Scientists have determined why smog forms at night during winters in the national capital, contrary to all the rules of atmospheric chemistry. For the past three years, New Delhi has been ranked the world's most polluted capital. Its high levels of air pollution have been linked with a large number of premature deaths.

In winter, the particulate matter levels exceed 500 micrograms per cubic meter of air. A team of researchers from the Paul Scherrer Institute (PSI) in Switzerland along with scientist from the Indian Institute of Technology (IIT) Kanpur and colleagues investigated the origin of these extremely high particulate levels in nighttime New Delhi in winter.

"The chemical processes that take place in the air at night are unique to the Indian capital and have not been observed anywhere else in the world," said Imad El-Haddad, an atmospheric chemist at PSI and one of the corresponding authors of the study.

The study, published in the journal Nature Geoscience, found that the trigger for the high levels of particulate matter is the fumes emitted when wood is burnt. Wood burning is common practice for around 400 million people living in the Indo-Gangetic Plain, who use wood for cooking and heating.

In the absence of strict regulations, materials other than wood are also burnt, sometimes including plastic and other waste materials, the resaerchers said. Such fires produce a mixture of gases containing countless chemical compounds, such as cresol, which our noses associate with the typical smell of fire, as well as sugar-like molecules from the burnt cellulose in the wood, they said.

These molecules, the team noted, cannot be seen in the air with the naked eye, even in high concentrations. However, as night falls the temperature in New Delhi drops so rapidly that some of the gas molecules condense and within a few hours clump together to form particles up to 200 nanometres across, which can be seen as a gray haze, they said.

"Condensation from gas to particulate phase resembles the way in which water droplets form on kitchen surfaces when one is cooking. Particles in the atmosphere act as large surfaces on which gases can condense," said Lubna Dada, an atmospheric scientist at PSI and one of the authors of the study.

This process, the researchers said, is very different from that in other places. Beijing, for example, is probably the best-studied megacity in the world in terms of its air pollution. However, in the atmosphere of the Chinese capital, particle formation follow different chemical pathways, according to the researchers.

In China, the gases from emissions such as traffic and wood burning react in the atmosphere during the day when they are exposed to light resulting in the formation of less volatile fumes capable of forming particles during the haze, they said.

Such a pathway was also expected in New Delhi, however the opposite happens. Haze formation from the condensation of directly emitted fumes occurs at night, without photooxidation, driven by increased emissions together with a sharp decrease in temperatures. "We have shown for the first time that semi-volatile gases can form such particles at night, contributing to the haze," Imad El Haddad added.??