Expect light rain on Wednesday but pollution may shoot up again, say experts

The high moisture content would trap the local pollutants hindering their dispersal process. This could help the pollution level to shoot up again from Thursday.

Delhi is likely to receive a drizzle on Wednesday, which might help in bringing down the pollution level of the city by some extent temporarily.

CPCB data states that pollution levels in Delhi had shot up after heavy rains during the monsoon months. But even then the air quality was never ‘good’ in June and fluctuated between moderate and poor categories. (Burhaan Kinu/HT PHOTO)

Experts, however, fear that the air quality is likely to become fouler once the rain is over. But they were sure that the pollution was unlikely to hit the ‘severe’ zone again.

The rain will also ensure that moisture level in the city’s air builds up again. The high moisture content would trap the local pollutants hindering their dispersal process. This could help the pollution level to shoot up again from Thursday.

The Indian Meteorological Department has forecasted that Delhi could once again witness some shallow fog and mist in the morning till Saturday. SAFAR, which is under the ministry of earth sciences, has also forecasted that levels of particulate matter could shoot up once the rain end.

“There can be a rise in pollution levels after the rain. This is because the moisture level will increase which will help trap pollutants. But this time we don’t expect the pollution to touch severe levels,” said D Saha head of the air quality laboratory of Central Pollution Control Board (CPCB).

Scientists said that fog and aerosol hazes likely amplify each other and form a vicious cycle.

“On one hand aerosol – a kind of pollutant – serves as ‘seeds’ that make it easier for fog to form, on the other hand the moisture droplets in the fog acts like small ‘chemical factories’ that help gaseous pollutants to graduate into haze causing aerosols,” said SN Tripathi, coordinator of the Centre for Environmental Science & Engineering at IIT Kanpur.

CPCB data states that pollution levels in Delhi had shot up after heavy rains during the monsoon months. But even then the air quality was never ‘good’ in June and fluctuated between moderate and poor categories.

Experts said that the rate of dispersal of pollutants depends much on the type of rain.

“Heavy rain with large and high velocity rain drops is able to clear pollutants more effectively than a drizzle with smaller water particles falling at a lower speed,” said Dilip Ganguly, assistant professor at Centre for Atmospheric Sciences in IIT-Delhi.

Usually rains wash away the particulate matter. But there are some particulate matters, whose size varies between 0.1 micron and 1 micron. They are not cleaned and linger even after heavy rains. They are hydrophobic and tend to bounce away whenever a rain drop hits them.

This phenomenon, experts say, is known as Green Field Gap.

“Not all types of rain is equally effective. Longer durations of drizzle are more effective that short lived heavy showers,” said Abhijit Chatterjee, assistant professor of environmental sciences section at Bose Institute in Kolkata.