Comment on acp-2022-832

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 submicron 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 factor EF_SML = 1.3&plusmn;0.2 and 1.1&plusmn;0.5 respectively). In contrast, the amino acids exhibited a higher enrichment in the SML with an averaging EF_SML of 2.4&plusmn;0.3 although being less surface-active than lipids. The same compounds studied in the seawater were found on the ambient submicron aerosol particles whereas the lipids were more pronounced enriched (EF_{<em>aer.</em>} = 1.6x10⁵) compared to the amino acids and carbohydrates (EF<em>_aer.</em> = 1.5x10³ and 1.3x10³ 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 composition of the single 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 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, in detail 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 here investigated aerosol particles.