2014 Research Experience for Undergrads
Analyzing oxidants of sulfate in the remote marine boundary layer using stable isotope analysis
This past summer I worked with Becky Alexander and her group to begin to analyze the sulfate collected in aerosol samples from the air above remote regions of the Indian Ocean and the Atlantic Ocean. We are interested in the chemical reactions that form particulate sulfate from gas-phase sulfur dioxide because these sulfate particles impact climate. Sulfate particles suspended in the atmosphere, or aerosols, directly affect climate by scattering away incoming shortwave radiation from the sun. Less understood are the ways in which sulfate particles interact with clouds, and thus with the earth’s radiative budget. The presence of aerosols in clouds creates more surfaces upon which cloud droplets may form, resulting in clouds made up of smaller particles, a larger total surface area, and greater ability to reflect solar radiation. These smaller particles can also lengthen the lifetime of the clouds because their smaller size makes their deposition more difficult. Depending on the oxidant in these reactions, and whether the reaction occurs in the gas phase, on particles, or while dissolved in water, the resulting sulfate will have different implications for new cloud and particle formation, as well as cloud lifetime and reflectivity. Developing a more accurate inventory of how sulfate is formed will help to model cloud-climate interactions and advance understanding of an uncertain factor in radiative forcing. My work this summer began work to study how hyphalous acids (HOCl and HOBr) may be oxidizing sulfate.
How, you might ask, do we take a sample of sulfate that has already formed and find out what reaction occurred to oxidize it? The tool that we used is light stable isotope analysis. This is analysis of isotopes that are relatively light and are not radioactive. We looked at the three different isotopes of oxygen which were isolated from the sampled sulfate. Previous lab experiments have established the characteristic isotopic ratios left by certain oxidants (in our case hydrogen peroxide, the hydroxyl radical, ozone, and hypohalous acids were of interest). Comparing the measured isotopic ratios in our samples to the ratios associated with the oxidants of interest provides information about which reactions are occurring.
The journey of a sample from filter to oxygen isotope ratios involves filtering the sample, removing organics, ion separation to isolate sulfate using an ion chromatographer, and ion exchange using resins to enable further removal of organics and conversion to a silver salt. Sample preparation is finished when silver sulfate powder is distributed into quartz capsules. The quartz capsules are then heated at 1000 degrees Celsius so that the oxygen present in the silver sulfate is released in gas phase and is carried by helium gas through several traps to remove other gases before arriving in the isotope ratio mass spectrometer. Using the unique ratio between the mass and charge of each of the three oxygen isotopes, the mass spectrometer separates the isotopes and records their relative concentrations.
My work included this sample preparation and analysis. It takes several days to fully prepare a sample, and at the end of the internship I had finished several calibrations and we had just analyzed our first sample (there are over one hundred)! I got to join the project as it was just beginning, so I am excited to hear about the developments that continue when I am gone. Along with lab work, I used the CAABA/MECCA box model to investigate potential feedbacks related to the formation of sulfate. Sulfate is an important source of acid in the troposphere. The oxidation of sulfate by hypohalous acids is acid catalyzed. The box model allows us to see if the increasing acidity of the atmosphere by production of sulfate leads to more production of sulfate by hypohalous acids. Further work with the model will investigate these interactions, though my work consisted of mainly debugging my model. As someone who had limited previous modeling experience, I found this component of the project particularly interesting as I became more comfortable with making modifications in the model.
I really enjoyed the opportunity to build on my lab experience, develop modeling skills, and learn about isotopic geochemistry. The Alexander group was extremely welcoming and supportive. I was lucky enough to join them on a lab group hike in Denny Creek during the last week of my internship and experience the beautiful Pacific Northwest. The community at JISAO was also collaborative and supportive; we six interns worked together on an oceanography outreach project for local elementary and middle school students. Meeting and working with the other interns and learning about their interests and projects was a great part of the summer. Thank you to JISAO for the wonderful opportunity to spend the summer at the University of Washington!
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