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In recent years, sea spray as well as the biological material it contains has received increased attention as a source of ice-nucleating particles (INPs). Such INPs may play a role in remote marine regions, where other sources of INPs are scarce or absent. In the Arctic, these INPs can influence water–ice partitioning in low-level clouds and thereby the cloud lifetime, with consequences for the surface energy budget, sea ice formation and melt, and climate. Marine aerosol is of a diverse nature, so identifying sources of INPs is challenging. One fraction of marine bioaerosol (phytoplankton and their exudates) has been a particular focus of marine INP research. In our study we attempt to address three main questions. Firstly, we compare the ice-nucleating ability of two common phytoplankton species with Arctic seawater microlayer samples using the same instrumentation to see if these phytoplankton species produce ice-nucleating material with sufficient activity to account for the ice nucleation observed in Arctic microlayer samples. We present the first measurements of the ice-nucleating ability of two predominant phytoplankton species: Melosira arctica, a common Arctic diatom species, and Skeletonema marinoi, a ubiquitous diatom species across oceans worldwide. To determine the potential effect of nutrient conditions and characteristics of the algal culture, such as the amount of organic carbon associated with algal cells, on the ice nucleation activity, Skeletonema marinoi was grown under different nutrient regimes. From comparison of the ice nucleation data of the algal cultures to those obtained from a range of sea surface microlayer (SML) samples obtained during three different field expeditions to the Arctic (ACCACIA, NETCARE, and ASCOS), we found that they were not as ice active as the investigated microlayer samples, although these diatoms do produce ice-nucleating material. Secondly, to improve our understanding of local Arctic marine sources as atmospheric INPs we applied two aerosolization techniques to analyse the ice-nucleating ability of aerosolized microlayer and algal samples. The aerosols were generated either by direct nebulization of the undiluted bulk solutions or by the addition of the samples to a sea spray simulation chamber filled with artificial seawater. The latter method generates aerosol particles using a plunging jet to mimic the process of oceanic wave breaking. We observed that the aerosols produced using this approach can be ice active, indicating that the ice-nucleating material in seawater can indeed transfer to the aerosol phase. Thirdly, we attempted to measure ice nucleation activity across the entire temperature range relevant for mixed-phase clouds using a suite of ice nucleation measurement techniques – an expansion cloud chamber, a continuous-flow diffusion chamber, and a cold stage. In order to compare the measurements made using the different instruments, we have normalized the data in relation to the mass of salt present in the nascent sea spray aerosol. At temperatures above 248 K some of the SML samples were very effective at nucleating ice, but there was substantial variability between the different samples. In contrast, there was much less variability between samples below 248 K. We discuss our results in the context of aerosol–cloud interactions in the Arctic with a focus on furthering our understanding of which INP types may be important in the Arctic atmosphere.

Coastal ecosystems play a crucial role in greenhouse gas (GHG) dynamics but are less studied than open oceans or terrestrial systems. This study measured concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in six shallow bays of the wider Stockholm Archipelago during spring (April) and autumn (September–October) 2024 using cavity ring-down spectroscopy combined with a water equilibration system. We explored how GHG levels relate to bay physical characteristics (i.e. topographic openness, sediment properties vegetation cover) and seawater properties (temperature, salinity, dissolved-oxygen saturation, chlorophyll-a, organic carbon, and nutrient concentrations), revealing significant seasonal variation of concentrations. Surface water pCO2 ranged from 225–1372 ppm, CH4 from 3.6–580 nmol L−1, and N2O from 8–20.8 nmol L−1 with pCO2 and CH4 higher in autumn and N2O higher in spring. CH4 concentrations below 250 nmol L−1 were negatively correlated with N2O, while higher CH4 levels showed a positive correlation, suggesting differences in the dominant sedimentary microbial pathways. Most bays acted as net GHG sinks in April and sources in September, with only one bay showing net source behaviour in both seasons. One bay that is subject to substantial human impacts (e.g. dredging, high nutrient loading, reduced vegetation cover) showed CO2-equivalent CH4 emissions that surpassed CO2 uptake in this particular bay. CO2-equivalent fluxes ranged from −195.2 to 793.6 mg CO2 eq. m−2 d−1 (median: 131.5 mg CO2 eq. m−2 d−1). This study is distinctive in simultaneously measuring all three major GHGs across multiple bays in relation to diverse environmental controls, offering a uniquely integrated understanding of coastal GHG dynamics. These findings highlight the variability and complexity of coastal ecosystems and demonstrate the importance of high-resolution measurements for accurate up-scaling of fluxes from these dynamic environments.

The chemical composition of cloud water can be used to infer the sources of particles upon which cloud droplets and ice crystals have formed. In order to obtain cloud water for analysis of chemical composition for elevated clouds in the pristine high Arctic, balloon-borne active cloud water sampling systems are the optimal approach. However, such systems have not been feasible to deploy previously due to their weight and the challenging environmental conditions. We have taken advantage of recent developments in battery technology to develop a miniaturised cloud water sampler for balloon-borne collection of cloud water. Our sampler is a bulk sampler with a cloud drop cutoff diameter of approximately 8 µm and an estimated collection efficiency of 70%. The sampler was heated to prevent excessive ice accumulation and was able to operate for several hours under the extreme conditions encountered in the high Arctic. We have tested and deployed the new sampler on a tethered balloon during the Microbiology-Ocean-Cloud-Coupling in the High Arctic (MOCCHA) campaign in August and September 2018 close to the North pole. The sampler was able to successfully retrieve cloud water samples that were analysed to determine their chemical composition as well as their ice-nucleating activity. Given the pristine conditions found in the high Arctic we have placed significant emphasis on the development of a suitable cleaning procedure to minimise background contamination by the sampler itself.

Sea spray aerosol particles are a recognised type of ice-nucleating particles under mixed-phase cloud conditions. Entities that are responsible for the heterogeneous ice nucleation ability include intact or fragmented cells of marine microorganisms as well as organic matter released by cell exudation. Only a small fraction of sea spray aerosol is transported to the upper troposphere, but there are indications from mass-spectrometric analyses of the residuals of sublimated cirrus particles that sea salt could also contribute to heterogeneous ice nucleation under cirrus conditions. Experimental studies on the heterogeneous ice nucleation ability of sea spray aerosol particles and their proxies at temperatures below 235 K are still scarce. In our article, we summarise previous measurements and present a new set of ice nucleation experiments at cirrus temperatures with particles generated from sea surface microlayer and surface seawater samples collected in three different regions of the Arctic and from a laboratory-grown diatom culture (Skeletonema marinoi). The particles were suspended in the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud chamber and ice formation was induced by expansion cooling. We confirmed that under cirrus conditions, apart from the ice-nucleating entities mentioned above, also crystalline inorganic salt constituents can contribute to heterogeneous ice formation. This takes place at temperatures below 220 K, where we observed in all experiments a strong immersion freezing mode due to the only partially deliquesced inorganic salts. The inferred ice nucleation active surface site densities for this nucleation mode reached a maximum of about 5×1010 m−2 at an ice saturation ratio of 1.3. Much smaller densities in the range of 108–109 m−2 were observed at temperatures between 220 and 235 K, where the inorganic salts fully deliquesced and only the organic matter and/or algal cells and cell debris could contribute to heterogeneous ice formation. These values are 2 orders of magnitude smaller than those previously reported for particles generated from microlayer suspensions collected in temperate and subtropical zones. While this difference might simply underline the strong variability of the number of ice-nucleating entities in the sea surface microlayer across different geographical regions, we also discuss how instrumental parameters like the aerosolisation method and the ice nucleation measurement technique might affect the comparability of the results amongst different studies.

We discuss concentrations of dissolved CH4, N2O, O2, NO3− and NH4+, and emission fluxes of CH4 and N2O for river sites in the western Congo Basin, Republic of Congo (ROC). Savannah, swamp forest and tropical forest samples were collected from the Congo main stem and seven of its tributaries during November 2010 (41 samples; wet season) and August 2011 (25 samples; dry season; CH4 and N2O only). Dissolved inorganic nitrogen (DIN: NH4+ NO3−; wet season) was dominated by NO3− (63 ± 19 % of DIN). Total DIN concentrations (1.5–45.3 µmol L−1) were consistent with the near absence of agricultural, domestic and industrial sources for all three land types. Dissolved O2 (wet season) was mostly undersaturated in swamp forest (36 ± 29 %) and tropical forest (77 ± 36 %) rivers but predominantly supersaturated in savannah rivers (100 ± 17 %). The dissolved concentrations of CH4 and N2O were within the range of values reported earlier for sub-Saharan African rivers. Dissolved CH4 was found to be supersaturated (11.2–9553 nmol L−1; 440–354 444 %), whereas N2O ranged from strong undersaturation to supersaturation (3.2–20.6 nmol L−1; 47–205 %). Evidently, rivers of the ROC are persistent local sources of CH4 and can be minor sources or sinks for N2O. During the dry season the mean and range of CH4 and N2O concentrations were quite similar for the three land types. Wet and dry season mean concentrations and ranges were not significant for N2O for any land type or for CH4 in savannah rivers. The latter observation is consistent with seasonal buffering of river discharge by an underlying sandstone aquifer. Significantly higher wet season CH4 concentrations in swamp and forest rivers suggest that CH4 can be derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers also exhibited both low (47 %) and high (205 %) N2O saturation but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always supersaturated (103–266 %) and for which the overall means and ranges of N2O were not significantly different. In swamp and forest rivers O2 saturation co-varied inversely with CH4 saturation (log %) and positively with % N2O. A significant positive correlation between N2O and O2 saturation in swamp rivers was coincident with strong N2O and O2 undersaturation, indicating N2O consumption during denitrification in the sediments. In savannah rivers persistent N2O supersaturation and a negative correlation between N2O and O2 suggest N2O production mainly by nitrification. This is consistent with a stronger correlation between N2O and NH4+ than between N2O and NO3−. Our ranges of values for CH4 and N2O emission fluxes (33–48 705 µmol CH4 m−2 d−1; 1–67 µmol N2O m−2 d−1) are within the ranges previously estimated for sub-Saharan African rivers but they include uncertainties deriving from our use of basin-wide values for CH4 and N2O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which is quite likely for CH4 (at least 20 %), we consider our emission fluxes for CH4 to be conservative.

Sea spray is one of the largest natural aerosol sources and plays an important role in the Earth’s radiative budget. These particles are inherently hygroscopic, that is, they take-up moisture from the air, which affects the extent to which they interact with solar radiation. We demonstrate that the hygroscopic growth of inorganic sea salt is 8–15% lower than pure sodium chloride, most likely due to the presence of hydrates. We observe an increase in hygroscopic growth with decreasing particle size (for particle diameters <150 nm) that is independent of the particle generation method. We vary the hygroscopic growth of the inorganic sea salt within a general circulation model and show that a reduced hygroscopicity leads to a reduction in aerosol-radiation interactions, manifested by a latitudinal-dependent reduction of the aerosol optical depth by up to 15%, while cloud-related parameters are unaffected. We propose that a value of κ s =1.1 (at RH=90%) is used to represent the hygroscopicity of inorganic sea salt particles in numerical models. Sea spray, one of the largest natural aerosol sources, plays an important role in the Earth’s radiative budget. Here the authors show that the ability of sea salt particles to take up water is smaller than for pure salt, with implications for the parameterization of the direct aerosol radiative effect.

We have developed an inorganic sea spray source function that is based upon state-of-the-art measurements of sea spray aerosol production using a temperature-controlled plunging jet sea spray aerosol chamber. The size-resolved particle production was measured between 0.01 and 10 μm dry diameter. Particle production decreased non-linearly with increasing seawater temperature (between −1 and 30 °C) similar to previous findings. In addition, we observed that the particle effective radius, as well as the particle surface, particle volume and particle mass, increased with increasing seawater temperature due to increased production of particles with dry diameters greater than 1 μm. By combining these measurements with the volume of air entrained by the plunging jet we have determined the size-resolved particle flux as a function of air entrainment. Through the use of existing parameterisations of air entrainment as a function of wind speed, we were subsequently able to scale our laboratory measurements of particle production to wind speed. By scaling in this way we avoid some of the difficulties associated with defining the \"white area\" of the laboratory whitecap – a contentious issue when relating laboratory measurements of particle production to oceanic whitecaps using the more frequently applied whitecap method. The here-derived inorganic sea spray source function was implemented in a Lagrangian particle dispersion model (FLEXPART – FLEXible PARTicle dispersion model). An estimated annual global flux of inorganic sea spray aerosol of 5.9 ± 0.2 Pg yr−1 was derived that is close to the median of estimates from the same model using a wide range of existing sea spray source functions. When using the source function derived here, the model also showed good skill in predicting measurements of Na+ concentration at a number of field sites further underlining the validity of our source function. In a final step, the sensitivity of a large-scale model (NorESM – the Norwegian Earth System Model) to our new source function was tested. Compared to the previously implemented parameterisation, a clear decrease of sea spray aerosol number flux and increase in aerosol residence time was observed, especially over the Southern Ocean. At the same time an increase in aerosol optical depth due to an increase in the number of particles with optically relevant sizes was found. That there were noticeable regional differences may have important implications for aerosol optical properties and number concentrations, subsequently also affecting the indirect radiative forcing by non-sea spray anthropogenic aerosols.

We discuss concentrations of dissolved CH.sub.4, N.sub.2 O, O.sub.2, NO.sub.3 .sup.- and NH.sub.4 .sup.+, and emission fluxes of CH.sub.4 and N.sub.2 O for river sites in the western Congo Basin, Republic of Congo (ROC). Savannah, swamp forest and tropical forest samples were collected from the Congo main stem and seven of its tributaries during November 2010 (41 samples; wet season) and August 2011 (25 samples; dry season; CH.sub.4 and N.sub.2 O only). Dissolved inorganic nitrogen (DIN: NH.sub.4 .sup.+ + NO.sub.3 .sup.- ; wet season) was dominated by NO.sub.3 .sup.- (63 ± 19 % of DIN). Total DIN concentrations (1.5-45.3 µmol L.sup.-1) were consistent with the near absence of agricultural, domestic and industrial sources for all three land types. Dissolved O.sub.2 (wet season) was mostly undersaturated in swamp forest (36 ± 29 %) and tropical forest (77 ± 36 %) rivers but predominantly supersaturated in savannah rivers (100 ± 17 %). The dissolved concentrations of CH.sub.4 and N.sub.2 O were within the range of values reported earlier for sub-Saharan African rivers. Dissolved CH.sub.4 was found to be supersaturated (11.2-9553 nmol L.sup.-1 ; 440-354 444 %), whereas N.sub.2 O ranged from strong undersaturation to supersaturation (3.2-20.6 nmol L.sup.-1 ; 47-205 %). Evidently, rivers of the ROC are persistent local sources of CH.sub.4 and can be minor sources or sinks for N.sub.2 O. During the dry season the mean and range of CH.sub.4 and N.sub.2 O concentrations were quite similar for the three land types. Wet and dry season mean concentrations and ranges were not significant for N.sub.2 O for any land type or for CH.sub.4 in savannah rivers. The latter observation is consistent with seasonal buffering of river discharge by an underlying sandstone aquifer. Significantly higher wet season CH.sub.4 concentrations in swamp and forest rivers suggest that CH.sub.4 can be derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers also exhibited both low (47 %) and high (205 %) N.sub.2 O saturation but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always supersaturated (103-266 %) and for which the overall means and ranges of N.sub.2 O were not significantly different. In swamp and forest rivers O.sub.2 saturation co-varied inversely with CH.sub.4 saturation (log %) and positively with % N.sub.2 O. A significant positive correlation between N.sub.2 O and O.sub.2 saturation in swamp rivers was coincident with strong N.sub.2 O and O.sub.2 undersaturation, indicating N.sub.2 O consumption during denitrification in the sediments. In savannah rivers persistent N.sub.2 O supersaturation and a negative correlation between N.sub.2 O and O.sub.2 suggest N.sub.2 O production mainly by nitrification. This is consistent with a stronger correlation between N.sub.2 O and NH.sub.4 .sup.+ than between N.sub.2 O and NO.sub.3 .sup.- . Our ranges of values for CH.sub.4 and N.sub.2 O emission fluxes (33-48 705 µmol CH.sub.4 m.sup.-2 d.sup.-1 ; 1-67 µmol N.sub.2 O m.sup.-2 d.sup.-1) are within the ranges previously estimated for sub-Saharan African rivers but they include uncertainties deriving from our use of basin-wide values for CH.sub.4 and N.sub.2 O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which is quite likely for CH.sub.4 (at least 20 %), we consider our emission fluxes for CH.sub.4 to be conservative.

We discuss concentrations of dissolved CH4, N2O, O2, NO3− and NH4+, and emission fluxes of CH4 and N2O for river sites in the western Congo Basin, Republic of Congo (ROC). Savannah, swamp forest and tropical forest samples were collected from the Congo main stem and seven of its tributaries during November 2010 (41 samples; wet season) and August 2011 (25 samples; dry season; CH4 and N2O only). Dissolved inorganic nitrogen (DIN: NH4++ NO3−; wet season) was dominated by NO3− (63 ± 19 % of DIN). Total DIN concentrations (1.5–45.3 µmol L−1) were consistent with the near absence of agricultural, domestic and industrial sources for all three land types. Dissolved O2 (wet season) was mostly undersaturated in swamp forest (36 ± 29 %) and tropical forest (77 ± 36 %) rivers but predominantly supersaturated in savannah rivers (100 ± 17 %). The dissolved concentrations of CH4 and N2O were within the range of values reported earlier for sub-Saharan African rivers. Dissolved CH4 was found to be supersaturated (11.2–9553 nmol L−1; 440–354 444 %), whereas N2O ranged from strong undersaturation to supersaturation (3.2–20.6 nmol L−1; 47–205 %). Evidently, rivers of the ROC are persistent local sources of CH4 and can be minor sources or sinks for N2O. During the dry season the mean and range of CH4 and N2O concentrations were quite similar for the three land types. Wet and dry season mean concentrations and ranges were not significant for N2O for any land type or for CH4 in savannah rivers. The latter observation is consistent with seasonal buffering of river discharge by an underlying sandstone aquifer. Significantly higher wet season CH4 concentrations in swamp and forest rivers suggest that CH4 can be derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers also exhibited both low (47 %) and high (205 %) N2O saturation but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always supersaturated (103–266 %) and for which the overall means and ranges of N2O were not significantly different. In swamp and forest rivers O2 saturation co-varied inversely with CH4 saturation (log %) and positively with % N2O. A significant positive correlation between N2O and O2 saturation in swamp rivers was coincident with strong N2O and O2 undersaturation, indicating N2O consumption during denitrification in the sediments. In savannah rivers persistent N2O supersaturation and a negative correlation between N2O and O2 suggest N2O production mainly by nitrification. This is consistent with a stronger correlation between N2O and NH4+ than between N2O and NO3−. Our ranges of values for CH4 and N2O emission fluxes (33–48 705 µmol CH4 m−2 d−1; 1–67 µmol N2O m−2 d−1) are within the ranges previously estimated for sub-Saharan African rivers but they include uncertainties deriving from our use of basin-wide values for CH4 and N2O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which is quite likely for CH4 (at least 20 %), we consider our emission fluxes for CH4 to be conservative.