Concerted measurements of free amino acids at the Cabo Verde islands: high enrichments in submicron sea spray aerosol particles and cloud droplets
Authors: Triesch Nadja , van Pinxteren Manuela , Engel Anja , Herrmann Hartmut 
Published: January 2021

Abstract. Measurements of free amino acids (FAAs) in the marine environment to
elucidate their transfer from the ocean into the atmosphere, to marine
aerosol particles and to clouds, were performed at the MarParCloud (marine biological production,
organic aerosol particles and marine clouds: a process chain) campaign
at the Cabo Verde islands in autumn 2017. According to physical and chemical
specifications such as the behavior of air masses, particulate MSA
concentrations and MSA∕sulfate ratios, as well as particulate mass
concentrations of dust tracers, aerosol particles predominantly of marine
origin with low to medium dust influences were observed. FAAs were
investigated in different compartments: they were examined in two types of
seawater underlying water (ULW) and in the sea surface microlayer (SML), as
well as in ambient marine size-segregated aerosol particle samples at two
heights (ground height based at the Cape Verde Atmospheric Observatory, CVAO, and
at 744 m height on Mt. Verde) and in cloud water using concerted
measurements. The ∑FAA concentration in the SML varied between
0.13 and 3.64 µmol L−1, whereas it was between 0.01 and 1.10 µmol L−1 in the ULW; also, a strong enrichment of ∑FAA
(EFSML: 1.1–298.4, average of 57.2) was found in the SML. In the
submicron (0.05–1.2 µm) aerosol particles at the CVAO, the
composition of FAAs was more complex, and higher atmospheric concentrations
of ∑FAA (up to 6.3 ng m−3) compared to the supermicron
(1.2–10 µm) aerosol particles (maximum of 0.5 ng m−3) were
observed. The total ∑FAA concentration (PM10) was between
1.8 and 6.8 ng m−3 and tended to increase during the campaign. Averaged
∑FAA concentrations in the aerosol particles on Mt. Verde were
lower (submicron: 1.5 ng m−3; supermicron: 1.2 ng m−3) compared to
the CVAO. A similar contribution percentage of ∑FAA to dissolved
organic carbon (DOC) in the seawater (up to 7.6 %) and to water-soluble
organic carbon (WSOC) in the submicron aerosol particles (up to 5.3 %)
indicated a related transfer process of FAAs and DOC in the marine
environment. Considering solely ocean–atmosphere transfer and neglecting atmospheric
processing, high FAA enrichment factors were found in both aerosol particles
in the submicron range (EFaer(∑FAA): 2×103–6×103) and medium enrichment factors in the
supermicron range (EFaer(∑FAA): 1×101–3×101). In addition, indications for a biogenic FAA
formation were observed. Furthermore, one striking finding was the high and
varying FAA cloud water concentration (11.2–489.9 ng m−3), as well as
enrichments (EFCW: 4×103 and 1×104 compared to the SML and ULW, respectively), which were reported here for
the first time. The abundance of inorganic marine tracers (sodium,
methanesulfonic acid) in cloud water suggests an influence of oceanic
sources on marine clouds. Finally, the varying composition of the FAAs in the
different matrices shows that their abundance and ocean–atmosphere transfer
are influenced by additional biotic and abiotic formation and degradation
processes. Simple physicochemical parameters (e.g., surface activity) are
not sufficient to describe the concentration and enrichments of the FAAs in
the marine environment. For a precise representation in organic matter (OM)
transfer models, further studies are needed to unravel their drivers and
understand their composition.