Natural aerosols and their interactions with clouds remain an important uncertainty within climate models, especially at the poles. Here, we study the behavior of sea salt aerosols (SSaer) in the Arctic and Antarctic within 12 climate models from CMIP6. We investigate the driving factors that control SSaer abundances and show large differences based on the choice of the source function, and the representation of aerosol processes in the atmosphere. Close to the poles, the CMIP6 models do not match observed seasonal cycles of surface concentrations, likely due to the absence of wintertime SSaer sources such as blowing snow. Further away from the poles, simulated concentrations have the correct seasonality, but have a positive mean bias of up to one order of magnitude. SSaer optical depth is derived from the MODIS data and compared to modeled values, revealing good agreement, except for winter months. Better agreement for AOD than surface concentration may indicate a need for improving the vertical distribution, the size distribution and/or hygroscopicity of modeled polar SSaer. Source functions used in CMIP6 emit very different numbers of small SSaer, potentially exacerbating cloud-aerosol interaction uncertainties in these remote regions. For future climate scenarios SSP126 and SSP585, we show that SSaer concentrations increase at both poles at the end of the 21st century, with more than two times mid-20th century values in the Arctic. The pre-industrial climate CMIP6 experiments suggest there is a large uncertainty in the polar radiative budget due to SSaer.

To improve our understanding of the impact of sea spray aerosols (SSA) on the Earth’s climate, it is critical to understand the physical mechanisms which determine the size-resolved sea spray aerosol source. Of the factors affecting SSA emissions, seawater salinity has perhaps received the least attention in the literature and previous studies have produced conflicting results. Here, we present a series of laboratory experiments designed to investigate the role of salinity on aerosol production from artificial seawater using a continuous plunging jet. During these experiments, the aerosol and surface bubble size distributions were monitored while the salinity was decreased from 35 to 0 g/kg. Three distinct salinity regimes were identified: 1) A high salinity regime, 10-35 g/kg, where decreasing salinity only resulted in minor reductions in particle number emissions but significant reductions in particle volume; 2) an intermediate salinity regime, 5-10 g/kg, with a local maximum in particle number; 3) a low salinity regime, < 5 g/kg, characterized by a rapid decrease in particle number as salinity decreased and dominated by small particles and larger bubbles. We discuss the implications of our results through comparison of the size-resolved aerosol flux and the surface bubble population at different salinities. Finally, by varying the seawater temperature at three specific salinities we have also generated a simple parameterization of the particle number concentration and effective radius as a function of seawater temperature and salinity that can be used to estimate the sea spray aerosol flux in low salinity regions like the Baltic Sea.

Poor understanding of aerosol-cloud interactions and cloud feedback processes in the Arctic climate system limit our ability to constrain future climate in the region and one important knowledge gap is the source of particles upon which cloud droplets and ice crystals have formed. If representative cloud-water can be obtained from Arctic clouds, it’s chemical composition can be analysed to infer the sources of particles present within it. However, the balloon-borne active cloud water sampling systems required to obtain such samples have not previously been feasible due to their weight and the challenging environmental conditions. Here we present a miniaturised cloud-water sampler for balloon-borne collection of cloud water which was deployed during the Microbiology-Ocean-Cloud-Coupling in the High Arctic (MOCCHA) campaign in August and September 2018 along with the deployment protocol required to obtain representative samples in the pristine conditions encountered. We present the chemical composition of the samples obtained as well as the ice-nucleating activity of the samples and discuss the implications of our results on aerosol-cloud interactions in the high Arctic.

The amount of ice versus supercooled water in clouds defines their radiative properties and role in climate feedbacks. Hence, knowledge of the concentration of ice-nucleating particles (INPs) is needed. Generally, the concentrations of INP is found to be very low in remote marine locations allowing clouds to persist in a supercooled state. However, little is known about the INP population in clouds at and around the summertime North Pole. We had expected that concentrations of INPs at the North Pole would have been very low given the distance from open ocean and terrestrial sources coupled with effective wet scavenging processes. Here we show that during summer 2018 (August and September) high concentrations of biological INPs (active at >-20°C) were present at the North Pole. In fact, INP concentrations were sometimes as high as those recorded in mid-latitude locations strongly impacted by highly active biological INPs, in strong contrast to the Southern Ocean. Furthermore, using a balloon borne sampler we demonstrated that INP concentrations were often different at the surface versus higher in the boundary layer where clouds form. Back trajectory analysis suggests that there were strong sources of INPs near the Russian coast, possibly associated with wind-driven sea spray production, whereas the pack ice, open leads, and the marginal ice zone were not sources of highly active INPs. These findings suggest that primary ice production, and therefore Arctic climate, is sensitive to transport from locations such as the Russian coast that are already experiencing marked climate change.