Chemical characterization of dissolved organic compounds from coastal sea surface microlayers (Baltic Sea, Germany)

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 monosaccharides, 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 and 1574 ng/l. The concentrations of carbohydrates were slightly higher, averaging 2900 ng/l. 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.海表微层（sea surface microlayer, SML）作为大气与海洋之间的边界层，其理化性质及其对气海交换过程的影响已被研究数十年之久。然而，针对这些过程的详尽描述仍有待完善。为更精准地表征海表微层的化学特征，本研究在波罗的海南部开展了一项案例研究，采集了三组海表微层样本及其对应的主体水体对照样本。对采集的样本开展了溶解有机碳、溶解总氮，以及多种含氮有机化合物与碳水化合物的检测分析，所涉物质包括脂肪胺、溶解游离氨基酸、溶解游离单糖、糖醇与单糖酐。据此建立了适用于脱盐与富集环节的合理分析流程。所有脂肪胺与半数以上的目标氨基酸（18种中的11种）均在样本中被检出，其平均浓度介于53纳克每升至1574纳克每升之间。碳水化合物的浓度则略高，平均可达2900纳克每升。通过计算海表微层与主体水体间的富集因子（enrichment factor, EF）可知，海表微层中溶解总氮的富集程度（EF值为1.1与1.2）高于溶解有机碳（EF值为1.0与1.1）。含氮有机化合物总体上在海表微层中呈现富集特征（EF值区间为1.9~9.2），而溶解碳水化合物则未出现富集，甚至出现贫化（EF值区间为0.7~1.2）。尽管本次检测的化合物平均仅占溶解有机碳总量的0.3%、占溶解总氮组分的0.4%，但上述研究结果仍凸显了单化合物分析对于解析海表微层的结构、功能及其向大气输送化合物的潜在能力的重要意义。