Atmospheric scientists study fires to resolve ice question in climate
models

Date:
August 26, 2020
Source:
Colorado State University
Summary:
Black carbon from fires is an important short-term climate
driver because it can affect the formation and composition of
clouds. Scientists are figuring out how.



FULL STORY ==========================================================================
When fossil fuel or biomass burns, soot -- also known as black carbon --
fills the air. Black carbon is an important short-term climate driver
because it absorbs solar energy and can affect the formation and
composition of clouds.


==========================================================================
The extent of black carbon's impact on clouds has been the subject
of debate for 30 years. A study recently published by Colorado State
University atmospheric scientists aims to settle the debate and improve
climate models.

Previous studies done in the laboratory conflicted on whether black
carbon was effective at ice nucleation, a process important to cloud
formation. Soot particles, like other types of aerosol particles in the
air, can act as the foundation for ice crystal growth. Lab results on soot ranged wildly from no ice nucleation activity to efficient ice formation.

"One reason these results could span such a range is that combustion
processes that form black carbon are extremely complicated and differ
depending on fuels burned, and on whether combustion is carefully
controlled, as in a diesel engine, or uncontrolled, as in wildfires," said Gregory Schill, first author on the study and a former NSF postdoctoral research fellow in the Department of Atmospheric Science.

Schill and his colleagues sampled smoke from wildfires and prescribed
burns, then filtered out soot particles using a technique he developed
with other members of Professor Sonia Kreidenweis and Paul DeMott's
research group. This work builds on Schill's previous investigation of
black carbon particles from diesel engine exhaust, conducted at the CSU
Engines and Energy Conversion Laboratory.

Combining the knowledge gained through these experiments, Schill and his colleagues simulated the contributions of black carbon ice-nucleating
particles versus other natural sources in a global model. They
found black carbon is not as important as previously thought for ice
particle formation in mid-level clouds, the clouds most responsible for precipitation over continents.



========================================================================== Natural sources, such as dust and sea spray, have more influence on
mid-level cloud properties. These cloud attributes variously affect
climate by reflecting sunlight, releasing precipitation and determining
how long the cloud persists.

"Our results suggest that black carbon, regardless of fuel types or
combustion conditions, have similar ice formation properties in mid-level clouds, and these are less efficient at forming ice compared to other non-anthropogenic sources," Schill said.

Atmospheric models have overestimated the role of black carbon as an ice- nucleating particle, and these findings correct that misunderstanding.

"This provides a clearer picture of the factors, both natural and anthropogenic, that might impact clouds and precipitation in a future
climate," Schill said.

The study eliminates black carbon as the primary suspect for ice
formation from smoke particles but leaves many unanswered questions
about how biomass burning affects clouds.

"Black carbon is only one component of a complex soup that makes up
smoke," Schill said. "We know that something in smoke can form ice
particles, but we do not fully understand what these cloud seeds are."
CSU atmospheric scientists are working on that problem, including a
study by the Kreidenweis/DeMott group that addresses biomass burning's contribution of such seeds to cloud levels. This nascent work is based
on samples taken during the WE-CAN campaign, in which scientists in
research aircraft flew into wildfire smoke. The new study's findings
confirm that lofted plumes have the same characteristics Schill found
in his ground-based studies.


========================================================================== Story Source: Materials provided by Colorado_State_University. Original
written by Jayme DeLoss. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Gregory P. Schill, Paul J. DeMott, Ethan W. Emerson, Anne Marie C.

Rauker, John K. Kodros, Kaitlyn J. Suski, Thomas C. J. Hill,
Ezra J. T.

Levin, Jeffrey R. Pierce, Delphine K. Farmer, Sonia
M. Kreidenweis. The contribution of black carbon to global
ice nucleating particle concentrations relevant to mixed-phase
clouds. Proceedings of the National Academy of Sciences, 2020;
202001674 DOI: 10.1073/ pnas.2001674117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200826101635.htm

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