Concerted measurements of free amino acids at the Cabo Verde islands: high enrichments in submicron sea spray aerosol particles and cloud droplets

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.


S1: Limit of quantification (LOQ) of the individual FAA in the matrices of the marine environment -on aerosol particles as atmospheric concentration in pg m -3 , as seawater concentration in nmol L -1 and as cloud water concentration in µg L -1 and ng m -3 analyte atmospheric concentration (a) seawater concentration (b) cloud water concentration (c) pg m -3 nmol L -1 µg L -1 ng m -3 For the LOQ, the concentration of the lowest but still quantifiable calibration stage of the external calibration was used and then extrapolated specifically for the various marine matrices: (a) The following factors were taken into account for the calculation of the atmospheric concentration of individual FAAs: the enrichment 5 factor caused by the reducing of the extract volume, the proportion of the investigated filter material (92 % of the filter material of each stage for the aqueous extract) and a median of the sampled air flow (110 m 3 ).
(b) For the calculation of the seawater concentration, the enrichment factor caused by the reducing of the desalinated sample volume was considered.
(c) For the calculation of the cloud water concentration in µg L -1 , the enrichment factor caused by the reducing of the cloud water volume 10 was regarded. Also, an averaged liquid water content (LWC) of 0.297 was taken into account to additionally calculate the cloud water concentration in ng m -3 .
15 Table S2: The wind speed, wind direction and chl-a concentration in the seawater samples as well as the FAA and sodium concentration and PM in the size-segregated aerosol particle samples (distinguished between submicron and supermicron size range) at the CVAO * The mean value of the measured wind speed/wind direction during the 24 h sampling time of the aerosol particles was considered ** Not on every matching sampling day the chl-a concentration in seawater was investigated. It was generally low but increased during the 5 campaign from 0.1 µg L -1 to 0.6 µg L -1 and is discussed in more detail by van Pinxteren et al., (2020). concentration of the analyte in the ULW was assumed to be LOQ/2, (LOQ is listed in Table S1).

Enrichment of FAA in SML 5
Looking at the individual amino acids (Table S3), Ser with a concentration range of 54-299 nmol L -1 (ULW) and 34-1237 nmol L -1 (SML) had usually the highest contribution to ∑FAA. Also Ala (ULW: 14-193 nmol L -1 , SML: 52-635 nmol L -1 ), Gly (ULW: 16-350 nmol L -1 , SML: 36-698 nmol L -1 ) and Asp (ULW: 3-83 nmol L -1 , SML: 14-286 nmol L -1 ) were included in higher concentrations as part of ∑FAA. This observed high contribution of Ser, Ala, Gly and Asp to ∑FAA in the ULW and especially in the SML is in accordance with the results of Kuznetsova et al. (2004) and Reinthaler et al. (2008). To compare the concentration of ∑FAA in the SML with the ULW samples, the enrichment factor in SML (EFSML) 10 was calculated using Eq. (1) and is shown in Fig. 1 (stars). Regarding the EFSML of ∑FAA, an enrichment of ∑FAA in SML between 1.1 and 298.4 could be observed (averaged EFSML of ∑FAA: 57.2). Although there is a wide variance in the EFSML for ∑FAA, our results are in good agreement with the literature. In the subtropical Atlantic, the EFSML of dissolved FAA which is between 7.6 and 229.4 (59.3±68.8) and between 6.2 and 26.1 (16.5±9.1) in western Mediterranean Sea were reported by Reinthaler et al. (2008). The EFSML depends on the measured concentration of ∑FAA in the SML and the ULW, and here daily variations could be observed during the campaign. Especially the higher EFSML on e.g. 6/10/2017 with 298.4 15 resulted from higher concentrations of ∑FAA in the SML (2224.9 nmol L -1 ) and a very low concentration in the ULW (6.23 nmol L -1 ).   Table S6, was considered.
(**) For the calculation of the percentage contribution of ∑FAA to TDN, the nitrogen content of the individual amino acids, listed in Table   S6, was regarded.
10 For the analysis, sodium corresponding seawater samples (ULW and SML) with n = 5 were investigated. In the SML, the sodium concentration was 12.53 ± 0.53 g L -1 whereas it was 12.45 ± 0.37 g L -1 in the ULW. Because of small relative standard deviations (4.2 % for SML and 2.9 % for ULW), the mean value of the sodium concentration in SML (12.53 g L -1 ) and ULW samples (12.45 g L -1 ) was used for the calculation of EFaer and EFcw. 5

Aerosol particles: dust and marine tracers
To investigate dust and marine impacts on especially the submicron particles we regarded several indicators specifically for the sub-and supermicron particles, such as the MSA concentration, the MSA/sulfate ratio, the fractional residence time of the air masses over water and ice and the size-resolved concentrations of the mineral dust tracers iron (Fe) and titanium (Ti), listed 5 in Table S8. Looking at the 96 h backward trajectories of the investigated air masses, it is obvious that all sampling days showed a very long (≥0.84) or a long (0.67) fractional residence time over water and ice. The sulfate concentration was 0.9±0.2 µg m -3 in the submicron size range and 1.0±0.4 µg m -3 in the supermicron size range. The measured sulfate concentrations in our study were in good agreement with the values of previous studies at the CVAO (Mueller et al., 2010;van Pinxteren et al., 2015). MSA, originating from the multiphase oxidation of dimethylsulfide (DMS) (Hoffmann et al., 2016), is 10 a tracer for marine aerosol particles to estimate the magnitude of biogenic contributions to local aerosol population (Miyazaki et al., 2011;van Pinxteren et al., 2015). The MSA concentration in the submicron size range varied between 7.9-20.8 ng m -3 and between 2.5-18.7 ng m -3 in the supermicron size range. These values are again in good agreement with previous studies at the CVAO (Mueller et al., 2010). In our study, the molar ratio of MSA to sulfate was on average 0.02±0.003 in the submicron and 0.01±0.006 in the supermicron aerosol particles. The MSA/sulfate ratio in the submicron size range was within the 15 MSA/sulfate ratios (0.02-0.04), reported for clean marine air over the Pacific Ocean (Nagao et al., 1999;Miyazaki et al., 2011) and consistent with the averaged MSA/sulfate ratio of PM1 samples (0.022) (van Pinxteren et al., 2015). In order to estimate potential dust influences during the campaign, mineral dust tracer as iron (Fe) and titanium (Ti) were considered. Considering the time-resolved trend of Fe and Ti values in the size-segregated aerosol particle samples, it could be noticed that the lowest concentration of Fe (7.0 ng m -3 , submicron size range) was detected on 4/10/2017 (Fe(PM10): 117.2 ng m -3 ). The Ti concentration 20 on that day was 0.1 ng m -3 in the submicron aerosol particles and 9.4 ng m -3 for PM10. When it comes to typical marine background concentrations of trace metals at the CVAO for PM10 aerosol particles with <25 ng m -3 for Fe and <6 ng m -3 for Ti (Fomba et al., 2013), especially the submicron aerosol particles on e.g. 4/10/2017 showed very low or no mineral dust influences. Moreover, it has to be noted that dust generally influences the supermicron particles to a larger extent than the submicron particles (Fomba et al., 2013). The MSA concentrations and the MSA/sulfate ratio were generally higher for the submicron particles showing a higher potential influence of marine sources to the submicron particles. The concentrations of the dust tracers in the submicron particles were low and significantly lower than in the supermicron particles except for the ones of the last sampling day (10/10/2017). This indicated that the submicron particles were mainly of marine origin during most of the time of the campaign but exhibited a dust impact to some extend on the last sampling day. 5 10     S11: Atmospheric concentration (ng m -3 ) of particulate matter (PM), WSOC, sodium (Na + ), MSA and ∑FAA in the sizesegregated aerosol particle samples at the CVAO and the MV station on sampling days (4/10/2017, 6/10/2017, 7/10/2017) and as an average of these three days; NAnot available data

The comparability of the different marine matrices
Spot measurements in the ocean (ULW, SML) were taken during the sampling time (24 h) of size-segregated aerosol particle samples. As a basis for a possible comparison, the origins of the aerosol particles and of the seawater samples had to be investigated in more detail. Due to the identified origin of the air masses using 96 h backward trajectories, the concentrations 10 of inorganic ions and mineral dust tracers on the aerosol particles were considered to be mainly of marine origin (section 3.2.1 and SI page 11/12: "Aerosol particles: dust and marine tracers"). An overview of backward trajectories during the entire campaign period can be found in the study by van Pinxteren et al., (2020). Regarding the seawater, it was shown that despite of a strong variability of ∑FAA in seawater samples, the measured ∑FAA concentrations in the ULW and especially in the SML samples are representative for the investigated marine region, the tropical Atlantic Ocean (section 3.1). van Pinxteren et 15 al. (2017) demonstrated that at the CVAO the air masses followed the water current (as described in Peña-Izquierdo et al.  (Fig. S1), it could be concluded that there is a probable connection between FAAs 20 as part of OM on the aerosol particles and in the SML.   were considered 5 ** the 'remote aerosol case' is valid for the size-segregated aerosol particle samples, sampled at the CVAO; OH radical concentrations of 3•10 -12 mol L -1

Eq. (3)
The calculation of the mean lifetime is based on the assumption that the OH radical concentrations react preferentially with the present amino acids. Within the marine environment, the presence of many different compounds implies that the OH 15 concentrations react not only with the amino acids but also with other compounds. Therefore, the mean lifetime given here is to be considered as a rough estimation method, which rather represents the lower limit of the lifetime of the individual amino acids. The parameter EFaer regards the transfer from the ocean onto the aerosol particles considering the enrichment processes of OM and compounds linked to OM (e.g. FAA) during bubble bursting. Chemical reaction processes during the transfer are not taken into account. 5