Speaker: Philippa Rickard (Heriot-Watt Univ)
Topic: The Secret Surface: Considering microlayer heterogeneity in control of air-water gas cycling

Despite the surface microlayer (SML; uppermost < 400 um water) covering ~ 70% of Earth's surface, it is typically overlooked in trace gas and carbon cycling predictions. Historically, the SML was assumed to be passive in air-water mass transfer processes but, in recent decades, its physico-chemical properties have been shown to significantly impact air-water gas transfer velocity (kw); the accumulation of surface-active organic matter (surfactants) in the SML alters the kinematic viscosity of the water surface and impedes molecular diffusion across the air-water interface. Consequently, the omission of SML dynamics in trace gas cycling predictions may be overestimating oceanic uptake rates of atmospheric CO2 by up to 9%, at the Ocean basin scale. Furthermore, different water bodies show different correlations of kw and surfactant concentration, which is indicative of SML compositional control of kw. Investigating SML compositional variability is a developing area of research, and one that must be incorporated into large-scale observations of the water surface if we are to accurately parameterise the spatiotemporal variability in SML dynamics and relationship with kw.