SCIENCE:

Researchers unmask climate secrets of sea spray and clouds

In the late 1990s, the Hydraulics Laboratory at the Scripps Institution of Oceanography nearly closed. The lab, founded in 1964, had lost its permanent funding.

Grant Deane, a physical oceanographer at the University of California, San Diego, stepped up to head the lab and rescue it from a possible shutdown. "I was a user of the facility at that time, but I had a broader vision for what could be done beyond my own work," Deane said.

Atmospheric chemist Kimberly Prather, who has just shed new light on one of the more difficult questions dealing with climate change, is glad for that. Prather, who heads up UC San Diego's Center for Aerosol Impacts on Climate and the Environment, has been working for years to better understand how aerosols -- tiny airborne particles that can reflect radiation and lead to cloud creation -- affect climate.

And a new experimental setup, housed in a specially designed wave flume at the Hydraulics Laboratory, just helped Prather and her fellow researchers take a giant leap forward in that task. They published their new approach in a paper this week in the journal Proceedings of the National Academy of Sciences.

Researchers have known for a long time that sea spray, the moisture flung off the top of collapsing waves, plays a role in reflecting light back to space and in creating clouds.

While that spray may appear like simple saltwater droplets to the casual observer leaning off the side of a boat, it's actually quite complex, with its droplets varying in size and composition.

Each droplet, tiny though it may be, can make different things happen in the atmosphere depending on how big it is and what it is made of. Some aerosol particles from sea spray are made of mostly salt, some of other chemicals, and some even have tiny marine life forms in them.

Sorting out the 'critters'

Prather calls these various life forms "critters." And she wants to know how they change cloud formation.

"What are the critters doing to the water, what are the changes in the chemistry of the water that are leading to a change in the sea spray that is then leading to a change in the ability to form a cloud?" she asked.

Prather has been trying to unravel this mystery by doing experiments in the open ocean, taking samples of sea spray in various ocean conditions, and working to figure out how that spray composition is influencing cloud properties and how clouds form.

To the layman this may sound easy, but in the natural world, things get complicated.

While on the ocean, Prather might notice a change in the spray she is measuring and see a change in cloud properties or cloud-forming particles. But because of factors like pollution or changes in the direction of the wind, it's difficult to isolate sea spray composition as the reason for a particular phenomenon.

"You're never really sure if the change is from what is coming directly from the ocean," she said.

The new experimental approach she and others developed allows researchers to measure those changes without any of those pesky complicating factors. In a wave flume at the Hydraulics Laboratory that took around a year to perfect, Deane, Prather and others now have the perfect system to analyze how sea spray affects cloud formation and other climate-related properties.

The reason it took so long to get the facility ready is that the flume has to be totally uncontaminated, so researchers could be sure they were analyzing only sea spray particles. This was harder than it seemed.

"Not that many particles are produced by breaking waves, surprisingly," Prather said. "And so if you have any contribution from your lab that sneaks in there somehow, the concentrations are many orders of magnitude higher and you can't even detect the signal from your breaking waves. It is a challenge."

The paper in PNAS is one of the earliest published results from this experiment, and one of its findings is that biology -- the "critters" -- does affect the ability of sea spray to seed clouds. In particular, the researchers found that adding bacteria cultures to the ocean led to a reduction in the spray's ability to form clouds.

Big question, little existing data

For climate models, understanding how clouds form over the ocean, which covers 71 percent of the Earth's surface, and which droplets reflect radiation back out to space is important, because clouds have an overall cooling effect on the planet.

Right now, said Vicki Grassian, chemistry professor at the University of Iowa and another author on the study, models represent the influence of sea spray on cloud formation incredibly poorly, because they treat it as just one thing: sodium chloride -- salt.

"It's pretty clear that sea spray aerosol is not just sodium chloride and there is actually a different range of different chemical compositions in sea spray aerosol and in different particle types," she said.

So while a lot of sodium chloride does come off the ocean, there are also a number of other particles that might behave exactly opposite of salt particles in terms of how they form clouds or reflect light, she said.

Grassian likened the simplicity of this approach, in the climate models, to modeling the human body, which is made up primarily of water, as a shapeless blob of H2O. "There's so much you would miss!" she exclaimed.

As for climate modelers, whose simplified system she is breaking apart, she says their response to the nuance she and others are finding in how sea spray behaves has been positive. "They love it."

Stephen Schwartz, a senior scientist at Brookhaven National Laboratory and an expert on sea spray aerosols who was not involved with the research, called the new laboratory approach "a great way to go," especially because guest researchers can come and use the setup to do more work on sea spray aerosols.

"It's an impressive first study coming out of this facility, and I expect a lot more really good science to come out of it," he added.

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