How Sea Spray Affects Clouds

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With the clamor over global warming in recent years, researchers have invested greater attention than ever into the behavior, composition, and formation of clouds.

Anyone who remembers fourth grade science class remembers the water cycle: Water from oceans, lakes, rivers, and reservoirs evaporate into the air, forming clouds, and is then dropped back down to the Earth’s surface in the form of precipitation. Given how much crap is in our oceans, though, it’s fair to wonder how much of it gets absorbed up into the clouds during this process.

A study published by the Pacific Northwest National Laboratory (PNNL) actually explored just that, and found that sea spray carried into the air by breaking waves transports the biochemistry of microscopic sea organisms that affect atmospheric aerosols—microscopic organisms that are invariably affected by the constantly changing chemistry of the ocean.

As the overview explains, PNNL researchers Dr. Susannah Burrows and Dr. Phil Rasch developed a new model that provides a better understanding of how microorganisms like phytoplankton affect the chemistry of oceans and sea spray, which in turn affect the makeup of the clouds it hits.

BTRtoday spoke with Rasch about the study, as well as the process and importance of creating a new model based more on the physical properties of the microorganisms and organic matter in the ocean.

BTRtoday (BTR): What is sea spray made of?

Phil Rasch (PR): Sea spray is comprised of Sodium Chloride, or salt, and organic compounds. Some of those compounds are lipids—organic compounds that are fatty acids or their derivatives, which are insoluble in water but soluble in organic solvents—as well as proteins (long chains of amino acids) and polysaccharides (chains of sugars).

BTR: Has sea spray always affected cloud formation?

PR: Yes, sea spray is a natural phenomenon, and it is always present and capable of affecting clouds. However, other kinds of aerosols can be more important.

BTR: Does sea spray affect oceans and clouds across the globe? Is it more concentrated where waters are warmer, or more polluted?

PR: Yes, these processes occur over, and near, all ocean surfaces. Where there are significant amounts of sea spray the organic compounds are more important. Where there is a lot of ocean life and less important where ocean ecosystems are less important. ? But the processes operate in all marine environments. Near land, many other aerosol types, like dust or pollution, are perhaps more important, just because there are a lot more of them.

BTR: For how long did the previous method of identifying the chemical composition of sea spray exist before your method was discovered?

PR: Scientists have been paying attention to the chemical composition of sea spray aerosols since the 1960s, at least. At that time they recognized that Sodium Chloride, or salt, was the major component of sea spray, but that there was a substantial fraction of organic compounds that accompanied the salt.

These organic compounds are now known to be the residue of marine life—bacteria, viruses, detritus—that are a natural part of the marine ecosystem. As this residue breaks down, it forms small bits of matter that are called “dissolved organic matter,” even though they are not liquid. Those bits just pass through very fine filters, so we sometimes call them dissolved.

BTR: Why was it important to base your method on a physical model, unlike previous methods?

PR: Historically, scientists have developed empirical relationships, which are described by equations that say, “if one sees this much chlorophyll, we expect to see this much organic matter in the sea spray.” But the real world is more complicated than that. The organic bits of matter in seawater that eventually become sea spray aerosol vary according to the sea life that became the little bits. And sea life varies from place to place, and it is not always correlated with chlorophyll appearance.

So we developed a model—a bunch of equations, essentially—that said “let’s put together a more realistic picture of how the process works.” That picture starts with developing equations that say, “if we see this kind of sea life in this region, it will eventually break down at a certain time to form this kind of dissolved organic matter.”

We identified a few different kinds of sea life that matter to this process. Each type develops different kinds of detritus. Each kind of detritus has different physical properties: different sizes, different propensity to combine with water, and different characteristics when it eventually nears the air-water interface that exists at the ocean surface.

Susannah developed more equations that described how air bubbles below the ocean surface would collect different kinds of material as they rise to the surface, and then what would happen to the material once the bubble reaches the surface and eventually bursts. It turns out that when a bubble bursts, it releases two different kinds of tiny droplets, one kind is called a “jet drop” and the other a “film drop.” Different amounts of organic material are attached to the small and large drops. And different amounts go onto the drop depending on the kind of material.

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