Aerosols such as dust, sea salt particles, bits of organic material, and even pollutants are what allows water vapor to congeal into clouds and the mix of aerosols in the sky helps determine what kinds of clouds form.
Clouds produce precipitation and regulate how much solar energy reaches Earth’s surface and since this process is so fundamental to understanding climate, aerosol research has become a key component of climate studies. Recently Lynn Russell, an atmospheric sciences researcher at Scripps Institution of Oceanography, UC San Diego, and colleagues decided some basic questions needed to be considered: What are pristine aerosols (ones not produced by human activity) and, in a heavily industrialized world, where can one still find them?
The answers are complicated, said Russell, but a new study and upcoming ones are an important first step.
"We have identified the baseline that can inform future studies of one of the most important variables in climate," she said.
Oceanic processes produce many atmospheric aerosols. The complex process is considered by scientists one of the most important unknowns in attempts to simulate and predict climate phenomena. The November study in the Journal of Geophysical Research, “Sources and composition of submicron organic mass in marine aerosol particles,” looked at the forces that determine what the oceans contribute naturally to atmospheric processes versus what they contribute when human-influenced sources of aerosols – ranging from ship exhaust to pollution originating on land – are added to the mix.
Amanda Frossard, a Scripps graduate student in Russell’s group, now a postdoctoral researcher at UC Berkeley and the study’s lead author, compared actual aerosol measurements with model-generated marine aerosol particles simulating what the oceans would be expected to produce naturally.
“The organic composition of atmospheric primary marine aerosols measured on five open ocean cruises is very similar to the organic composition of the model-generated marine aerosol particles,” Frossard said. “This indicates that we confirmed the origin of a fraction of the marine aerosol measured.”
A key variable in the comparison is a class of chemical compounds known as alkanes, produced by burning of fuel or plant material. Because these compounds can also be produced naturally and because they so quickly mix with naturally produced aerosols in the atmosphere, teasing out natural sources from human sources has been difficult for scientists. As a consequence, some measurements that have been used to set guidelines for models may have been misinterpreted. By making chemically specific measurements of organic components in the study, the researchers provided a better characterization of the marine contribution and of its regional and seasonal differences.
“Previous studies looking at the composition of pristine marine aerosol particles may have included some particles from man-made, or anthropogenic, sources,” Frossard said.
In predicting climate change, global climate models need to include not only increases in greenhouse gases since pre-industrial times but also changes in aerosol concentrations and composition, the researchers said. Because the air in marine regions is influenced and polluted by shipping and urban emissions, it is challenging to sample air that is representative of pristine marine conditions. Such conditions are characterized by sea spray particles and gaseous emissions from the ocean surface, which include components from the organisms that inhabit the surface waters. The researchers have developed a new, more accurate way to discriminate between these marine sources and those associated with the products of fossil fuel combustion on land and at sea.
With a better understanding of actual pristine marine aerosol contributions, researchers are better able to simulate pre-industrial atmospheric aerosols and that allows them to better constrain recent changes in climate, Russell said.
As far as where pristine aerosols could be found in nature today, the researchers concluded that there are almost no places where marine samples are truly pristine. Even samples from remote ocean regions in the Southern Hemisphere contained human-contributed chemicals, they said.
More answers could come this year during a multi-institutional project led by Oregon State University called the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES) in which Russell is a participant. The project is anticipated to provide data on seasonal aerosol characteristics and those related to episodic events such as phytoplankton blooms.
Besides Russell and Frossard, contributors to the study include Susannah Burrows of Pacific Northwest National Laboratory, Scott Elliott of Los Alamos National Laboratory, Timothy Bates of the University of Washington, and Patricia Quinn of NOAA Pacific Marine Environmental Laboratory.