Today is the last first day of a station, since we reached station five around midnight last night. I think the entire ship was glad to finally have a day to sleep as we traveled yesterday! It was business as usual for the aerosol people though, since we do not have to deploy most of our instruments—they are running 24/7 and collecting air and particles for us. There are many aerosol people looking for specific compounds, including biologically produced chemicals like dimethyl sulfide. In contrast, I am looking for a needle in a haystack. Rather than looking at water vapor clouds, I study ice clouds.
These are clouds that tend to form higher in the atmosphere where it is colder. These colder temperatures can cause the water vapor to freeze into ice crystals, changing the properties of the clouds. I am interested in the individual ice crystals and when they form. The water vapor normally freezes at very cold temperatures in pure air—around -40 °C. But wait, doesn’t water freeze at 0 °C? The answer is yes, but not if the droplet is small enough, which these are. These small droplets instead get supercooled and freeze at much colder temperatures. If there are particles in the air, sometimes ice can form at warmer temperatures, up to about -5 °C. We call these particles ice nuclei, and they can be anything from pollen to dust to proteins. This is what I am looking for in my samples. We do this by taking ambient samples (samples collected from a tall mast on the bow of the ship) all the time, and while we are on station we take seasweep samples.
The seasweep is a structure that creates bubbles that then burst and create aerosols that are sucked up by a tube that goes to our van and to my filters. As an oceanographer rather than an atmospheric scientist, these are the samples I am most interested in. These samples represent everything the sea could produce as aerosols, and I am hoping these samples have lots of ice nuclei. However, I won’t know what I have collected until I get back to the lab, since my instruments do not travel well. So for now I keep collecting samples.
Written by Elise Wilbourn