Solid aerosols found in Arctic atmosphere could impact cloud formation
and climate

Date:
March 28, 2022
Source:
University of Michigan
Summary:
Solid aerosols can change how clouds form in the Arctic. And, as
the Arctic loses ice, researchers expect to see more of these unique
particles formed from oceanic emissions combined with ammonia from
birds, which will impact cloud formation and climate. Additionally,
understanding the characteristics of aerosols in the atmosphere
is critical for improving the ability of climate models to predict
current and future climate in the Arctic and beyond.



FULL STORY ==========================================================================
The Arctic is rapidly losing sea ice, and less ice means more open water,
and more open water means more gas and aerosol emissions from the ocean
into the air, warming the atmosphere and making it cloudier.


==========================================================================
So when researchers from the lab of University of Michigan aerosol
scientist Kerri Pratt collected aerosols from the Arctic atmosphere
during summer 2015, Rachel Kirpes, then a doctoral student, discovered
a curious thing: Aerosolized ammonium sulfate particles didn't look like typical liquid aerosols.

Working with fellow aerosol scientist Andrew Ault, Kirpes discovered
that ammonium sulfate particles, which should have been liquid, were
actually solid.

The team's results are published in the Proceedings of the National
Academy of Sciences.

Solid aerosols can change how clouds form in the Arctic. And, as the
Arctic loses ice, researchers expect to see more of these unique particles formed from oceanic emissions combined with ammonia from birds, which
will impact cloud formation and climate. Additionally, understanding the characteristics of aerosols in the atmosphere is critical for improving
the ability of climate models to predict current and future climate in
the Arctic and beyond.

"The Arctic is warming faster than anywhere else in the world. As we
have more emissions from open water in the atmosphere, these types of
particles could become more important," said Pratt, associate professor
of chemistry, and earth and environmental sciences. "These types of observations are so critical because we have so few observations to even evaluate the accuracy of models of the Arctic atmosphere.

"With so few observations, sometimes you get surprises like this when you
make measurements. These particles didn't look like anything we had ever
seen in the literature, in the Arctic, or anywhere else in the world."
The aerosols observed in the study were up to 400 nanometers, or about
300 times smaller than the diameter of a human hair. Ault, associate
professor of chemistry, says that aerosols in the Arctic are typically
assumed to be liquid.



==========================================================================
Once the relative humidity of the atmosphere reaches 80% -- about the
level of a humid day -- the particle becomes liquid. When you dry the
aerosol back out, it doesn't turn into a solid until the relative humidity
is about 35%-40%.

Because the air over the Arctic Ocean -- or any ocean -- is humid,
researchers expect to see liquid aerosols.

"But what we saw is a pretty new phenomenon where a small particle
collides with our droplets when it's below 80% humidity, but above 40% humidity.

Essentially, this provides a surface for the aerosol to solidify
and become a solid at a higher relative humidity than you would have
expected," Ault said.

"These particles were much more like a marble than a droplet. That's
really important, particularly in a region where there haven't been a lot
of measurements because those particles can eventually end up acting as
the seeds of clouds or having reactions happen on them." Additionally,
the researchers say, the size, composition and phase of atmospheric
aerosols impact climate change through water uptake and cloud formation.

"It's our job to keep helping modelers refine their models," Ault
said. "It's not that the models are wrong, but they always need more
new information as events on the ground change, and what we saw was
something completely unexpected." Pratt's team collected aerosols
in August-September 2015 in Utqia?vik, the northernmost point of
Alaska. To do this, they used what's called a multistage impactor,
a device that has several stages that collect particles according to
their size. Kirpes later analyzed these particles in Ault's lab using microscopy and spectroscopy techniques that can examine the composition
and phase of particles less than 100 nanometers in size.

"If we were to go back several decades when there was ice near the
shore, even in August and September, we would not be observing these
particles. We're observing the consequences of this climate already
changing," Pratt said. "We need to have the reality captured in
models that simulate clouds and the atmosphere, which are critical for understanding the energy budget of the Arctic atmosphere, for this place
that is changing faster than anywhere else."

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


========================================================================== Journal Reference:
1. Rachel M. Kirpes, Ziying Lei, Matthew Fraund, Matthew J. Gunsch,
Nathaniel W. May, Tate E. Barrett, Claire E. Moffett, Andrew
J. Schauer, Becky Alexander, Lucia M. Upchurch, Swarup China,
Patricia K. Quinn, Ryan C. Moffet, Alexander Laskin, Rebecca
J. Sheesley, Kerri A. Pratt, Andrew P. Ault. Solid organic-coated
ammonium sulfate particles at high relative humidity in the
summertime Arctic atmosphere. Proceedings of the National Academy
of Sciences, 2022; 119 (14) DOI: 10.1073/pnas.2104496119 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220328165339.htm

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