Scientists have discovered why smog forms at night in the national capital during winter, defying all the rules of atmospheric chemistry. For the past three years, New Delhi has been ranked as the world’s most polluted capital. Its high level of air pollution is associated with a large number of premature deaths.
In winter, 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 a scientist from the Indian Institute of Technology (IIT) Kanpur and colleagues investigated the origin of these extremely high levels of particulate matter in New Delhi at night 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 study’s corresponding authors. .
A study published in the journal Nature Geoscience found that the trigger for the high levels of particulate matter is fumes released when wood is burned. Burning wood is a common practice for about 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 burned, sometimes including plastics and other waste materials, the researchers said. Such fires produce a mixture of gases containing a myriad of chemical compounds, such as cresol, which our noses associate with the typical smell of fire, as well as sugar-like molecules from the burned cellulose in wood, they said.
These molecules, the team noted, cannot be seen in air with the naked eye, even in high concentrations. But as night falls, the temperature in New Delhi drops so quickly that some gas molecules condense and clump together within hours to form particles up to 200 nanometers in diameter that can be seen as a gray haze, they said.
“Condensation from the gas to the particle phase resembles the way water droplets form on a kitchen surface during 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.
The process, the researchers said, is very different from that in other places. For example, Beijing is probably the best-studied megacity in the world when it comes to air pollution. However, according to scientists, the formation of particles in the atmosphere of the Chinese capital takes place through different chemical pathways.