Connecting the Atmosphere and the Ocean through Reactions at the Sea Surface Microlayer

Abstract

The reaction of ozone with chemical species at the ocean surface is a source of volatile molecules, including molecular iodine (I2) and volatile organic compounds (VOCs), including organoiodine molecules. The volatiles released affect air quality and climate by impacting aerosol composition and the atmospheric oxidative budget.One major goal of this thesis was to understand the factors which affect the formation of VOCs from gas-surface ozonolysis. To accomplish this, axenic Thalassiosira pseudonana cultures were used to generate a reproducible and realistic surface ocean proxy. When the culture was exposed to gas-phase ozone, VOCs were observed which were further oxidized to form secondary organic aerosol. The VOC type and yield was linked to the phytoplankton growth cycle, emphasizing the importance of biological processes on VOC precursors. The second and third projects focused on understanding the interaction of ozone with the iodide component of seawater, and how that affected the release of VOCs from ozonolysis. Using a halide solution that matched seawater composition, the ozone loss rate was strongly influenced by iodide depletion. These findings were substantiated with a kinetic multilayer model. Uncertainties in the efficiency of mixing processes in the ocean makes assessment of the importance of iodide depletion in the environment an open question. The formation of gaseous I2 from the ozonolysis of iodide was suppressed in the presence of chloride and phytoplankton organics, which could not be modelled by known chemistry. A sensitivity analysis indicated two key aqueous phase reaction rate constants which could be changed to replicate the trend of decreasing gaseous I2yield in the presence of chloride. Attempts to determine why phytoplankton organics suppress the I2 yield involved measuring I2uptake to the T. pseudonana cultures. However, experimental limitations caused by the rapid hydrolysis of I2 into HOI prevented firm conclusions being made. Finally, preliminary studies comparing the VOC yield from photochemistry and ozonolysis on real Arctic Ocean samples are presented. Overall, this thesis highlights the factors that should be considered when determining the importance of ozonolysis to VOC and iodine emission to the marine atmosphere.