Concerted measurements of lipids in seawater and on submicrometer aerosol particles at the Cabo Verde islands: biogenic sources, selective transfer and high enrichments
Authors: Triesch Nadja , van Pinxteren Manuela , Frka Sanja , Stolle Christian , Spranger Tobias , Hoffmann Erik Hans , Gong Xianda , Wex Heike , Schulz-Bull Detlef , Gašparović Blaženka , Herrmann Hartmut 
Published: March 2021

Abstract. In the marine environment, measurements of lipids as representative species
within different lipid classes have been performed to characterize their
oceanic sources and their transfer from the ocean into the atmosphere to
marine aerosol particles. The set of lipid classes includes hydrocarbons
(HC); fatty acid methyl esters (ME); free fatty acids (FFA); alcohols (ALC);
1,3-diacylglycerols (1,3 DG); 1,2-diacylglycerols (1,2 DG);
monoacylglycerols (MG); wax esters (WE); triacylglycerols (TG); and
phospholipids (PP) including phosphatidylglycerols (PG),
phosphatidylethanolamine (PE), phosphatidylcholines (PC), as well as
glycolipids (GL) which cover sulfoquinovosyldiacylglycerols (SQDG),
monogalactosyl-diacylglycerols (MGDG), digalactosyldiacylglycerols (DGDG)
and sterols (ST). These introduced lipid classes have been analyzed in the
dissolved and particulate fraction of seawater, differentiating between
underlying water (ULW) and the sea surface microlayer (SML) on the one hand.
On the other hand, they have been examined on ambient submicrometer aerosol
particle samples (PM1) which were collected at the Cape Verde
Atmospheric Observatory (CVAO) by applying concerted measurements. These
different lipids are found in all marine compartments but in different
compositions. Along the campaign, certain variabilities are observed for the
concentration of dissolved (∑DLULW: 39.8–128.5 µg L−1, ∑DLSML: 55.7–121.5 µg L−1) and
particulate (∑PLULW: 36.4–93.5 µg L−1, ∑PLSML: 61.0–118.1 µg L−1) lipids in the seawater of the
tropical North Atlantic Ocean. Only slight SML enrichments are observed for
the lipids with an enrichment factor EFSML of 1.1–1.4 (DL) and 1.0–1.7
(PL). On PM1 aerosol particles, a total lipid concentration between
75.2–219.5 ng m−3 (averaged: 119.9 ng m−3) is measured. As also
bacteria – besides phytoplankton sources – influence the lipid
concentrations in seawater and on the aerosol particles, the lipid abundance
cannot be exclusively explained by the phytoplankton tracer
(chlorophyll a). The concentration and enrichment of lipids in the SML are
not related to physicochemical properties which describe the surface
activity. On the aerosol particles, an EFaer (the enrichment factor on
the submicrometer aerosol particles compared to the SML) between 9×104–7×105 is observed. Regarding the individual lipid
groups on the aerosol particles, a statistically significant correlation
(R2=0.45, p=0.028) was found between EFaer and
lipophilicity (expressed by the KOW value), which was not present for
the SML. But simple physicochemical descriptors are overall not sufficient
to fully explain the transfer of lipids. As our findings show that
additional processes such as formation and degradation influence the
ocean–atmosphere transfer of both OM in general and of lipids in particular,
they have to be considered in OM transfer models. Moreover, our data suggest
that the extent of the enrichment of the lipid class constituents on the
aerosol particles might be related to the distribution of the lipid within
the bubble–air–water interface. The lipids TG and ALC which are preferably
arranged within the bubble interface are transferred to the aerosol
particles to the highest extent. Finally, the connection between ice
nucleation particles (INPs) in seawater, which are already active at higher
temperatures (−10 to −15 ∘C), and the lipid classes
PE and FFA suggests that lipids formed in the ocean have the potential to
contribute to (biogenic) INP activity when transferred into the atmosphere.