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
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DOI：
10.5194/bg-13-3981-2016
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发表时间：
2016-07
期刊：
影响因子：
4.9
通讯作者：
R. Pereira;K. Schneider-Zapp;R. Upstill‐Goddard
中科院分区：
文献类型：
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作者：
R. Pereira;K. Schneider-Zapp;R. Upstill‐Goddard
摘要。了解海气交换的物理和生物地球化学控制是建立预测区域和全球尺度微量气体通量和反馈的生物地球化学模型的必要条件。为此，我们报告了一项实验的结果，该实验旨在约束海洋表面微层（SML）中表面活性剂对气体传输速度（kw; cm h - 1）的影响，季节性（2012-2013）沿着20公里的沿海样带（英国东北部）。我们测量了SML和地下水（SSW）中的总表面活性剂活性（SA）、显色性溶解有机物（CDOM）和叶绿素a (Chl a)，并使用定制设计的海气交换箱评估了相应的kw值。时间变率大于空间变率。总体而言，所有样品之间的SA差异为5倍（0.08至0.38 mg L -1 T-X-100），夏季SML最高。SML SA富集因子（EFs）相对于SSW为~ 1.0 ~ 1.9，除了两个值（0.75；0.89:2013年2月）。对应的k660（海水中CO2在20°C时的kw）范围为6.8 ~ 22.0 cm h−1。膜因子R660（海水的k660与“干净”，即无表面活性剂的实验室水的k660之比）与SML SA呈强相关（r≥0.70,p≤0.002，各n = 16）。相对于干净的实验室水，高SML SA通常对应于k660抑制约14%至51%，突出了由于这些沿海水域中表面活性剂的变化而导致的气体交换的强烈时空梯度。在评价海洋微量气体源和汇时，应考虑到这种可变性。CDOM的总吸光度（250 ~ 450 nm）、CDOM的光谱斜率比（SR = S275 ~ 295∕S350 ~ 400）、250:365 nm CDOM吸收比（E2: E3）和Chl a都是SML和SSW中有机质数量和组成的时空信号。这促使我们假设R660的时空变化及其与SA的关系是SML DOM池中表面活性剂部分成分差异的结果，值得进一步研究。
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.