In the summer of 2015, samples were collected from the Arctic atmosphere that contained aerosols – particles suspended in the atmosphere that aren’t a part of the natural composition. Think of it like particles suspended in a liquid, but instead of solids in a liquid, aerosols are liquids suspended in a gas.

Now, in a new paper published in the Proceedings of the National Academy of Sciences, then-doctoral student Rachel Kirpes analyzed those aerosols and discovered that the ammonium sulfate particles weren’t liquid – they were solid. This is not a good thing. Solid aerosols change how clouds form. Combined with the ongoing loss of Arctic sea ice, which results in more open water and more aerosols released from the ocean, we end up with a warmer atmosphere and more clouds.

So why do we expect liquid aerosols and how did we end up with solid ones instead? We expect them because the humidity over any ocean is pretty high, more than 80% as we have a constant exchange of water into water vapor. Generally, the aerosols stay liquid until the humidity drops below about 40%, which is unlikely to happen over the water. Further, co-author Andrew Ault explains: …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. 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.

All this new information means we can add another factor to our climate models, which were already looking kind of grim.

More Information

University of Michigan press release

“Solid organic-coated ammonium sulfate particles at high relative humidity in the summertime Arctic atmosphere,” Rachel M. Kirpes et al., 2022 March 28, PNAS