Explained: What Causes Fog And Haze In Delhi In The Winters?

For three consecutive mornings, a thick layer of fog blanketed Delhi and its neighbouring states of Uttar Pradesh, Punjab and Haryana. On Monday morning a dense layer of fog enveloped the national capital and adjoining areas with the minimum temperature settling at eight degrees Celsius, normal for this time of the year.

On Tuesday, a van and a truck collided in Noida, claiming several lives, due to poor visibility. And now the foggy winter is likely to recur over the Indo-Gangetic plains over the next several days. 

What causes fog and haze in Delhi?

The Air Quality Index (AQI) in the national capital has consistently and drastically reduced, leaving the government scrambling for solutions.

Meteorologists explain that the reasons for a dense fog are generally four --- low winds, low temperatures, availability of moisture and pollution particles which act as surfaces for condensation. The presence of pollutants accentuates foggy conditions.

Fog is basically suspended water droplets or ice crystals in the air very close to the surface.

Many studies in the past have identified PM2.5 (particulate matter or aerosol particles with a diameter of fewer than 2.5 micrometres) as a major pollutant, responsible for haze and fog formation over Indo-Gangetic plains including Delhi. 

Further, in 2021, a study led by researchers from the Indian Institute of Technology Madras (IIT-M) found out that the burning of plastic waste is primarily responsible for haze and fog formation in northern India.

The study explains that complex chemical reactions involving Hydrochloric Acid (HCl), which is directly emitted into the atmosphere from the burning of plastic waste and from a few industrial processes, are primarily responsible for high PM2.5 chloride and the subsequent haze and fog formation over Delhi during chilly winter nights.
 
It should also be noted that during the winter season, the inversion height, the level beyond which pollutants cannot be pushed further up in the atmosphere, gets reduced. As a result, this leads to the accumulation of pollutants near the ground. Hence, water vapour gets more particles near the ground to cluster around and condense.

Delhi also experiences direct emission of atmospheric hydrochloric acid (HCl) from small-scale industries such as metal-, e-waste-, and plastic-processing as well as biomass- and plastic-contained waste burning. This HCl then reacts with excess ammonia in the atmosphere to form particulate ammonium chloride (NH4Cl).

The current situation in Delhi

On Tuesday, visibility dropped to 50 metres at the Palam and Safdarjung airports between 5:30 am and 7 am, an IMD official said.

On Tuesday, 11 trains were reported running late by one to five hours. Flight operations, however, were unaffected, an airport official said.
Satellite images showed a thick layer of fog over Punjab, Haryana, Delhi, north Rajasthan and Uttar Pradesh.

"Due to moisture and light winds at lower tropospheric levels over Indo-Gangetic plains, dense to very dense fog in many/some pockets very likely over Punjab, Haryana, Chandigarh, Delhi, Himachal Pradesh and Uttar Pradesh in night/morning hours during the next three days and dense fog in isolated pockets for subsequent two days," the Met office had said in a statement on Monday.