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

Date:
March 31, 2022
Source:
Wiley
Summary:
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 shows.



FULL STORY ==========================================================================
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.


==========================================================================
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.


========================================================================== Story Source: Materials provided by Wiley. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Guangzhi He, Jinzhu Ma, Biwu Chu, Renzhi Hu, Hao Li, Meng Gao,
Yuan Liu,
Yonghong Wang, Qingxin Ma, Pinhua Xie, Guoxian Zhang, Xiao Cheng
Zeng, Joseph S. Francisco, Hong He. Generation and Release of OH
Radicals from the Reaction of H2O with O2 over Soot. Angewandte
Chemie International Edition, 2022; DOI: 10.1002/anie.202201638 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220331121234.htm

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