Big Result from a Tiny Ocean

A team of UC San Diego researchers that built a replica ocean-atmosphere system in an on-campus laboratory found evidence that bacteria play a key role in influencing the formation of clouds through their effect on sea spray composition.

The scientists used a novel approach to produce sea spray capturing the chemical complexity of tiny atmospheric particles called aerosols that can influence climate by absorbing or reflecting sunlight and seeding clouds.

“After many decades of attempting to understand how the ocean impacts the atmosphere and clouds above it, it became clear a new approach was needed to investigate the complex ocean-atmosphere system—so moving the chemical complexity of the ocean to the laboratory represented a major advance that will enable many new studies to be performed,” said Kimberly Prather, Distinguished Chair in Atmospheric Chemistry at the University of California, San Diego and director of the Center for Aerosol Impacts on Climate and the Environment.

Prather led a team of more than 30 scientists involved in this project. They reported their findings in the online edition of the Proceedings of the National Academy of Sciences on April 22.

Tiny air bubbles form in the ocean when waves break and then rise to the surface and burst, releasing gases and aerosols into the atmosphere. This study demonstrated how sea spray aerosols come in a wide variety of sizes and shapes with chemical complexity ranging from simple salts to complex biological mixtures to bacterial cells.

For decades, scientists have been studying how their chemical make-up affects their ability to take up water, seed clouds, and react in the atmosphere. It is has been difficult to isolate and study marine aerosols in the real world, however, because aerosols from other sources overwhelm field measurements.  Before one can fully understand human impacts on climate, scientists must first understand how natural aerosols such as sea spray form over the ocean, which covers 71 percent of Earth’s surface, Prather said.

The scientists built a closed ocean-atmosphere system in a wave tank at the Hydraulics Lab at UCSD’s Scripps Institution of Oceanography in 2011. They pumped in seawater directly from the Pacific Ocean and mechanically generated waves. By filtering the air within the wave chamber, the team eliminated contamination from other sources allowing scientists to probe sea spray aerosol directly for the first time after it was produced by breaking waves.

Over five days, the team systematically altered biological communities within the flume by adding various combinations of cultures of marine bacteria and microscopic marine algae, or phytoplankton.

Then, as a hydraulic paddle sent waves breaking over an artificial shoal, instruments positioned along the 33-meter long flume measured the chemistry of the seawater, air, and aerosols.

As the seawater composition evolved and bacteria levels increased, the resulting sea spray aerosols showed a major change in composition leading to a reduction in their ability to form clouds. In particular, a day after new cultures were added, bacteria levels rose five-fold and the cloud-seeding potential of the sea spray fell by about a third.  These changes were happening even as the concentration of phytoplankton fell, along with levels of chlorophyll-a, the pigment essential to photosynthesis. This is an important finding because current estimates of biological activity in surface waters of the ocean rely on instruments aboard satellites that measure the color of the sea surface, which changes along with levels of chlorophyll-a.  As the climate continues to change, the microorganisms in the ocean will also change, and thus developing novel methods to link changes in ocean chemistry with atmospheric chemistry and climate is critical to understanding the earth-climate system.

“Once the ocean-atmosphere system is isolated, we can now systematically probe how changes in the seawater due to biological activity affect the composition and climate properties of the sea spray aerosol,” said Prather, a professor in the Department of Chemistry and Biochemistry who holds a joint appointment at Scripps Institution of Oceanography.

The findings demonstrate the value of the center’s novel approach for sorting through the interdependent factors governing the effects of the ocean and its impact on sea spray, clouds, and climate, she added.

– Susan Brown and Robert Monroe