already higher. This warm air above the cold air acts like a lid and traps the pollutants and does not let them escape into the atmosphere.
The effects of inversion are stronger at night, which is why air quality worsens overnight. This is also why people are asked to avoid morning walks especially during winters due to exposure to higher concentrations of pollutants at that time.
In summers, the air in the lower part of the atmosphere is warmer and lighter; so, it can rise up and carry pollutants away from the ground and mixes them with cleaner air in the upper layers of the atmosphere (”vertical mixing”).
Another reason for high pollution levels during winters is a decrease in wind speed, which further slows down the dispersal of pollutants. The pollutants therefore remain close to the surface and worsen the air quality. The ‘trough like topography’ in North India, due to hills and mountain ranges coupled with stagnant air during winters also traps the toxic air over a large part of the country.