Amino acids, carbohydrates, and lipids in the tropical oligotrophic Atlantic Ocean: sea-to-air transfer and atmospheric in situ formation

Abstract

This study examines carbohydrates, amino acids, and lipids as important contributors to organic carbon (OC) in the tropical Atlantic Ocean at the Cape Verde Atmospheric Observatory (CVAO). The above compounds were measured in both surface seawater and in ambient sub-micron aerosol particles to investigate their sea-to-air transfer, including their enrichment in the sea surface microlayer (SML), potential atmospheric in situ formation or degradation, and their oceanic contribution to the ambient marine aerosol particles. In bulk seawater and the SML, similar distributions among species were found for the lipids and carbohydrates with moderate SML enrichments (enrichment factors EFSML = 1.3 ± 0.2 and 1.1 ± 0.5 respectively). In contrast, the amino acids exhibited a higher enrichment in the SML with an average EFSML of 2.3 ± 0.4, although they are less surface-active than lipids. The same compounds studied in the seawater were found on the ambient sub-micron aerosol particles, whereas the lipids' enrichment was more pronounced (EFaer.=1.6×105) compared to the amino acids and carbohydrates (EFaer.=1.5×103 and 1.3×103 respectively), likely due to their high surface activity and/or the lipophilic character. Detailed molecular analysis of the seawater and aerosol particles revealed changes in the relative abundance of the individual organic compounds. They were most pronounced for the amino acids and are likely related to an in situ atmospheric processing by biotic and/or abiotic reactions. On average, 49 % of the OC on the aerosol particles (=∧97 ng m−3) could be attributed to the specific components or component groups investigated in this study. The majority (43 %) was composed of lipids. Carbohydrates and amino acids made up less than 1 % of the OC. This shows that carbohydrates, at least when resolved via molecular measurements of single sugars, do not comprise a very large fraction of OC on marine aerosol particles, in contrast to other studies. However, carbohydrate-like compounds are also present in the high lipid fraction (e.g. as glycolipids), but their chemical composition could not be revealed by the measurements performed here. Previously determined OC components at the CVAO, specifically amines, oxalic acid, and carbonyls, comprised an OC fraction of around 6 %. Since the identified compounds constituted about 50 % of the OC and belong to the rather short-lived biogenic material probably originating from the surface ocean, a pronounced coupling between ocean and atmosphere was indicated for this oligotrophic region. The remaining, non-identified OC fraction might in part contain recalcitrant OC; however, this fraction does not constitute the vast majority of OC in the aerosol particles investigated here.