While common causes of pollution in north India are the smoke from power generators and power plants, vehicles, burning of crop stubble in the region of Punjab and Haryana, etc., the geographic particulars make the north-Indian cities like Delhi more susceptible to air pollution that the rest of the country, to the extent of making it severy unhealthy to breathe and visibility limited to as low as 400 metres on some days in the winter season.
Anti-cyclonic Weather Conditions
The atmospheric circulation over north India happens to be such that the areas surrounding the national capital Delhi come in a zone of Anti-cyclonic weather conditions during winter. This condition keeps the air movement calm, and aggravate building up of pollutant concentration in lower troposphere, as calm surface winds are not able to disperse pollutants over a larger region.
Westerlies blowing over north India bring dust from the Persian Gulf dust storms. Though the dust storms in the gulf are common in summers, they are particularly problematic to India during the winters, when they are picked up by the westerlies which blow through north India and give rise to smog formation over the region. Though the contribution of Gulf storms in polluting north Indian air is not so substantial on a regular day, a SAFAR (System of Air Quality and Weather Forecasting and Research) report attributed 40% of pollutant concentration in Delhi to gulf dust storms on a peak day in November 2017. Climate change and desertification are further intensifying these dust storms.
High density of population in the Gangetic valley
The Gangetic Plain is among the world’s most densely populated areas. Dense and high population translates into concentration of pollution over pockets of areas. Construction activities are a major employer for workers, and a major cause of pollution in urban centres; dust from transportation of uncovered dry cement, grinding of building materials in open, etc, go unchecked.
Burning of crop residue in the states of Haryana and Punjab
Stubble burning in Haryana and Punjab has been cited as a major cause of pollution in the Delhi region. Agriculture in these states is highly mechanised with very limited role of manual labour. Farm sizes too are larger in these states. For these reasons, farmers find burning stubble to be a cheap and easy way to dispose of the crop residue. Recently, due to environmental awakening, the government has started providing alternate means for farmers to dispose of their crop residue, such as using it for generating biofuel- ethanol.
See real-time air pollution and wind map of earth here.