Sea surface microlayer
Photos by Audrey Tong September 17, 2024
UD professor and students head out on the Hugh R. Sharp Research Vessel to study the boundary between the ocean and the air
The boundary between the ocean and the atmosphere, where the air and sea interact, is known as the sea surface microlayer. Understanding how nutrients, pollutants and organic matter are exchanged between the air and the sea also is vital for better understanding the role they play in regulating carbon cycles and climate. The problem, however, is that studying the sea surface microlayer is an extremely time-intensive undertaking that can present many challenges.
To better understand the air-sea interactions of biogeochemically important constituents such as gases and different particles, researchers from the University of Delaware and the University of Georgia (UGA) recently conducted a research cruise aboard UD’s Research Vessel Hugh R. Sharp to study the sea surface microlayer in the North Atlantic Ocean.
The research is funded by the National Science Foundation and the research team has taken two prior cruises that were cut short due to stormy weather and the COVID-19 pandemic.
Andrew Wozniak, associate professor in the School of Marine Science and Policy (SMSP), is the UD project lead while the project lead for UGA is Amanda Frossard, associate professor in the Department of Chemistry at UGA.
Wozniak said the sea surface microlayer is a unique micro-environment.
“It accumulates organic material of a certain kind and then it reaches the air-sea interface and accumulates there, and that creates interesting physical properties that influence the exchange of materials back and forth,” said Wozniak. “We thought that if we can do a better job of understanding how the chemistry of the organic matter changes in space and time due to the ocean processes and biology, then we’ll have a better understanding of how these gases exchange and how the particles that are emitted to the atmosphere can influence atmospheric chemistry.”
Frossard explained that the research team is interested in studying what are known as surfactants, compounds that reduce the surface tension of a liquid such as seawater and accumulate in surface waters and the sea surface microlayer due to their affinity for surfaces and interfaces, such as rising bubbles in the ocean.
One of the main goals of the project is to look at and better understand these surfactants.
“We want to understand what surfactants are in the ocean, how they partition to the microlayer and what affects their concentration and their composition,” said Frossard. “We're collecting samples here, some that we're processing on the ship, but we're taking all of them back to the lab to do different analyses. We'll use these results to understand the sea surface microlayer and further understand air-sea gas exchange as well as the emission of particles from the ocean into the atmosphere.”
Glass Panels
Researching the sea surface microlayer is anything but easy. As seen from the previous cruises which ran into unforeseen complications, oceanography work requires persistence amidst a sea of challenges.
For the experiment, researchers on board the ship conducted a suite of biological and chemical analyses which requires a lot of material.
The surface microlayer is incredibly thin, on the order of 100 micrometers thick, which is approximately the thickness of a piece of copy paper. One of the ways the researchers collect the material is by dipping a glass plate into the water and then taking a squeegee to scrape the water that stuck to the plate into a bottle.
“As you can imagine, that takes a really long time,” said Wozniak. “What we want is about one and a half liters of water, and every time you do that, you collect about seven milliliters.”
Working with the crew aboard the R/V Sharp—Jon Swallow, Timothy Deering and James Warrington—the team refined their collection technique to help improve upon their method of sampling.
They used a Rosette sampler, a device used for water sampling in deep water. That type of sampler usually goes off the side of a ship equipped with bottles so that when it goes down, it comes back up with bottles of water.
For this experiment, however, they equipped the Rosette with pieces of glass.
“We extend the sampler off the back of the boat, dip it in, lift it up and bring it back on board and then scrape the glass,” said Wozniak. “We do that over and over to collect enough water to combine for a sample. It takes us about two hours to do this and we count the number of times we’re doing this, so we have a record of how many it takes to collect the samples.”
Wozniak said the idea is that when the glass plate goes down into the water and rises back up, the material at the surface is the last thing it’s touching. Those surfactant molecules are attracted to the glass plate, and they adhere to it. As it’s coming out, some of what is dripping off is mostly that underlying water.
“They’re called surfactant molecules because they reduce surface tension and that surface tension property is going to influence the turbulence at that air-water interface,” said Wozniak. “That turbulence is a key factor in how gases can go back and forth. With more turbulence, you’re going to have more exchange of materials.”
Currently, the way for quantifying how something like carbon dioxide is exchanged back and forth is based on wind speed because the wind is going to influence that turbulence. But those models have big margins of error, and the thought is those errors are in part due to the influence of other factors like these surfactants.
“By scraping this off of the glass, you’ve consolidated that thin layer,” said Wozniak. “This was our solution to study the sea-surface microlayer and it really is a great example of a collaboration between the ship crew and the scientists to solve a real challenge.”
Once the samples were collected, the researchers conducted their analyses either onboard the ship or back in the laboratories at their home institutions.
Students from UD involved in the cruise include Felix Agblemanyo, a doctoral student studying oceanography in UD’s School of Marine Science and Policy who is a member of Wozniak’s lab group and was conducting the research as part of his PhD dissertation, Audrey Tong, an undergraduate student on-board the expedition to help with the science and communications work, and Tia Ouyang, a doctoral student and also a member of the Wozniak lab. Ava Grove, an undergraduate student getting her first exposure to oceanographic research, rounded out the UD research team.
There were five members of the University of Georgia team, as Frossard was joined by four of her graduate students: Daniel Ammer, Amber Birt, Ellie Smerznak, and Mara Shields.
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