A Comparison of Atlantic, Pacific and Instant Ocean(TM) Seawaters using Palmitic Acid and Hexadecanol Monolayers as Model Atmospheric Films
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Date
2018-05
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The Ohio State University
Abstract
Sea spray aerosol (SSA) is a major factor in global climate change modeling but is still one of the most uncertain forcing factors. Organic and ionic chemical species within SSA account for only some of the complexity, but can alter properties of these aerosols that affect their interaction with atmospheric systems. Palmitic acid (PA) is found in SSA, originating from cellular membranes in the sea surface microlayer (SSML). Hexadecanol (Hex) is an efficient ice nucleator and is found in the SSML, produced from biological activity. The importance of the SSML as a site for heterogeneous reaction and transfer of chemical species to SSA cannot be understated, so ongoing study is necessary. Since there are so many species in the SSML, creating an effective proxy for use in laboratory work is important to be able to isolate distinct chemical players. Within this thesis, the changes in the surface behavior of PA and Hex on pure water, Atlantic seawater, Pacific seawater, and Instant Ocean (IO) are compared using Langmuir pressure-area (Π-A) isotherms, Brewster angle microscopy (BAM), and infrared reflection-absorption spectroscopy (IRRAS). On Atlantic and Pacific seawater relative to pure water, PA showed condensation at low surface pressure (Π) (Δmean molecular area (MMA) = 2-3 Å2/molecule). As area decreased, there was a loss of phase change with increasing Π, monolayer expansion at high Π, leading to collapse at higher Π than on pure water. BAM images on these seawater subphases showed minimal aggregate formation after collapse in contrast with pure water. The PA monolayer was more compressible on real seawater than pure water. The C-H and carboxyl IRRAS modes changed depending on system pH and organics, and the PA monolayer packed hexagonally. These dynamics can be slated to a balance between expansion-inducing organics and contraction-inducing pH and ionic effects from the seawaters. On these same subphases, Hex showed expansion relative to pure water throughout the isotherms with a retention of phase change on seawater. BAM images showed some aggregation on Pacific and Atlantic seawater, and the Hex monolayer was again more compressible than on pure water. Peaks in IRRAS were analyzed in the CH stretching and CH scissoring regions, showing similar organic content and hexagonal packing on the real seawater subphases. Hex interacted less strongly with the subphases than PA, as expected because of the alcohol headgroup on Hex that maintained neutrality regardless of system pH. On IO, PA and Hex were greatly expanded and showed 3D aggregation earlier that remained throughout compression. Both lipids were most compressible on IO out of any subphase, meaning the surfaces could change their packing efficiently to offer the least resistance to compression. For PA on IO, IRRAS results were irreproducible, which is likely due to interfacial aggregation even at low Π. For Hex, IRRAS results were also variable, but less so than PA, again showing the tendency of PA to interact with the subphase more strongly. Based on these results, IO does not work as a proxy for real seawater in our lab. Alternatively, a house-made artificial seawater (ASW) matched real seawater data best based on preliminary trials, but was hard to maintain within desired specifications. In the future, the ASW should be explored further for seawater study in our lab. After a suitable proxy has been determined, the fundamental chemical interactions in the SSML that influence global climate can be probed in even more representative systems in the lab.
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Keywords
Atmosphere, Surface, proxy systems, climate