Imad El Haddad is pleased that the processes in New Delhi's atmosphere, which have never been observed elsewhere, can now be better understood. Credit: Paul Scherrer Insitute/Mahir Dzambegovic
Key points:
- Researchers have discovered the cause of a strange fog that settles at night over New Delhi.
- The team found a chemical process is taking place in the Indian capital due to the frequent burning of plastics and other materials.
- The study is based on four years of data from two separate measuring stations.
A team of researchers from Switzerland and India scientists have solved the mystery of why smog forms at night in the Indian capital New Delhi, contrary to all the rules of atmospheric chemistry. Their surprising results were recently published in the journal Nature Geoscience.
For the past three years, New Delhi has been ranked the world’s most polluted capital. Its high levels of air pollution are responsible for a large number of premature deaths. In winter, the particulate matter levels exceed 500 micrograms per cubic meter of air. To get some idea of this magnitude, compare this value with the Chinese capital Beijing. In that smog-plagued metropolis, one cubic meter of air contains “only” 70 micrograms of particulates; whereas in Zurich the figure is just 10 micrograms per cubic meter.
A team at the Paul Scherrer Institute (Switzerland) began investigating the extremely high particulate levels and found an extraordinary explanation.
“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.
In the study, the team found that the trigger for the high levels of particulate matter is the fumes emitted when wood is burnt. For more than 400 million people living in the Indo-Gangetic Plain, wood burning is common practice. They use the wood for cooking and heating, but the absence of strict regulations has led to other materials being burnt as well—including plastic and other waste materials.
Such fires produce a mixture of gasses that contain numerous chemical compounds, such as cresol, which we associate with the typical smell of fire. These molecules normally cannot be seen in the air with the naked eye. However, as night falls in New Delhi, the temperature drops rapidly and some of the gas molecules condense and clump together to form particles that are visible as a gray haze.
For the study, the research team set up a station in the center of New Delhi with measuring equipment that included instruments to determine the number and size of the particles and their chemical composition. A second measuring station was also set up in the city with scaled-down equipment to verify that the formation of particulates is a localized phenomenon.
Awareness of the severity of air pollution in India has certainly increased and an ambitious Clean Air Program has been initiated.
The ability of local researchers to share their knowledge with one another is key to identifying the sources of pollution. However, according to study researchers, there is still a long way to go to improve the air quality, because that entails social changes and general public awareness.