Take in the air
Aerosol study seeks clean atmospheric baseline at Palmer Station
Posted November 22, 2013
Scientists head to Antarctica for all sorts of reasons. Some are interested in ice cores that contain clues to ancient atmospheric conditions and past climate. Others study the continent’s iconic penguins and seals. A few even use the clear, crisp atmosphere above for astrophysical research into the mysteries of the universe.
It’s the clean polar air that has drawn Gregory Roberts from Scripps Institution of Oceanography at the University of California, San Diego to Palmer Station . But it’s the atmosphere much closer to Earth that is the focus of his research.
Roberts is interested in aerosols and the roles they play in atmospheric chemistry and physics, as well as the formation and characteristics of clouds . Clouds remain a big data gap in global models that scientists build to mimic the biosphere in order to predict future climate change. The pristine air in Antarctica offers a way to learn more about the natural composition of aerosols in the atmosphere that influence cloud dynamics.
Photo Credit: Gregory Roberts
Gregory Roberts stands in front of the PAEROS instrument package, which collects data on aerosols.
“It’s impossible to find a clean environment in the Northern Hemisphere,” Roberts said while at Palmer Station, where he and Scripps colleague Craig Corrigan had installed an instrument package to collect data on aerosols and other atmospheric conditions.
“We need to know what the natural Earth system does in order for us to make any sort of guesses for what anthropogenic activity is going to be doing to climate change,” he explained.
Aerosols are very tiny particles suspended in the atmosphere. There are a number of natural sources, including sea salt, sulfates,as well as manmade aerosols from pollution, such as black carbon from soot.
These small particles play a big role in climate. Aerosols scatter sunlight directly back into space. They also modify the size and number of cloud particles, known as cloud condensation nuclei (CCN), changing how the clouds reflect and absorb sunlight.
The concentration of anthropogenic sulfate aerosols in the atmosphere has grown rapidly since the beginning of the Industrial Revolution. They cause the number of cloud droplets to increase but make the droplet sizes smaller, increasing the cloud’s albedo or the amount of radiation reflected back to space.
“If you can regulate the number of droplets that a cloud can form, you can regulate the cloud brightness,” explained Roberts, who has a joint appointment at the National Center for Scientific Research in France.
Palmer’s pristine maritime environment makes it an ideal place to understand the natural aerosols emitted by the ocean, including from biological sources. In particular, Roberts is interested in the aerosols eventually produced from phytoplankton , free-floating microscopic plants found in the upper water column.
Phytoplankton and marine bacteria produce a gas called dimethyl sulfide (DMS), as well as the more familiar greenhouse gas carbon dioxide. A component of that distinctive salty ocean odor, DMS oxidizes into sulfates in the atmosphere, possibly an important source for aerosols and CCN. Lab-based studies have also found that marine organisms alter the chemical composition of sea spray in ways that influence its ability to form clouds over the ocean .
It’s been nearly 20 years since aerosol measurements were last made at Palmer Station, according to Roberts. That short-term study found spikes in aerosol concentrations during the prime summer month of January and February when phytoplankton blooms erupt across the surface of the ocean.
The new Scripps study will run about five months with more powerful instruments to monitor the atmosphere as the seasons turn from winter sea ice, which muffles any biological activity, to the open water characteristic of summer along the western edge of the Antarctic Peninsula where Palmer Station is located.
“By comparing the two [seasons], we’ll be able to get a much better idea of what the biological contribution is,” Roberts said.
The instrument package, called PAEROS for Portable AERosol Observing System, is located only a few hundred meters behind Palmer Station. Weighing 40 kilograms and packable in a large suitcase, PAEROS was developed by Roberts’ lab using a variety of internally-developed and commercial sensors, many modified so they could be integrated into the lightweight package.
Instruments for measuring various physical and chemical characteristics of aerosol, including one that characterizes their cloud-forming ability and chemical signature, were miniaturized by Roberts’ team for PAEROS. Another sensor that detects black soot in the air showed that soot concentrations increased on a weather front that passed by Palmer Station in early November.
Photo Credit: Gregory Roberts
The PAEROS instrument package was installed on the research vessel LAURENCE M. GOULD, during the transit from South America to Antarctica.
“We were able to detect air masses coming from South America all the way to Palmer Station,” Roberts said. “Otherwise, there is no black carbon here, which is amazing. That’s concrete proof that this is a really, really clean environment that you can’t find anywhere else on Earth.”
The project is funded for one field season as a proof of concept, according to Roberts. He had originally proposed to use PAEROS on a ship, and it was installed aboard the research vessel Laurence M. Gould and used during the crossing between South America and Palmer Station.
“The results are very interesting, especially this gradient across the Drake [Passage],” he said, explaining that concentrations of natural and manmade aerosols dropped as the ship traveled farther south. But the Gould’s own exhaust from its smokestack interfered with the measurements.
Palmer Station is a much better location, Roberts added. If the experiment proves successful, he said it would be useful to do airborne measurements – much of his fieldwork in the past has involved both manned and unmanned airplanes – in conjunction with the ground-based instruments.
“What would be very interesting to look at are the aerosol measurements from the ground and what the cloud properties are above,” he said. “That would be the direction of future projects if we can come back.”
NSF-funded research in this article: Gregory Roberts, University of California, San Diego, Award No. 1246152 .