EAGER: Characterization of Surface Tension During the Western Atlantic Climate Study (WACS)

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Physicochemical interactions involving marine organic matter (OM) in the surface ocean, at the air-sea interface, and in the atmosphere have important implications for radiative transfer and climate. Current understanding of OM dynamics in the surface ocean and corresponding interactions with wave processes and marine aerosol production largely neglect the physical processes that occur within a wave-breaking bubble plume and at the sea surface: Aerosol production in Earth system models is parameterized by wind-speed alone, and OM is often prognostically linked to observable parameters such as chlorophyll-a, which do not reflect observed variability. The poor state of current understanding reflected in these highly simplified approaches results from the lack of (1) relevant observations of surfactant characteristics at the ocean surface and (2) a reliable theoretical framework that captures the associated non-linearity.Seawater OM exhibits variable surfactant characteristics, which result from nutrient-mediated microbial processes in and below the surface mixed layer, modulated by seasonal stratification. This leads to the hypothesis that there is a significant surfactant signature associated with these surface and at-depth DOC pools that is reflected in the observed population of nascent ocean-derived aerosols. It is to test this hypothesis as part of a NOAA- and NSF-funded cruise from Boston, MA to Bermuda during August 2012 using measurements of surface tension in conjunction with direct observation of in situ surfactants in seawater and a large suite of ancillary measurements of water column, atmospheric boundary layer, and aerosol properties.This EAGER award will specifically support novel application of the maximum bubble pressure (MBP) method to characterize surface tension at the natural air-seawater interface. The method involves measuring pressure within gas bubbles at the end of a fine capillary tube of a controlled size and rate injected into the water column. The surface tension for bubbles with variable lifetimes at the capillary tip as the bubbles are generated is governed by the diffusion rate of surfactants from the bulk liquid to the bubble surface, based on the Laplace equation for a film-mediated bubble. The exploratory MBP measurements, if successful, will represent the first step toward building a representative map of ocean surfactants, which will be useful both to atmospheric scientists and chemical oceanographers. This will prove valuable in improving Earth system models used in climate prediction as well as ocean wave and weather forecast models that directly affect commerce and transportation. A postdoctoral scholar will gain experience with the method and benefit from participation on a research cruise with a world-class team of atmospheric chemists and oceanographers.