Algae Help Drive Aerosol Formation Over the Ocean

Clouds are generally thought to help cool the planet. In reality, different kinds and sizes of clouds forming in different regions and altitudes can reflect or absorb the sun’s energy in different ways. Even the very process by which clouds form is more complicated than often presented.

The simple story is that water vapor in the atmosphere condenses into liquid as the temperature and pressure change. But that process relies on the presence of tiny, floating particles, or aerosols, that provide a surface onto which water vapor can condense. Vast numbers of these particles are constantly forming thanks to an array of complex chemical reactions as sulfur, nitrogen, and hydrocarbon gases are emitted from Earth’s surface and interact in the atmosphere.

That makes these aerosols one of the biggest unknowns in models of Earth’s climate. It also means that understanding particle formation, especially over the planet’s vast oceans, is essential to creating models that accurately capture the complex chemistry of the atmosphere and how it’s changing.

Steve Archer, a senior research scientist at Bigelow Laboratory for Ocean Sciences, has teamed up with Coty Jen, an associate professor in chemical engineering at Carnegie Mellon University, to unravel this process in the coastal waters of the Gulf of Maine. Combining biological, chemical, and weather data, the team is working to understand what kinds of particles are forming, how quickly, and how that process is controlled by microscopic marine algae.

“We’re trying to understand the composition of the cocktail of gases coming off the ocean and how that relates to the biological community that are emitting those gases,” said Archer, who also directs Bigelow Analytical Services. “That’s the first step to understanding how the process might shift as the climate changes.”

“Emissions from these microbes are poorly quantified now — let alone as waters warm and the chemistry changes with the climate,” added Jen, who is a member of the Center for Atmospheric Particle Studies at Carnegie Mellon. “The information we’re collecting will be critical in predicting how ocean microbial communities ultimately influence the climate.”

Biological processes can speed up this process by releasing reactive compounds, including ammonia and amines, which can be produced from the breakdown of organic matter, and iodine-containing gases, which are naturally produced by seaweed. Dimethyl sulfide, for example, is produced by plankton in the ocean. In fact, it’s the largest natural source of sulfur to the atmosphere. There, it undergoes a chemical reaction to create sulfate aerosols, which can evolve into the seed particles, called cloud condensation nuclei, onto which water vapor condenses.

Weather adds a layer of complexity as wind can blow man-made gases out to sea. Rain and fog can also influence the emissions from algae and remove particles from the air before they have a chance to grow.

Unsurprisingly, these dynamic chemical reactions, and how they’re interacting with the ocean’s biology, is a challenging scientific question. But it’s also an important one since the number and type of cloud condensation nuclei directly influences the kinds of clouds that form.

“It’s been long known that planktonic biology contributes to aerosol and cloud formation over the oceans to an extent that it actually influences global climate, but the processes and extent of that influence is still poorly understood,” Archer said. “By collaborating in this research, we’re hoping to unravel some of the complexity and contribute to reducing uncertainty in the climate models.”

A particle sizer and counter have been running since the spring at Bigelow Laboratory’s shoreside facility, enabling the researchers to literally count these aerosols as they form. It will continue running through the winter to reveal how particle formation changes over the year and if it’s influenced by seasonal changes in plankton growth.

This fall, several graduate students from Carnegie Mellon also set up a month-long experiment in the shore facility with an array of sophisticated instruments. With mass spectrometers, they’re able to go beyond counting particles and determine what compounds they’re made from. Scientists at Bigelow Laboratory’s Center for Aquatic Cytometry are also helping the research team count and characterize the phytoplankton community to understand the biological processes at play. Meanwhile, a weather station is collecting data, including wind speed and direction, to help them differentiate gases being emitted from the ocean versus those blowing from land. Next summer, they intend to run a similar experiment to get even more data.

The ultimate goal is to improve climate model predictions by incorporating this improved understanding of what kinds of particles are forming and how quickly — and the various feedback loops between particle formation, climate, and algae.

Though it’s still relatively early, the team has already made some interesting findings, including showing that particle formation is faster at low tide, likely due to the large amounts of reactive iodine emitted by exposed seaweeds. Surprisingly, they’ve also observed new particle formation at low tide even at night, which Jen points out is unusual.

“Typically, you need sunlight to convert the emissions from seaweed into gaseous compounds that form atmospheric particles,” she said. “The working theory is the seaweeds are actually emitting tiny particles themselves, but we’re excited to keep digging into the observations to help figure that out.”

For that reason, the team hopes to replicate their experiments offshore at some point. A goal of the recent experiments was, in fact, to test out prototype instruments that could eventually be deployed on Bigelow Laboratory’s research vessel, the R/V Bowditch, to help unpack differences in aerosol formation at sea versus on the coast.

“The low tide appears to be responsible for large amounts of small particles, which only rarely grow into cloud condensation nuclei sizes. It’s a preliminary finding, but there’s a lot going on we have yet to unravel,” Jen added. “It’s a reminder of how complex this process is, so we’re excited to keep digging into what happens next.”

Photos: Carnegie Mellon PhD students Joy Kiguru (top photo), Ziheng Zeng (middle photo), and Christine Troller (bottom photo) set up monitoring equipment in a container by Bigelow Laboratory’s shoreside facility. Malena Rybacki and Dallan Schoenberger, other students in Jen’s lab, also visited Bigelow Laboratory at different times to help run the experiments alongside Archer and Research Associate Gabriella Iacono.