ALBERT

Description: MILAN was a multidisciplinary, international study examining how the diel variability of sea-surface microlayer biogeochemical properties potentially impacts ocean-atmosphere interaction, in order to improve our understanding of this globally important process. The sea-surface microlayer (SML) at the air-sea interface is 〈 1 mm deep but it is physically, chemically and biologically distinct from the underlying water and the atmosphere above. Wind-driven turbulence and solar radiation are important drivers of SML physical and biogeochemical properties. Given that the SML is involved in all ocean-atmosphere exchanges of mass and energy, its response to solar radiation, especially in relation to how it regulates the air-sea exchange of climate-relevant gases and aerosols, is surprisingly poorly characterised. MILAN (sea-surface MIcroLAyer at Night) was an international, multidisciplinary campaign designed to specifically address this issue. In spring 2017, we deployed diverse sampling platforms (research vessels, radio-controlled catamaran, free-drifting buoy) to study full diel cycles in the coastal North Sea SML and in underlying water, and installed a land-based aerosol sampler. We also carried out concurrent ex situ experiments using several microsensors, a laboratory gas exchange tank, a solar simulator, and a sea spray simulation chamber. In this paper we outline the diversity of approaches employed and some initial results obtained during MILAN. Our observations of diel SML variability, e.g. the influence of changing solar radiation on the quantity and quality of organic material, and diel changes in wind intensity primarily forcing air-sea CO2 exchange, underline the value and the need of multidisciplinary campaigns for integrating SML complexity into the context of air-sea interaction.

Description: The project MarParCloud (Marine biological production, organic aerosol Particles and marine Clouds: a process chain) aims to improve our understanding of the genesis, modification and impact of marine organic matter (OM) from its biological production, to its export to marine aerosol particles and, finally, to its ability to act as ice-nucleating particles (INPs) and cloud condensation nuclei (CCN). A field campaign at the Cape Verde Atmospheric Observatory (CVAO) in the tropics in September–October 2017 formed the core of this project that was jointly performed with the project MARSU (MARine atmospheric Science Unravelled). A suite of chemical, physical, biological and meteorological techniques was applied, and comprehensive measurements of bulk water, the sea surface microlayer (SML), cloud water and ambient aerosol particles collected at a ground-based and a mountain station took place. Key variables comprised the chemical characterization of the atmospherically relevant OM components in the ocean and the atmosphere as well as measurements of INPs and CCN. Moreover, bacterial cell counts, mercury species and trace gases were analyzed. To interpret the results, the measurements were accompanied by various auxiliary parameters such as air mass back-trajectory analysis, vertical atmospheric profile analysis, cloud observations and pigment measurements in seawater. Additional modeling studies supported the experimental analysis. During the campaign, the CVAO exhibited marine air masses with low and partly moderate dust influences. The marine boundary layer was well mixed as indicated by an almost uniform particle number size distribution within the boundary layer. Lipid biomarkers were present in the aerosol particles in typical concentrations of marine background conditions. Accumulation- and coarse-mode particles served as CCN and were efficiently transferred to the cloud water. The ascent of ocean-derived compounds, such as sea salt and sugar-like compounds, to the cloud level, as derived from chemical analysis and atmospheric transfer modeling results, denotes an influence of marine emissions on cloud formation. Organic nitrogen compounds (free amino acids) were enriched by several orders of magnitude in submicron aerosol particles and in cloud water compared to seawater. However, INP measurements also indicated a significant contribution of other non-marine sources to the local INP concentration, as (biologically active) INPs were mainly present in supermicron aerosol particles that are not suggested to undergo strong enrichment during ocean–atmosphere transfer. In addition, the number of CCN at the supersaturation of 0.30 % was about 2.5 times higher during dust periods compared to marine periods. Lipids, sugar-like compounds, UV-absorbing (UV: ultraviolet) humic-like substances and low-molecular-weight neutral components were important organic compounds in the seawater, and highly surface-active lipids were enriched within the SML. The selective enrichment of specific organic compounds in the SML needs to be studied in further detail and implemented in an OM source function for emission modeling to better understand transfer patterns, the mechanisms of marine OM transformation in the atmosphere and the role of additional sources. In summary, when looking at particulate mass, we see oceanic compounds transferred to the atmospheric aerosol and to the cloud level, while from a perspective of particle number concentrations, sea spray aerosol (i.e., primary marine aerosol) contributions to both CCN and INPs are rather limited.

Description: Transparent exopolymer particles (TEPs) exhibit the properties of gels and are ubiquitously found in the world oceans. TEPs may enter the atmosphere as part of sea-spray aerosol. Here, we report number concentrations of TEPs with a diameter 〉 4.5 µm, hence covering a part of the supermicron particle range, in ambient aerosol and cloud water samples from the tropical Atlantic Ocean as well as in generated aerosol particles using a plunging waterfall tank that was filled with the ambient seawater. The ambient TEP concentrations ranged between 7×102 and 3×104 #TEP m−3 in the aerosol particles and correlations with sodium (Na+) and calcium (Ca2+) (R2=0.5) suggested some contribution via bubble bursting. Cloud water TEP concentrations were between 4×106 and 9×106 #TEP L−1 and, according to the measured cloud liquid water content, corresponding to equivalent air concentrations of 2–4×103 #TEP m−3. Based on Na+ concentrations in seawater and in the atmosphere, the enrichment factors for TEPs in the atmosphere were calculated. The tank-generated TEPs were enriched by a factor of 50 compared with seawater and, therefore, in-line with published enrichment factors for supermicron organic matter in general and TEPs specifically. TEP enrichment in the ambient atmosphere was on average 1×103 in cloud water and 9×103 in ambient aerosol particles and therefore about two orders of magnitude higher than the corresponding enrichment from the tank study. Such high enrichment of supermicron particulate organic constituents in the atmosphere is uncommon and we propose that atmospheric TEP concentrations resulted from a combination of enrichment during bubble bursting transfer from the ocean and a secondary TEP in-situ formation in atmospheric phases. Abiotic in-situ formation might have occurred from aqueous reactions of dissolved organic precursors that were present in particle and cloud water samples, whereas biotic formation involves bacteria, which were abundant in the cloud water samples. The ambient TEP number concentrations were two orders of magnitude higher than recently reported ice nucleating particle (INP) concentrations measured at the same location. As TEPs likely possess good properties to act as INPs, in future experiments it is worth studying if a certain part of TEPs contributes a fraction of the biogenic INP population.

Description: 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.