Surfactant control of gas transfer velocity along an offshore coastal transect: results from a laboratory gas exchange tank

Abstract

Abstract. Understanding the physical and biogeochemical controls of air–sea gas exchange is necessary for establishing biogeochemical models for predicting regional- and global-scale trace gas fluxes and feedbacks. To this end we report the results of experiments designed to constrain the effect of surfactants in the sea surface microlayer (SML) on the gas transfer velocity (kw; cm h−1), seasonally (2012–2013) along a 20 km coastal transect (North East UK). We measured total surfactant activity (SA), chromophoric dissolved organic matter (CDOM) and chlorophyll a (Chl a) in the SML and in sub-surface water (SSW) and we evaluated corresponding kw values using a custom-designed air–sea gas exchange tank. Temporal SA variability exceeded its spatial variability. Overall, SA varied 5-fold between all samples (0.08 to 0.38 mg L−1 T-X-100), being highest in the SML during summer. SML SA enrichment factors (EFs) relative to SSW were ∼ 1.0 to 1.9, except for two values (0.75; 0.89: February 2013). The range in corresponding k660 (kw for CO2 in seawater at 20 °C) was 6.8 to 22.0 cm h−1. The film factor R660 (the ratio of k660 for seawater to k660 for “clean”, i.e. surfactant-free, laboratory water) was strongly correlated with SML SA (r ≥ 0.70, p ≤ 0.002, each n = 16). High SML SA typically corresponded to k660 suppressions ∼ 14 to 51 % relative to clean laboratory water, highlighting strong spatiotemporal gradients in gas exchange due to varying surfactant in these coastal waters. Such variability should be taken account of when evaluating marine trace gas sources and sinks. Total CDOM absorbance (250 to 450 nm), the CDOM spectral slope ratio (SR = S275 − 295∕S350 − 400), the 250 : 365 nm CDOM absorption ratio (E2 : E3), and Chl a all indicated spatial and temporal signals in the quantity and composition of organic matter in the SML and SSW. This prompts us to hypothesise that spatiotemporal variation in R660 and its relationship with SA is a consequence of compositional differences in the surfactant fraction of the SML DOM pool that warrants further investigation.

Estimation of bubble-mediated air–sea gas exchange from concurrent DMS and CO<sub>2</sub> transfer velocities at intermediate–high wind speeds

“…W A ), while W B may vary widely due to surfactant concentration with little or no impact upon bubble-mediated gas exchange (e.g. Pereira et al, 2016). In this case, k w should be more strongly correlated with W A than W B or W T .…”

On the imprint of surfactant‐driven stabilization of laboratory breaking wave foam with comparison to oceanic whitecaps

“…For example, recent relevant measurements have been made by Pereira et al . [] along an offshore coastal transect in the North Sea, off the North East coast of England. Surfactant activity was found to vary 5‐fold between 0.08 mg L −1 and 0.38 mg L −1 seasonally, and to vary by a factor of 2 spatially along a single 20 km transect, with values typically increasing with increasing proximity to the coast.…”

Study on organic matter fractions in the surface microlayer in the Baltic Sea by spectrophotometric and spectrofluorometric methods

“…The humic substances contribute significantly both to the CDOM pool in the water column as well as to surfactant concentrations, especially in coastal ocean, estuaries and semi-enclosed marine basins that are impacted by terrestrial runoff and marine traffic. Therefore optical methods could be used efficiently to determine of natural and anthropogenic organic surface active substances in the SML (Drozdowska et al Pereira et al, 2016;Frew et al,2004;Zhang et al, 2009;McKnight et al, 1997;Guéguen et al, 2007).…”