In a major joint project with top Indian scientists, PSI researchers have determined why smog forms at night in the Indian capital New Delhi, contrary to all the rules of atmospheric chemistry. Their results have now been 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.
Where do these extremely high particulate levels come from in nighttime New Delhi in winter? A team of researchers from the Laboratory for Atmospheric Chemistry at PSI has been investigating this question together with local scientists, including members of the Indian Institute of Technology Kanpur. They 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,” says Imad El-Haddad, an atmospheric chemist at PSI and one of the corresponding authors of the study. In their study, the team 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.