Scientists discover a way Earth's atmosphere cleans itself
Chemist helped shed light on the formation of an air-clearing molecule


Date:
April 7, 2023
Source:
University of California - Irvine
Summary:
Human activities emit many kinds of pollutants into the air, and
without a molecule called hydroxide (OH), many of these pollutants
would keep aggregating in the atmosphere.


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FULL STORY ========================================================================== Human activities emit many kinds of pollutants into the air, and without
a molecule called hydroxide (OH), many of these pollutants would keep aggregating in the atmosphere.


==========================================================================
How OH itself forms in the atmosphere was viewed as a complete story,
but in new research published in Proceedings of the National Academy of Sciences, a research team that includes Sergey Nizkorodov, a University of California, Irvine professor of chemistry, report that a strong electric
field that exists at the surface between airborne water droplets and
the surrounding air can create OH by a previously unknown mechanism.

It's a finding that stands to reshape how scientists understand how
the air clears itself of things like human-emitted pollutants and
greenhouse gases, which OH can react with and eliminate. "You need OH
to oxidize hydrocarbons, otherwise they would build up in the atmosphere indefinitely," said Nizkorodov.

"OH is a key player in the story of atmospheric chemistry. It initiates
the reactions that break down airborne pollutants and helps to remove
noxious chemicals such as sulfur dioxide and nitric oxide, which
are poisonous gases, from the atmosphere," said Christian George, an atmospheric chemist at the University of Lyon in France and lead author
of the new study. "Thus, having a full understanding of its sources and
sinks is key to understanding and mitigating air pollution." Before, researchers assumed that sunlight was the chief driver of OH formation.

"The conventional wisdom is that you have to make OH by photochemistry or
redox chemistry. You have to have sunlight or metals acting as catalysts," Nizkorodov said. "What this paper says in essence is you don't need any
of this. In the pure water itself, OH can be created spontaneously by
the special conditions on the surface of the droplets." The team built
on research from Stanford University scientists led by Richard Zare that reported spontaneous formation of hydrogen peroxide on the surfaces of
water droplets. The new findings help interpret the unexpected results
from the Zare group.

The team measured OH concentrations in different vials -- some containing
an air-water surface and others containing only water without any air --
and tracked OH production in darkness by including a "probe" molecule
in the vials that fluoresces when it reacts with OH.

What they saw is that OH production rates in darkness mirror those and
even exceed rates from drivers like sunlight exposure. "Enough of OH will
be created to compete with other known OH sources," said Nizkorodov. "At
night, when there is no photochemistry, OH is still produced and it is
produced at a higher rate than would otherwise happen." The findings, Nizkorodov reported, alter understanding of the sources of OH, something
that will change how other researchers build computer models that attempt
to forecast how air pollution happens.

"It could change air pollution models quite significantly," Nizkorodov
said.

"OH is an important oxidant inside water droplets and the main assumption
in the models is that OH comes from the air, it's not produced in
the droplet directly." To determine whether this new OH production
mechanism plays a role, Nizkorodov thinks the next step is to perform
carefully designed experiments in the real atmosphere in different parts
of the world.

But first, the team expects the results to make a splash in the
atmospheric research community.

"A lot of people will read this but will not initially believe it and will either try to reproduce it or try to do experiments to prove it wrong,"
said Nizkorodov. "There will be many lab experiments following up on
this for sure." He added that UCI is a prime place for such science to continue happening, because other labs at UCI, like that of Ann Marie
Carlton, professor of chemistry, focus their efforts on the role water
droplets play in the atmosphere.

This project, which was funded by the European Research Council, involved researchers from France's University Claude Bernard, China's Guangdong University of Technology, and Israel's Weizmann Institute.

* RELATED_TOPICS
o Earth_&_Climate
# Air_Quality # Pollution # Air_Pollution #
Environmental_Science # Environmental_Issues # Atmosphere
# Water # Environmental_Awareness
* RELATED_TERMS
o Atmospheric_dispersion_modeling o Meteorite o Air_pollution
o Water_resources o Water_pollution o Pollution o
Planetary_boundary_layer o Ionosphere

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


========================================================================== Journal Reference:
1. Kangwei Li, Yunlong Guo, Sergey A. Nizkorodov, Yinon Rudich, Maria
Angelaki, Xinke Wang, Taicheng An, Sebastien Perrier, Christian
George.

Spontaneous dark formation of OH radicals at the interface of
aqueous atmospheric droplets. Proceedings of the National Academy
of Sciences, 2023; 120 (15) DOI: 10.1073/pnas.2220228120 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/04/230407133443.htm

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