“Oceans are major contributors to atmospheric aerosols and for a long time they were modeled and studied in laboratories as if they were only made of sodium chloride,” said Vicki Grassian, a chemistry professor at the University of Iowa and co-director of the Center for Aerosol Impacts on Climate and the Environment, or CAICE, based at the University of California, San Diego. “But there’s a much richer chemistry that we need to understand.”
The surface of the sea teems with a mélange of organic material – viruses, bacteria and the detritus of marine life. When bubbles burst upward from the watery depths, organic matter can cling to their surface and rise into the atmosphere as sea spray. Although the particles are within easy reach – within 50 nanometers of the water’s surface – parsing out their effect under complex oceanic conditions is a challenge.
DIY AerosolsTo control that complexity, researchers at CAICE make their own aerosols. Funded as part of the National Science Foundation’s Centers for Chemical Innovation program, the center is home to a giant flume of ocean water. This simulated ocean setup allows scientists to create sea spray aerosols, or SSAs, and capture them for further study.
“A wide range of molecules come out of seawater, and we sample these particles as a function of size,” said Grassian. So far, researchers including University of Iowa student Olga Laskina and postdoctoral associate Richard Cochran have detected more than 280 organic compounds with EMSL’s high resolution mass spectrometers.
Beyond chemically cataloguing these compounds, CAICE scientists also want to understand how the ocean’s changing biology affects the composition of SSAs. For example, in one experiment they created two plankton blooms and analyzed aerosol particles at time intervals throughout the two events. After analyzing those samples, the scientists found each bloom produced a different outcome depending on the degree of bacterial degradation in the water. These results showed for the first time the role of bacteria on SSAs, said Grassian. Prior to these studies, only chlorophyll was thought to be a measure of biology activity in seawater that could impact SSA.
Figuring out what these SSAs actually look like as they leave the ocean’s surface is the work of CAICE researcher Joseph Patterson, an assistant project scientist in Nathan Gianneschi’s laboratory at the UC San Diego. With a background in synthetic polymer nanomaterials, Patterson collects lab-made aerosols and views them with a cryogenic electron microscopy technique he developed specifically for aerosols. Unlike other methods that require dehydrating the particles, this electron microscopy technique gets high-resolution images of “wet,” intact particles.
“These images provide the most direct observations of what’s inside these aerosols and how they exist,” said Patterson. One surprise was finding an abundance of complex, multilamellar vesicles that contained small particles. Although the particles have not yet been identified, he suspects the materials are phospholipid membranes containing enzymes and proteins that eventually end up in SSAs.
“One big drive behind working at EMSL are the highly specialized microscopes,” said Patterson. “With them, we have chemical analysis capabilities and we’re able to localize specific bioparticles to measure how they affect the overall elemental composition of aerosols.”
Next, he plans to look at aerosols with a transmission electron microscope, which gives high spatial resolution and elemental information, and then go to Nanoscale secondary ion mass spectrometry, or NanoSIMS, for more detailed analysis of the surface.
An artificial marine system also helps Daniel Knopf, a professor of atmospheric sciences at the State University of New York at Stony Brook, to study the effects of phytoplankton on aerosols. With his colleague, marine microbiologist Josephine Aller, they create phytoplankton blooms under controlled conditions and measure parameters such as cell counts and amounts of organic carbon produced. In parallel experiments, they interrogate the composition of lab-generated aerosols with single particle microspectroscopy tools to analyze how plankton blooms modulate SSA composition.
In addition, Knopf wants to understand the potential of organic material to become ice nuclei that instigate cold cloud formation. “Ice nucleation represents a big conundrum in atmospheric science because only a few particles have this ability – maybe only one particle in one liter of air – and we don’t know why,” said Knopf.
Although the ice nuclei capabilities of mineral dust are well known, Knopf – a physicist by training – wondered if marine scientists could help discover other sources of ice formation at the ocean surface. In previous work, fragments of plankton cells were capable of increasing ice formation under atmospheric conditions. More recently, Knopf was part of an international collaboration of scientists who sampled sea surfaces from around the world and showed enhanced ice formation with plankton exudate, or emitted fluid, less than 200 nanometers in size. The identity of this minute amount of “ice active” material lurking at the surface of the sea remains a mystery. Knopf wants its spectral fingerprint; with that he can search for similar features and freezing profiles among the samples collected from the sea or generated in the lab. Beyond getting that identification, analyzing sea spray particles with EMSL’s computer-controlled scanning electron microscopy and energy dispersive X-ray spectroscopy could show the surface structures that contribute to this enhanced ice formation.
“The computer-controlled systems let us look at hundreds of particles automatically, which helps us be statistically certain about the elemental compositions in lab-made and field-collected SSAs,” said Knopf.
In the FieldA new method developed by Knopf, and colleagues who include EMSL scientists Alex Laskin and Bingbing Wang, allows real-time analysis of ice nucleation in particles. In a Department of Energy-sponsored effort, researchers collected particles from central California to run experiments in this high-resolution ice nuclei environmental scanning electron microscope. With this, they can see where the nucleus forms and then find the particle again to conduct organic functionality investigation or elemental particle position, explained Knopf.
“You have to imagine these particles, about two orders of magnitude smaller than average hair, are deposited on substrates and with this new instrument we can directly – in situ – see the ice nucleation as it happens,” said Knopf.
This work could answer many questions, such as: What is the nucleation mechanism? Is there liquid water present or not? On which particle did ice preferentially form?
In one case, the research team found aged sea salt particles could nucleate ice, hinting that the ocean could serve as a source of SSAs, which not only affect warm clouds, but also clouds at lower temperatures where ice forms.
“It’s amazing to discover there may be ice nucleating material within the top hundred micrometers of the ocean, which affects what’s going on up to 10 kilometers in the atmosphere,” said Knopf.
By the SeaThe Arctic is another site that may yield key information about the impact of SSAs on cloud formation. From the decks of the Arctic Ocean research vessel, the CCGS Amundsen, environmental chemist Allan Bertram takes sea spray aerosols directly from the atmosphere. He also collects samples from the top microlayer of the Arctic Ocean.
The Arctic is an area with a climate that is warming faster than anywhere else on Earth. With naturally low aerosol levels, the region is more sensitive to environmental impacts. Bertram’s research, funded by NETCARE, or the Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments, is in the early phases. After obtaining preliminary measurements of ice nucleation properties at the University of British Columbia, he is getting samples ready to go to EMSL for scanning electron microscopy and energy dispersive X-ray spectroscopy analysis to get at particle morphology and chemical composition. Bertram also plans to put the samples through the novel ice nuclei environmental scanning electron microscope analysis.
Already, Bertram’s work collecting samples by air from the Northwest Territories shows there are many ice-nucleating particles that may originate from mineral dust in Asia. That is another complication for Arctic climate that needs to be included in our models, he said. If the ocean’s microlayers also have ice nuclei capabilities, that could further affect cloud formation and precipitation in these regions.
“Both of those are important points,” said Bertram. “But we need EMSL’s capabilities to help us dig deeper on a molecular level and confirm our hypotheses.”
Modeling the “Plausibilities”Instead of generating aerosols to examine, Pacific Northwest National Laboratory, or PNNL, scientist Susannah Burrows creates models to understand what might happen at the surface of the bubbles traversing the air-water interface.
Field samples show that under certain conditions almost 75 percent of SSAs can be coated with organic matter, a finding at odds with models which predicted coatings with only up to 40 percent of organic material. Contributing to that conundrum, most of the coating is made of polysaccharides, compounds which are so water soluble they should not float at the surface like fats or other oils. Further, if the polysaccharides are dissolved in water, then how can they latch onto bubbles at the surface and get launched into the air?
To explain that conflict, Burrows worked with Scott Elliott, an ocean biogeochemist at Los Alamos National Laboratory, and Philip Rasch, the chief scientist for climate science at PNNL, to develop a cooperative adsorption model. This model theorized a thin layer of lipids at the ocean’s surface attracted the dissolved polysaccharides. By sticking to the lipids, the polysaccharides could glom onto bubbles when they burst at the surface of the sea.
To test that hypothesis, Burrows turned to Hongfei Wang, a chief scientist at EMSL who developed a high-resolution vibrational sum-frequency generation spectroscopy to improve analysis of complex surface interfaces. In turn, Wang recruited Rob Walker, Montana State University, for his expertise in surface chemistry.
“This idea of organic enrichments was plausible, but there was no experimental basis for knowing whether or not that was true, let alone how powerful that effect could be,” said Walker.
In a series of experiments with proxy molecules for sugars and phospholipids, Walker and his students used the high-resolution vibrational sum-frequency generation spectroscopy to reveal that glucosamine, a common sugar in plant cell membranes, could stick to a layer of lipids. Moreover, that interaction modified the interfacial water structure and the organization of lipid molecules at the water’s surface.
“Until now, we assumed these molecules didn’t interact with each other,” said Burrows. “But this mechanism gives us some basis for creating a representation that we can use in our models.”
Although the model system worked, “the landscape before us is largely unexplored,” Walker cautioned. They initially worked with DPPC, short for the phospholipid dipalmitoylphosphatidylcholine, but in reality there is a heterogeneous mixture of lipids in the ocean. Does a mix of lipids enhance or inhibit cooperative adsorption? We have no idea, said Walker, and there is a long list of other environmental variables.
“We know it can work. Next, we need to know what matters about how it works,” said Walker. He is planning more studies at EMSL to test more complex models. To date, this research has been supported by the National Science Foundation and an EMSL user proposal.
Closing the GapThere remains much to learn about sea spray aerosols and their role in cloud formation. However, the close collaboration among scientists working on fundamental experiments and modeling studies are a unique aspect of research efforts at EMSL, said John Shilling, interim lead for EMSL’s Atmospheric Aerosol Systems Science Theme.
“There are no degrees of separation between experimentalists and modelers at our facility,” he said. “I think that enhances our research about the effect of these aerosols on our climate.”
EMSL, the Environmental Molecular Sciences Laboratory, is a national scientific user facility sponsored by the Department of Energy's Office of Science. Located at Pacific Northwest National Laboratory in Richland, Wash., EMSL offers an open, collaborative environment for scientific discovery to researchers around the world. Its integrated computational and experimental resources enable researchers to realize important scientific insights and create new technologies.
Author: Elizabeth Devitt is a science journalist and freelance writer.
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