Investigating chemical composition at the sea-air interface: A subsampling approach for marine surface microlayer analysis

The marine surface microlayer (SML) is distinct from the subsurface water by physical, chemical and biological properties. Being the interface, the SML regulates mass and energy transfer between the ocean and the overlying atmosphere. Given the wide surface area covered by oceans, even small change in flux may have a significant global impact. While there is a general agreement on the importance of SML chemical composition, sampling and analyzing it still poses a major challenge. Common practices for chemical analysis of SML involve water sampling by a mesh screen, glass plate or rotating drum that are applied in-situ. These methods can be applied only in calm water and are prone to collect a relatively thick layer of surface water. The present work examines an alternative approach, including water subsampling combined with the application of a polycellulose-ester membrane filter to solely sample the SML. Compounds adsorbed to the filter are then extracted and analyzed by solid phase micro extraction and gas chromatograph mass spectrometry. As part of the method evaluation, the effect of sorption affinity, competition between solutes, as well as correlation between dissolved concentrations and adsorbed loads on the filter were tested. Overall, the proposed methodology allows semi-quantitative analysis of the volatile and semi volatile organic compounds (VOCs and SVOCs) profile of the SML, offering high efficiency, ability to sample in various sea conditions, and lower noise in the collected data relative to the traditional SML sampling methods.