Delhi air pollution: Is it smog or fog? Opinions differ

The National Capital Region is going through this year’s worst spell of air pollution — even worse than the post-Diwali haze that engulfed the city. Most experts stopped short of terming the phenomenon as “smog” because they said smoke — the primary ingredient of smog — was still missing. However, a senior official of the Delhi Pollution Control Committee (DPCC) differed, and said that this was indeed smog.

What is the difference between fog and mist?

Fog and mist are formed when water vapour in the air condenses. During condensation, molecules of water vapour combine to form tiny water droplets that hang in the air.

“The only difference between fog and mist is that during fog visibility is reduced to less than one kilometre, and during mist it is more than one kilometre,” said a senior officer of the regional meteorological department.

On Tuesday the visibility at IGI Airport had dropped to around 200m around 5.30am.

Was it smog then?

Experts said that smog is combination of smoke and fog.

“It was definitely fog, but not smog. For smog, you need smoke in the air. For smoke, presence of gases such as sulphur dioxide (SO2) in high doses in the air is a must. Being acidic in nature, SO2 reacts and forms complex compounds which helps in the formation of fog,” said D Saha head of the air quality laboratory at Central Pollution Control Board.

The levels of SO2 have, however, dropped over the past few months in Delhi for several reasons such as witching over to clean fuels under BS1V, shutting down of power plants, banning of furnace oil and pet coke among others.

While the permissible limit of S02 is 80 micrograms per metre cube, in Delhi it is well below the limit. In Anand Vihar, widely considered a “pollution hotspot”, the level of SO2 was around 13.8 micrograms per-metre-cube.

“Apart from the usual culprits such as SO2, other gases and elements such as soot and organic gases such as benzene, toluene and formaldehyde also help in forming smoke. The main sources are burning of fossil fuels and industries,” said SN Tripathi, coordinator of the Centre for Environmental Science & Engineering at IIT Kanpur.

But experts said that as their primary sources such as polluting industrial units, diesel generators and power plants have been cut down recently, chances are less that SO2 would build up again.

Opinions differ

A senior official from DPCC, however, insisted that it was smog and not a simple fog as other experts claim. “Fog, in which you have only water particles, is white in colour. But in Delhi, the colour of the fog is grey. There are definitely pollutants and toxic gases in the air. We can term it as smog.”