Soot as a surprising source of haze-building hydroxyl radicals
Haze is formed when a cocktail of various gaseous pollutants is
oxidized and forms particulate matter diffusing sunlight. This process
is mainly mediated by hydroxyl radicals (OH), and researchers have now
discovered a new route to their formation. This newly discovered
radical-building mechanism could also offer new perspectives for air
purification and the energy industry, as the study published in Angewandte Chemie shows.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Haze consists of fine particulate matter containing soot. It is
formed when gaseous pollutants, which are from industrial emissions,
vehicle exhausts, and other sources, are converted to condensable
matter. “This condensation is remarkably accelerated under the action of
OH radicals,” says Joseph S. Francisco from the University of
Pennsylvania in Philadelphia, USA, who is co-author of the study.
The commonly known sources for OH radicals, such as nitrogen oxide
and ozone, only partly account for the vast haze events which keep
occurring in haze-afflicted regions such as the megacities of East and
South Asia.
In a cooperation, the teams of Hong He at the Chinese Academy of
Sciences, Xiao Cheng Zeng at the University of Nebraska-Lincoln, USA,
and Francisco have now taken a closer look at the chemical activity of
soot particles. Soot originates from diesel engine exhaust fumes or is
spread by slash-and-burn practices or forest fires. However, to date,
soot particles consisting of uncombusted carbon have been considered
more as a sink of hydroxyl radicals, rather than a source.
Despite this, Francisco and the team’s new experiments showed that
soot particles can produce OH radicals if air and water vapor are blown
over the particles while being irradiated with light.
It was expected, though, that hydroxyl species formed in this process
would not leave the surface of the soot and would quickly react again.
However, energy calculations showed that the hydroxyl exhibited
“roaming-like features”, as the authors stated it: they migrated over
the surface, ultimately leaving it.
The results of their study led the team to the conclusion that soot
particles play an active role in smog formation. But the researchers
aren’t stopping there: since it seems that light radiation is sufficient
to decompose water molecules into radicals, this material could
potentially be used to develop metal-free carbocatalysts. Such
soot-based catalysts could either help purify the air from pollutants
such as nitrogen oxide and volatile organic compounds (VOCs), or they
could be used to generate chemical energy from light energy. This could
pave the way for an environmentally friendly form of artificial
photosynthesis.
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About the Authors
Dr. Joseph S.
Francisco is President Distinguished Professor at the Department of
Chemistry of the University of Pennsylvania. The research in his
laboratory focuses on kinetics and photochemistry of novel transient
species in the gas phase, in aerosol, and at the ice-quasi liquid layer,
to address the questions of how structures correlate to reactivity and
photochemical mechanisms.
Dr. Hong He is Professor of the Research Center for Eco-environmental
Sciences, Chinese Academy of Sciences, China. His research team focuses
on the heterogeneous transformation and elimination of pollutants in
environmental catalysis and multi-phase reaction process in atmospheric
chemistry. His team develops theories of haze chemistry and the
technology for the control of mobile/stationary source of indoor air
pollution.