Physicochemical effects of the marine microlayer on air-sea gas transport

McKenna, Sean P.; Bock, Erik J.

doi:10.1007/3-540-33271-5_9

Cited by 11 publications

(6 citation statements)

References 25 publications

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“…The SML is itself overlain by the surface nanolayer (SNL); this is ∼ 1 to 10 nm thick and can be a monolayer, it also comprises of surface-active substances and it may be enriched in carbohydrates during summer (Lass et al, 2013). Its physicochemical properties differ from those of the SML, providing an additional diffusion barrier and modifying the viscoelasticity of the air-sea interface (McKenna and Bock, 2006). This reduces the rate of air-sea gas exchange by wave damping and by attenuating turbulent energy transfer (Liss and Duce, 1997).…”

Section: Introductionmentioning

confidence: 99%

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

2016

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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.

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Section: Introductionmentioning

confidence: 99%

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

2016

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“…It is a place where certain surface-active substances as well as bouyant organics tend to accumulate. The properties of this layer may differ considerably from those of the underlying microlayer and determine the viscoelastic properties of the water-air interface (McKenna and Bock, 2006).…”

Section: Introductionmentioning

confidence: 99%

Seasonal signatures in SFG vibrational spectra of the sea surface nanolayer at Boknis Eck Time Series Station (SW Baltic Sea)

2013

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The very thin sea surface nanolayer on top of the sea surface microlayer, sometimes just one monomolecular layer thick, forms the interface between ocean and atmosphere. Due to the small dimension and tiny amount of substance, knowledge about the development of the layer in the course of the year is scarce. In this work, the sea surface nanolayer at Boknis Eck Time Series Station (BE), southwestern Baltic Sea, has been investigated over a period of three and a half years. Surface water samples were taken monthly by screen sampling and were analyzed in terms of organic content and composition by sum frequency generation spectroscopy, which is specifically sensitive to interfacial layers. A yearly periodicity has been observed with a pronounced abundance of sea surface nanolayer material (such as carbohydrate-rich material) during the summer months. On the basis of our results we conclude that the abundance of organic material in the nanolayer at Boknis Eck is not directly related to phytoplankton abundance alone. We speculate that indeed sloppy feeding of zooplankton together with photochemical and/or microbial processing of organic precursor compounds is responsible for the pronounced seasonality

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“…Vertical velocity fluctuations near the interface are considered vital to gas transfer enhancement. Decreased vertical transport of fresh fluid towards the water surface results in a thicker boundary layer and thus a reduced transfer velocity (McKenna and McGillis, 2004).…”

Section: Surfactantsmentioning

confidence: 99%

Measurements of air–sea gas transfer velocities in the Baltic Sea

2019

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Abstract. Heat transfer velocities measured during three different campaigns in the Baltic Sea using the active controlled flux technique (ACFT) with wind speeds ranging from 5.3 to 14.8 m s−1 are presented. Careful scaling of the heat transfer velocities to gas transfer velocities using Schmidt number exponents measured in a laboratory study allows us to compare the measured transfer velocities to existing gas transfer velocity parameterizations, which use wind speed as the controlling parameter. The measured data and other field data clearly show that some gas transfer velocities are much lower than those based on the empirical wind speed parameterizations. This indicates that the dependencies of the transfer velocity on the fetch, i. e., the history of the wind and the age of the wind-wave field, and the effects of surface-active material need to be taken into account.

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Physicochemical effects of the marine microlayer on air-sea gas transport

Cited by 11 publications

References 25 publications

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

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

Seasonal signatures in SFG vibrational spectra of the sea surface nanolayer at Boknis Eck Time Series Station (SW Baltic Sea)

Measurements of air–sea gas transfer velocities in the Baltic Sea

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