Chloride-rich particles linked to visibility reduction over Delhi

Researchers at the Indian Institute of Technology (IIT) Madras have found chloride to be the highest inorganic fraction in particulate matter, primarily responsible for haze and fog formation in Northern India, including the national capital.

The study, published in the journal Nature Geoscience, not only provides the scientific explanation for the source of high chloride in PM2.5 mass over Delhi but also quantifies its role in haze and fog formation and visibility reduction.

"With the results from first couple of days, it was very clear to us that New Delhi is different; because generally for a polluted urban region like Delhi, one would expect sulfate to be highest inorganic fraction of particulate matter; however, we found chloride to be the highest inorganic fraction of particulate matter," R. Ravikrishna from IIT Madras, said in a statement on Wednesday.

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

However, the role of PM2.5 and detailed chemistry of haze and fog formation over the national capital was poorly understood, the team said.

For the study, the team deployed state-of-the-art instruments to measure the chemical composition and other important properties of PM2.5, along with relative humidity and temperature in New Delhi, which were operated round the clock for one month with extreme care and dedicated expertise.

The findings were surprising for the researchers and unfolded the mystery of high chloride in PM2.5 and scientifically unravelled its precise role in fog and haze formation over New Delhi.

"We realised that despite absolute PM2.5 mass burden over New Delhi being much less than other polluted megacities around the world, including Beijing, the pollution and atmospheric chemistry of New Delhi is much more complex to understand," said researcher Sachin S. Gunthe, Associate Professor, Department of Civil Engineering, IIT Madras.

"This work put forward importance of measurements and modelling approaches to scientifically conclude that half of the water uptake and visibility reduction by aerosol particles around the national capital is caused by the hydrogen chloride (HCl) emissions, which is locally emitted in New Delhi potentially due to plastic contained waste burning and other industrial processes," Gunthe added.