Strange chemical process behind Delhi’s unique extreme night-time smog discovered

Phenomenon behind poor air quality is unique and not observed elsewhere in the world, study says

Atmospheric chemical reactions that drive New Delhi’s unique extreme night-time smog have been uncovered by scientists.

Delhi has been ranked the world’s most polluted city for the last three years and has gained infamy for its air pollution and harsh winter conditions that have drastically reduced the quality of life for residents of the national capital.

The city’s smog has also been linked to a large number of premature deaths.

In winter, observations have shown particulate matter levels in the Indian capital can exceed 500 micrograms per cubic metre of air – an extremely poor level of air quality that can cause severe health impacts.

Scientists, including those from the Indian Institute of Technology Kanpur, have sought to understand where the extremely high particulate levels at night come from.

They found the chemical processes behind the poor air quality during night-time are unique for the Indian capital and have not been observed anywhere else in the world.

The findings, published in the journal Nature Geoscience, suggest that the trigger for the high levels of particulate matter is the fumes emitted when wood is burnt.

Wood burning is a common practice for around 400 million people living in the Indo-Gangetic Plain for cooking and heating, and sometimes materials such as plastic and other waste materials are also burned.

A bird flies next to the smog-covered Akshardham temple in New Delhi on 4 November 2022 (REUTERS)

The fires from the burning contain a mixture of chemical compounds and gases that condense as night falls and temperatures drop.

Within a few hours at night, these gas molecules clump together to form particles up to 200 nanometers across that can be seen as a grey haze.

“Condensation from gas to particulate phase resembles the way in which water droplets form on kitchen surfaces when one is cooking,” said study co-author Lubna Dada from Paul Scherrer Institute in Switzerland.

“Particles in the atmosphere act as large surfaces on which gases can condense.”

This process, scientists said, is very different from how haze forms in other places.

Citing the example of Beijing, they said gases from emissions like traffic and wood burning in the Chinese capital react in the atmosphere during the day when they are exposed to light.

Traffic moves slowly amidst smog on a flyover in New Delhi on 29 November 2022 (REUTERS)

This leads to the formation of less volatile fumes capable of forming particles during the haze, researchers explained.

In Delhi, however, haze formation occurs at night, without sunlight, driven by increased emissions together with a sharp decrease in temperatures, the study noted.

“We have shown for the first time that semi-volatile gases can form such particles at night, contributing to the haze,” said Imad El-Haddad, another study author.

“Our work implies that regulating uncontrolled biomass-combustion emissions may help inhibit nocturnal haze formation and improve human health in India,” the scientists concluded in the study.