Characterization of aerosol particles at Cabo Verde close to sea level and at the cloud level – Part 2: Ice-nucleating particles in air, cloud and seawater

Ice-nucleating particles (INPs) in the troposphere can form ice in clouds via heterogeneous ice nucleation. Yet, atmospheric number concentrations of INPs (NINP) are not well characterized, and, although there is some understanding of their sources, it is still unclear to what extend different sources contribute or if all sources are known. In this work, we examined properties of INPs at Cabo Verde (a.k.a. Cape Verde) from different environmental compartments: the oceanic sea surface microlayer (SML), underlying water (ULW), cloud water and the atmosphere close to both sea level and cloud level. Both enrichment and depletion of NINP in SML compared to ULW were observed. The enrichment factor (EF) varied from roughly 0.4 to 11, and there was no clear trend in EF with ice-nucleation temperature. NINP values in PM10 sampled at Cape Verde Atmospheric Observatory (CVAO) at any particular ice-nucleation temperature spanned around 1 order of magnitude below −15 ∘C, and about 2 orders of magnitude at warmer temperatures (>−12  ∘C). Among the 17 PM10 samples at CVAO, three PM10 filters showed elevated NINP at warm temperatures, e.g., above 0.01 L−1 at −10 ∘C. After heating samples at 95 ∘C for 1 h, the elevated NINP at the warm temperatures disappeared, indicating that these highly ice active INPs were most likely biological particles. INP number concentrations in PM1 were generally lower than those in PM10 at CVAO. About 83±22 %, 67±18 % and 77±14 % (median±standard deviation) of INPs had a diameter >1 µm at ice-nucleation temperatures of −12, −15 and −18 ∘C, respectively. PM1 at CVAO did not show such elevated NINP at warm temperatures. Consequently, the difference in NINP between PM1 and PM10 at CVAO suggests that biological ice-active particles were present in the supermicron size range. NINP in PM10 at CVAO was found to be similar to that on Monte Verde (MV, at 744 m a.s.l.) during noncloud events. During cloud events, most INPs on MV were activated to cloud droplets. When highly ice active particles were present in PM10 filters at CVAO, they were not observed in PM10 filters on MV but in cloud water samples instead. This is direct evidence that these INPs, which are likely biological, are activated to cloud droplets during cloud events. For the observed air masses, atmospheric NINP values in air fit well to the concentrations observed in cloud water. When comparing concentrations of both sea salt and INPs in both seawater and PM10 filters, it can be concluded that sea spray aerosol (SSA) only contributed a minor fraction to the atmospheric NINP. This latter conclusion still holds when accounting for an enrichment of organic carbon in supermicron particles during sea spray generation as reported in literature

Biofilm-like habitat at the sea-surface: A mesocosm study, Cruise No. POS537, 14.09.2019 – 04.10.2019, Malaga (Spain) – Cartagena (Spain) - BIOFILM

Biofilm-like properties can form on sea surfaces, but an understanding of the underlying processes leading to the development of these biofilms is not available. We used approaches to study the development of biofilm-like properties at the sea surface, i.e. the number, abundance and diversity of bacterial communities and phytoplankton, the accumulation of gel-like particles and dissolved tracers. During the expedition POS537 we used newly developed and free drifting mesocosms and performed incubation experiments. With these approaches we aim to investigate the role of light and UV radiation as well as the microbes themselves, which lead to the formation of biofilms. With unique microbial interactions and photochemical reactions, sea surface biofilms could be biochemical reactors with significant implications for ocean and climate research, e.g. with respect to the marine carbon cycle, diversity of organisms and oceanatmosphere interactions

Marine organic matter in the remote environment of the Cape Verde islands-an introduction and overview to the MarParCloud campaign

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-molecularweight 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. © Author(s) 2020

Marine organic matter in the remote environment of the Cape Verde islands – an introduction and overview to the MarParCloud campaign

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

Chlorophyll a, particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphorous (POP) and Surfactants measurements from the central arctic

As part of the MOCCHA 2018 campaign onboard I/B Oden, Sea surface microlayer (SML) and underlying water (ULW) samples were taken from an open lead system in the central arctic. Sampling occured during the autumn freeze-up from 18.08.18 - 08.09.18. Latitude ranged from 88.7644-89.5474 degrees and longitude ranged from 38.1087-45.7652 degrees as the ice pack moved over time, elevevation remained 0. Sea surface microlayer samples were collected using the glass plate technique (Harvey and Burzell, 1972, doi:10.4319/lo.1972.17.1.0156) via rotating glass disks equipped to a catamaran and underlying water samples were collected from 1 meter depth. Seven variables were sampled for; transparent exopolymer particles (TEP), coomassie stainable particles (CSP), surfactants, chlorophyll a, particulate organic carbon (POC), particulate organic nitrogen (PON) and particulate organic phosphorous (POP). Their concentrations are presented here in micrograms per litre. sea surface microlayer (SML) and undrlaying water (ULW) samples were brought back to a wet lab onboard I/B Oden. All samples were stored in their respective states until processing and analysis could be done after the expedition in laboratories of the University of Oldenburg. Chlorophyll a (Chl a) was measured by filtering 500 - 1000mL of seawater on to pre-combusted (4 h, 450°C) GF/F filters (Whatman). The filters were stored frozen (-18 °C). Chl a was then analysed after the expedition using a fluorometer (Jenway 6285, precision of 0.01± < 1 ng/mL) following Wasmund et al. (2006). Samples for particulate organic carbon (POC), nitrogen (PON) and phosphorous (POP), collectively particulate organic matter (POM), were filtered onto acid washed and pre-combusted GF/F filters (Whatman) and stored at -18 ◦C. For analysis, filters for POC and PON were dried at 60°C for 3days, put in tin capsules and measured using an elemental analyser (Thermo, Flash EA 1112 and Elementar Analysensysteme, precision of 0.01 ‰). POP was measured by molybdate reaction after digestion with potassium peroxydisulfate (K2S2O8) solution (Wetzel and Likens, 2000)

Transparent exopolymer particles (TEP), coomassie stainable particles (CSP) and Surfactants measurements from the central artic

As part of the MOCCHA 2018 campaign onboard I/B Oden, Sea surface microlayer (SML) and underlying water (ULW) samples were taken from an open lead system in the central arctic. Sampling occured during the autumn freeze-up from 18.08.18 - 08.09.18. Latitude ranged from 88.7644-89.5474 degrees and longitude ranged from 38.1087-45.7652 degrees as the ice pack moved over time, elevevation remained 0. Sea surface microlayer samples were collected using the glass plate technique (Harvey and Burzell, 1972, doi:10.4319/lo.1972.17.1.0156) via rotating glass disks equipped to a catamaran and underlying water samples were collected from 1 meter depth. Seven variables were sampled for; transparent exopolymer particles (TEP), coomassie stainable particles (CSP), surfactants, chlorophyll a, particulate organic carbon (POC), particulate organic nitrogen (PON) and particulate organic phosphorous (POP). Their concentrations are presented here in micrograms per litre. sea surface microlayer (SML) and undrlaying water (ULW) samples were brought back to a wet lab onboard I/B Oden. All samples were stored in their respective states until processing and analysis could be done after the expedition in laboratories of the University of Oldenburg. Transparent exopolymer particles (TEP) were measured by filtering seawater, in triplicates, onto 0.2 µm polycarbonate filters under low vacuum (3.0.CO;2-7) and measures surfactants as bulk concentrations. Under stirring for roughly 30 seconds, surfactants accumulate at the hanging mercury drop electrode at a potential of -0.6 V versus an Ag/AgCl reference electrode. The frequency of the A.C. voltage was 170 Hz and the p-p amplitude was 10 mV. With a sampling rate of 20 mV s-1, the phase-shifted signal of the A.C. current was measured to quantify the amount of surfactants accumulating on the Hg drop. For sample evaluation, 10ml of unfiltered seawater were collected and stored at 4°C in the dark until they could be analysed back in the home laboratory. Each sample was measured three to four times using a standard addition technique involving the non-ionic surfactant Triton X-100 (Sigma Aldrich, Germany) as the standard. Surfactant concentrations are therefore expressed as the equivalent concentration of Triton X-100 addition (µg Teq L-1)

Biochemical parameters in the SML and ULW from 3 different regions

Biochemical data from 3 different regions; Cape Verde, the Baltic Sea, and Norwegian fjords/Sea. discreet samples for TEP, Chl a, POC, PON and nutrients as well as 2 and 24 hour average data on sea state parameters; temperature, salinity, PAR and wind speed. Vertical profile data for TEP, POC, TCN, small autotrophs and Chl a are also presented for depths of 0-2m, sampled in the Baltic and Norwegian Seas