Investigations into Biological Influences on the Carbon Isotopic Composition (d13C) of Nascent Sea Spray Aerosol and Ocean-Aerosol Transfer of Organic Material

Atmospheric aerosols remain the largest uncertainty in assessments of the anthropogenic influence on Earth’s radiative budget. Aerosols affect Earth’s radiative budget directly, by reflecting and absorbing incoming solar radiation, and indirectly, by serving as nuclei for water and ice cloud formation. Current estimates of anthropogenic impacts on the direct and indirect aerosol effects are hindered by an inadequate understanding of how naturally produced aerosols contribute to both processes. Sea spray aerosol (SSA), formed by oceanic wave breaking, represents the largest source of natural aerosol to the atmosphere by mass. The climate-relevant properties of SSA, such as hygroscopicity, reflectivity, and ice nucleation ability, are highly dependent on the amount and composition of the organic material transferred from the seawater into these aerosols. The composition of this organic material is strongly influenced by microbial activity in the seawater, highlighting the importance of employing new techniques to examine the impact of oceanic biological activity on the organic material transferred into SSA. Carbon isotopic analysis has been frequently employed to differentiate between anthropogenic and natural sources of aerosol carbon, but has not previously been applied to study ocean-aerosol transfer of organic material. In my dissertation work, I measured carbon isotopic compositions (d13C) of seawater and SSA organic material during two laboratory phytoplankton blooms, identifying an increased contribution of “freshly-produced” carbon to SSA, strongly controlled by the microbial loop. I further demonstrate that not accounting for this biological influence on the d13C value of nascent SSA can lead to significant underestimates in the contribution of anthropogenic aerosol carbon to the marine environment (Chapter 2). Building from this work, I investigated differences between submicron and supermicron SSA during a wave channel mesocosm experiment. The stark differences between these two SSA fractions, with supermicron SSA heavily influenced by biological activity and submicron SSA primarily influenced by surface-active anthropogenic compounds originating from the coastal seawater, reveals the importance of biology and ocean-aerosol transfer processes on SSA organic composition and d13C (Chapter 3). Finally, I explored biologically induced changes in seawater submicron particulates (SMPs) in the bulk seawater and SSML to better understand how seawater biology may influence the transfer of ice nucleating entities into SSA (Chapter 4)