Chemical Characterization of Dissolved Organic Compounds from Coastal Sea Surface Microlayers (Baltic Sea, Germany) Manuela van Pinxteren, † Conny Mü ller, † Yoshiteru Iinuma, † Christian Stolle, ‡ and Hartmut Herrmann †, * † Leibniz-Institut fü r Troposphä renforschung (IfT), Permoserstrasse 15, D-04318 Leipzig, Germany ‡ Leibniz-Institute for Baltic Sea Research Warnemuende (IOW), Seestrasse 15, 18119 Rostock, Germany * S Supporting Information ABSTRACT: The physicochemical properties of the sea surface microlayer (SML), i.e. the boundary layer between the air and the sea, and its impact on air- sea exchange processes have been investigated for decades. However, a detailed description about these processes remains incomplete. In order to obtain a better chemical characterization of the SML, in a case study three pairs of SML and corresponding bulk water samples were taken in the southern Baltic Sea. The samples were analyzed for dissolved organic carbon and dissolved total nitrogen, as well as for several organic nitrogen containing compounds and carbohydrates, namely aliphatic amines, dissolved free amino acids, dissolved free mono- saccharides, sugar alcohols, and monosaccharide anhydrates. Therefore, reasonable analytical procedures with respect to desalting and enrichment were established. All aliphatic amines and the majority of the investigated amino acids (11 out of 18) were found in the samples with average concentrations between 53 ng L -1 and 1574 ng L -1 . The concentrations of carbohydrates were slightly higher, averaging 2900 ng L -1 . Calculation of the enrichment factor (EF) between the sea surface microlayer and the bulk water showed that dissolved total nitrogen was more enriched (EF: 1.1 and 1.2) in the SML than dissolved organic carbon (EF: 1.0 and 1.1). The nitrogen containing organic compounds were generally found to be enriched in the SML (EF: 1.9-9.2), whereas dissolved carbohydrates were not enriched or even depleted (EF: 0.7-1.2). Although the investigated compounds contributed on average only 0.3% to the dissolved organic carbon and 0.4% to the total dissolved nitrogen fraction, these results underline the importance of single compound analysis to determine SML structure, function, and its potential for a transfer of compounds into the atmosphere. ■ INTRODUCTION Exchange processes between air and sea play an essential role as oceans cover a substantial area of the planet. In this context, the sea surface microlayer (SML) represents a physical boundary layer between air and seawater formed due to different physicochemical properties of the two layers. 1 The SML has been described as a structured marine compartment that is very important for atmospheric processes; however, its character- istics and dynamics remain largely enigmatic. 1,2 Recent investigations suggest that the SML is stable up to a wind speed of 10 m s -1 and is therefore existent at the global average wind speed of 6.6 m s -1 . 3 One of the main interests in a better understanding of the SML is attributed to the fact that enrichment of inorganic and organic matter can occur in the SML and how this might affect biogeochemical cycles. 1 As the SML is the interface for all gaseous, liquid, and particulate mass transfer between the ocean and the atmosphere, the SML may play a key role in the export of organic material from the ocean to the atmosphere due to processes such as bubble bursting. Although important processes near the air-sea interface are not limited to a strictly defined microlayer but rather take place over gradients of varying thicknesses, the SML is often operationally defined as the first 1000 μm of the ocean surface. 1 The SML is either described as a series of sublayers of dry and wet surfactants 4 or as a gelatinous-like matrix. 5 Independent of the model, the accumulation of surface active material, such as fatty acids, lipids, proteins, monosaccharides, and amino acids is evident. Kuznetsova et al. reported an enrichment of amino acids (free dissolved and combined) in the SML by a factor up to 50. 6 Further investigations showed enrichment of anthropogenic hydrophobic compounds such as PCBs 2,7 and tin organic compounds 8 in the SML. The focus of this work was a detailed chemical character- ization of dissolved organic material of three SML samples and the corresponding bulk water taken in the southern Baltic Sea in summer 2006 and in winter and spring 2008. Although a total of three paired samples is quite a low number, first indications of trends regarding the concentrations and enrichment factors for different organic substances can still be obtained from this limited data basis. Besides the determination Special Issue: Marine Boundary Layer: Ocean Atmospheric Inter- actions Received: December 15, 2011 Revised: March 30, 2012 Accepted: April 4, 2012 Published: April 4, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 10455 dx.doi.org/10.1021/es204492b | Environ. Sci. Technol. 2012, 46, 10455-10462