Why there is sudden rise in Delhi's air pollution: Temperature inversion ?

Everybody is aware of the 'Kadwi Hawa' of Delhi but what makes the air pollution situation in Delhi worse during winter months? We all know that industrial emissions, vehicle exhaust, construction activity, etc, are the biggest contributors to pollution in Delhi throughout the whole year, but the situation specifically gets worse during winter months when Air Quality Index (AQI) crosses its maximum limit. What aggravates the already bad situation? There are three major factors to it:
  • Inversion: Usually the temperature in the atmosphere decreases with height, hence warmer air close to the earth's surface moves upwards and the air does not get stagnant. However, during inversion cold air gets trapped close to the surface overlain by a layer of warm air. This phenomenon cap the movement of air. During winter months sunrays do not sufficiently heat the surface which causes the trapping of this cold air and with it the pollution in Delhi.
  • Stubble burning: After harvesting paddy during the October-November months, there is plant residue left on the ground which is called stubble. Farmers in Punjab and Haryana burn the stubble, which is one of the biggest contributors to the deterioration of air quality in Delhi. This burning emits particulate matter (PM), CO, CO2, NO2, and other harmful pollutants in huge numbers.
  • Wind pattern: In winter 72 % of the wind blows into Delhi from the northwestern part of India and Pakistan, these winds are not too strong during the monsoon season. These winds bring pollutants emitted by stubble burning in the adjoining states of Punjab and Haryana.
The sudden increase in Delhi air pollution can be attributed to the coincidence of all these factors. Stubble burning is followed by the northwestern winds blowing into Delhi. These winds bring with them the pollutants caused by stubble burning which get trapped in Delhi's air because of the temperature inversion. 

This all happens usually at the onset of winter because at this time temperature is not sufficiently low to condense the water vapor and not sufficiently high to evaporate it. It is this temperature that just traps the particulate matter and remains suspended in the atmosphere because of the stagnant atmospheric conditions.

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