Winter months in Delhi come with enough problems. While freezing temperatures are an obvious nuisance, smog in the air adds to the troubles. Hazy vision due to smog, pollution are serious concerns for people living in Delhi. But what causes this haze?

A recent study by the Indian Institute of Technology (IIT) Madras suggests that Chloride-rich particles resulting from plastic burning may be primarily responsible for haze and fog formation in Northern India, including Delhi, during the winter months.

The study, published in the journal Nature Geoscience, may help develop better policies to improve the air quality and visibility in North India.

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

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

The new study found that chloride-rich particles were the highest inorganic fraction in particulate matter, primarily responsible for haze and fog formation in the region.

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

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

The latest study greatly enhances our understanding about the precise role of PM2.5 in the chemistry of fog formation, which will help policymakers to frame the better policies for improving the air quality and visibility over the national capital.

The researchers noted that during the winter season, most of the Indo Gangetic Plain invariably is engulfed in a dense fog and haze, particularly during the months of December and January.

Over the national capital, dense fog negatively impacts the air and surface transport resulting in huge financial losses and jeopardise human lives, they said.

The study explained that complex chemical reactions involving HCl, which is directly emitted in the atmosphere from plastic contained waste burning and few industrial processes, is primarily responsible for high PM2.5 chloride and the subsequent haze and fog formation over Delhi during chilly winter nights.

The team, including researchers from the Harvard University, US, and Manchester University in the UK, deployed state-of-the-art instruments to measure the chemical composition and other important properties of PM2.5, and the relative humidity and temperature in Delhi.

Professor R Ravikrishna, from the Department of Chemical Engineering, IIT Madras, noted that with the results from the first couple of days, it was very clear to us that Delhi is different.

"Generally for a polluted urban region like Delhi, one would expect sulfate to be a highest inorganic fraction of particulate matter, however, we found chloride to be the highest inorganic fraction of particulate matter," Ravikrishna, who was also part of the study, said.

The researchers explained that HCl from various sources combines with ammonia, which is emitted in great amounts over this region.

The resulting ammonium chloride (NH4Cl) condenses to aerosol and exponentially increase the water uptake ability of aerosol particles resulting in the increase in size, eventually leading to dense fog formation, they said.

In the absence of the excess chloride, the fog formation otherwise would be suppressed significantly, according to the researchers.

The study emphasised that plastic burning emits toxic substances in the atmosphere impacting human health, and these emissions are for the first time linked to visibility and climate.

Plastic-contained waste burning can emit highly toxic chemicals called 'dioxins', which can accumulate in food chain causing severe problems with reproduction and immune system, the researchers said

"Given that we find plastic burning as a potential cause of the reduced visibility, we hope these findings will help policymakers to efficiently enforce and implement policies that are already in place towards regulating open burning of plastic contained-waste and other potential chlorine sources," Gunthe added.

(With PTI inputs.)