The Arctic is affected by global warming like no other place on Earth. Sea ice is shrinking rapidly and there is more open water, resulting in more gases and aerosols being released from the ocean into the atmosphere, further warming it and causing more cloud formation. In a recent study published in the journal Proceedings of the National Academy of Sciences, researchers in northern Alaska detected solid aerosols in the Arctic atmosphere, where typically only liquid aerosols should be present.
The University of Michigan researchers collected aerosol samples in Utqiaġvik, at the northern tip of Alaska, in 2015 and were surprised when then-doctoral student Rachel Kirpes, the lead author of the study, later discovered aerosolized ammonium sulfate particles in the lab that were solid rather than liquid as expected.
Aerosol particles play an important role in cloud formation because they serve as condensation nuclei. However, solid and liquid particles behave differently, and a higher proportion of solid aerosol particles over the Arctic Ocean could alter cloud formation in the Arctic, which in turn affects climate. With ice loss in the Arctic, researchers expect more of these unusual particles to form from oceanic emissions combined with ammonia from bird feces. In addition, understanding the properties of aerosols in the atmosphere is critical to improving climate models.
“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,” explains Kerri Pratt, associate professor of chemistry and earth and environmental sciences and co-author of the study. “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.”
Aerosol particles become liquid when the relative humidity reaches 80 percent. When the aerosol is dried again, it only becomes solid again at a relative humidity of 35-40%. Because the air over the ocean is humid, liquid aerosols are expected in the atmosphere above.
“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,” says Andrew Ault, a professor of chemistry at the University of Michigan and co-author of the study. “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.”
“It’s our job to keep helping modelers refine their models,” Dr Ault says. “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.”
Dr Pratt is convinced that the solid aerosol particles could not have been observed several decades ago, when there was still ice near the coast, and that they are a result of the changing climate. “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.”
Julia Hager, PolarJournal
Link to the study: Rachel M. Kirpes, Ziying Lei, Matthew Fraund et al. Solid organic-coated ammonium sulfate particles at high relative humidity in the summertime Arctic atmosphere. PNAS 119 (14) e2104496119, 2022. https://doi.org/10.1073/pnas.2104496119
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