How is mist formed? Soot as a surprising source of haze-forming hydroxyl radicals


Haze is formed when a cocktail of various gaseous pollutants is oxidized and forms particulate matter that disperses sunlight. This process is primarily mediated by hydroxyl radicals (OH), and researchers have now discovered a new pathway for their formation. As the study is published, this newly discovered radical-forming mechanism could also offer new perspectives for the air purification and energy industry applied Chemistry shows.

The mist consists of fine particles containing soot. It forms when gaseous pollutants, which are from industrial emissions, vehicle exhaust and other sources, are converted into condensed matter. “This condensation is remarkably accelerated under the action of OH radicals,” said Joseph S. of the University of Pennsylvania in Philadelphia, United States. Francisco, who is a co-author of the study.

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Commonly known sources for OH radicals, such as nitrogen oxides and ozone, are only partially responsible for the giant haze events that occur in haze-prone regions such as megacities in East and South Asia.

In a collaboration, the teams of Hong He at the Chinese Academy of Sciences, Xiao Cheng Zeng and Francisco at the University of Nebraska-Lincoln, USA, have now taken a closer look at the chemical activity of soot particles. Soot comes from diesel engine exhaust fumes or is spread by slash-and-burn practices or wildfires. However, to date, soot particles consisting of unbonded carbon have been considered more as a sink rather than a source of hydroxyl radicals.

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Despite this, new experiments by Francisco and team have shown that soot particles can produce OH radicals if air and water vapor are blown over the particles when irradiated with light.

However, it was expected that the hydroxyl species formed in this process would not leave the surface of the soot and would quickly re-react. However, energy calculations showed that the hydroxyl exhibited “roaming-like features”, as the authors put it: they migrated to the surface, eventually leaving it.

The results of their study led the team to conclude that soot particles play an active role in the formation of smog. But the researchers aren’t stopping there: Since it appears that light radiation is enough to decompose water molecules into radicals, this material could potentially be used to develop metal-free carbocatalysts. Such soot-based catalysts can either help purify the air from pollutants such as nitrogen oxides and volatile organic compounds (VOCs), or they can be used to generate chemical energy from light energy. This could pave the way for an eco-friendly form of artificial photosynthesis.

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