Very tiny droplets in sea spray can have a big impact on the atmosphere by helping seed clouds 鈥� and there is now a new way to explain how they form in such large numbers.
Researchers have long known that sea spray droplets less than a micrometre across 鈥� known as submicron drops 鈥� are created in large numbers by roiling seas, but they haven鈥檛 been able to explain how or why.
at Fudan University in China and his colleagues have discovered that the answer may lie below the surface of the sea, where air bubbles collide before they rise and burst to form the sea spray.
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The researchers produced air bubbles of different sizes in a container of water by pushing air through a needle or a porous glass filter at its bottom. They then let the bubbles rise to the water鈥檚 surface and measured how many submicron drops each produced in the air above the container.
The researchers expected that, to get more of these drops, they would have to make the bubbles particularly large, as that would also make the water membrane that envelops the air inside the bubble larger. However, they found that what was crucial was how the bubbles interacted with each other as they rose through the water.
Based on high-speed camera recordings of the bubbles, the researchers realised that when lots of air was going through the needle quickly, many bubbles formed in rapid succession and could collide and merge before reaching the surface. These violent collisions created bubbles filled with submicron drops 鈥� so when they reached the surface and burst, an unusually large number of these drops flew into the air.
To get a better idea of whether this could happen in the ocean, the researchers plunged a jet of water into their container. This helped them produce breaking waves at the surface similar to those seen in the ocean. Some of the bubbles produced in the water by these breaking waves again got caught in subsurface collisions 鈥� and when those bubbles rose and burst, they produced about five times as many submicron drops as bubbles that hadn鈥檛 been involved in subsurface collisions. Consequently, Wang and his colleagues argue that most submicron drops above oceans are produced this way.
at Boston University says that sea spray researchers have previously focused on the physics of what happens to a bubble when it reaches the surface, but the new experiment shows that physical phenomena below the surface also matter. 鈥淲e often present it as these are bubbles bursting at the surface and inside of them is just perfect air. And this study is saying, well, hang on, the bubble has history here,鈥� he says.
鈥淭his is a new hypothesis to a long-standing question of where the smallest [drops in] sea salt aerosol come from. Whether it is correct or not remains to be confirmed by subsequent studies,鈥� says at Princeton University. Answering the question more definitively may have implications for cloud coverage in maritime environments including the Southern Ocean, says Deike.
arXiv
