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The Atmospheric Radiation Measurement Mobile Facility‐2 was installed onboard the research vessel Aurora Australis to measure aerosol properties during the 2017–2018 Measurement of Aerosols, Radiation, and CloUds over the pristine Southern ocean (MARCUS) Experiment, providing unique data on aerosols latitudinal and seasonal variation, including south of 60°S where previous observations are scarce. Data from a Cloud Condensation Nuclei (CCN) counter and Ultra‐High‐Sensitivity Aerosol Spectrometer show that both the number concentration (NCCN) and size distribution of CCN‐active aerosols, with diameters (D) between 60 nm < D < 1,000 nm are different over the North Southern Ocean (NSO) (50°S–60°S) and the South Southern Ocean (SSO) (62°S–68°S). The average NSO NCCN at 0.2% and 0.5% supersaturation were 28% and 49% less than that over the SSO. This increase of CCN over the SSO is caused by the increase of aerosols with 60 nm < D < 200 nm, consistent with calculations of Aerosol Scattering Angstrom Exponents derived from a nephelometer. Aerosol hygroscopicity growth factor measured by the Hygroscopic Tandem Differential Mobility Analyzer stayed close to 1.41 for aerosols with 50 nm < D < 250 nm over the SSO, but increased from 1.30 to 1.67 over the NSO, indicating different chemical compositions. Both CCN and Ice Nucleating Particles (INPs) showed a stronger variation with season than with latitude. The variation of heat‐labile and presumably proteinacous INPs suggests an increase of ice nucleating‐active microbes in summer. Plain Language Summary: The Atmospheric Radiation Measurement Mobile Facility‐2 was installed onboard an ice breaker to measure small particles suspended in the air (aerosols), particularly those with potential influences on cloud formation and evolution. The 2017–2018 Measurement of Aerosols, Radiation, and CloUds over the pristine Southern ocean (MARCUS) measurement campaign provides unique data on the latitudinal and seasonal variation of the suspended particles, including south of 60°S where previous observations are scarce. Data show that both the number concentration and size distribution of particles that serve as embryos of cloud droplets are different over the North Southern Ocean (NSO) (50°S–60°S) and the South Southern Ocean (SSO) (62°S–68°S). There are greater concentrations of these embryos over the SSO for increased total amount of suspended particles there. These observations are consistent with data collected by other instruments that show differences over the NSO and SSO in how these suspended particles scatter radiation and how they absorb water vapor, which shows the particles have different sizes and chemical compositions respectively. The concentrations of both particles that serve as embryos for cloud drops and ice crystals both vary more with season than latitude, which has an implication for the energy balance of the Southern Ocean. Key Points: Machine Learning is applied to identify ship stack contamination of ship‐borne aerosol measurementsSurface Cloud Condensation Nuclei number concentration and its seasonal variation over 62°S–68°S is higher than over 50°S–60°SIce Nucleating Particles over the Southern Ocean originate from primarily organic and biological sources during MARCUS [ABSTRACT FROM AUTHOR]

Ice crystal formation in mixed-phase clouds is initiated by specific aerosol particles, termed ice-nucleating particles (INPs). Only a tiny fraction of all aerosol particles are INPs, providing a challenge for contemporary INP measurement techniques. Models have shown that the presence of INPs in clouds can impact their radiative properties and induce precipitation formation. However, for a qualified implementation of INPs in models, measurement techniques able to accurately detect the temperature-dependent INP concentration are needed. Here we present measurements of INP concentrations in ambient air under conditions relevant to mixed-phase clouds from a total of 10 INP methods over 2 weeks in October 2018 at the Puy de Dôme observatory in central France. A special focus in this intercomparison campaign was placed on having overlapping sampling periods. Although a variety of different measurement principles were used, the majority of the data show INP concentrations within a factor of 5 of one another, demonstrating the suitability of the instruments to derive model-relevant INP data. Lower values of comparability are likely due to instrument-specific features such as aerosol lamina spreading in continuous-flow diffusion chambers, demonstrating the need to account for such phenomena when interpreting INP concentration data from online instruments. Moreover, consistently higher INP concentrations were observed from aerosol filters collected on the rooftop at the Puy de Dôme station without the use of an aerosol inlet. [ABSTRACT FROM AUTHOR]

Supercooled liquid clouds are ubiquitous over the Southern Ocean (SO), even to temperatures below −20°C, and comprise a large fraction of the marine boundary layer (MBL) clouds. Earth system models and reanalysis products have struggled to reproduce the observed cloud phase distribution and occurrence of cloud ice in the region. Recent simulations found the microphysical representation of ice nucleation and growth has a large impact on these properties, however, measurements of SO ice nucleating particles (INPs) to validate simulations are sparse. This study presents measurements of INPs from simultaneous aircraft and ship campaigns conducted over the SO in austral summer 2018, which include the first in situ observations in and above cloud in the region. Our results confirm recent observations that INP concentrations are uniformly lower than measurements made in the late 1960s. While INP concentrations below and above cloud are similar, higher ice nucleation efficiency above cloud supports model simulations that the dominant INP composition varies with height. Model parameterizations based solely on aerosol properties capture the mean relationship between INP concentration and temperature but not the observed variability, which is likely related to the only modest correlations observed between INPs and environmental or aerosol metrics. Including wind speed in addition to activation temperature in a marine INP parameterization reduces bias but does not explain the large range of observed INP concentrations. Direct and indirect inference of marine INP size suggests MBL INPs, at least during Austral summer, are dominated by particles with diameters smaller than 500 nm. Plain Language Summary: Although Antarctica is remote, the continent and the Southern Ocean (SO) that surrounds it play a fundamental role in shaping regional and global climate. The clouds in this region are unique, with less ice and more liquid water present at low temperatures than in other areas. This is likely related to very low concentrations of rare aerosol particles called ice nucleating particles (INPs), which cause liquid water droplets in clouds to freeze. Largely due to a lack of observations, SO clouds are poorly represented in global models, and the interactions between aerosol particles and clouds are one of the largest remaining uncertainties. This study presents results of INP measurements from several recent field campaigns over the SO, including the first observations within and above clouds in the region. Our results suggest different types of particles are present below and above clouds, which have varying ability to nucleate ice. They also highlight the need for additional measurements of INP composition and size, which are key variables needed to improve model simulations. Key Points: First vertically resolved measurements of ice nucleating particles (INPs) over the Southern Ocean, including in‐cloud observationsCorrelation between normalized INP concentrations and wind speed suggests marine active site density is variableHigher ice nucleation efficiency observed above cloud, consistent with an increasing influence of mineral dust with height [ABSTRACT FROM AUTHOR]

Rapid Arctic climate warming, amplified relative to lower-latitude regions, has led to permafrost thaw and associated thermokarst processes. Recent work has shown permafrost is a rich source of ice-nucleating particles (INPs) that can initiate ice formation in supercooled liquid clouds. Since the phase of Arctic clouds strongly affects the surface energy budget, especially over ice-laden surfaces, characterizing INP sources in this region is critical. For the first time, we provide a large-scale survey of potential INP sources in tundra terrain where thermokarst processes are active and relate to INPs in the air. Permafrost, seasonally thawed active layer, ice wedge, vegetation, water, and aerosol samples were collected near Utqiaġvik, Alaska, in late summer and analyzed for their INP contents. Permafrost was confirmed as a rich source of INPs that was enhanced near the coast. Sensitivity to heating revealed differences in INPs from similar sources, such as the permafrost and active layer. Water, vegetation, and ice wedge INPs had the highest heat-labile percentage. The aerosol likely contained a mixture of known and unsurveyed INP types that were inferred as biological. Arctic water bodies were shown to be potential important links of sources to the atmosphere in thermokarst regions. Therefore, a positive relationship found with total organic carbon considering all water bodies gives a mechanism for future parameterization as permafrost continues to thaw and drive regional landscape shifts. [ABSTRACT FROM AUTHOR]

Single‐ and multi‐layer clouds are commonly observed over the Southern Ocean in varying synoptic settings, yet few studies have characterized and contrasted their properties. This study provides a statistical analysis of the microphysical properties of single‐ and multi‐layer clouds using in‐situ observations acquired during the Southern Ocean Cloud‐Radiation Aerosol Transport Experimental Study. The relative frequencies of ice‐containing samples (i.e., mixed and ice phase) for multi‐layer clouds are 0.05–0.25 greater than for single‐layer clouds, depending on cloud layer height. In multi‐layer clouds, the lowest cloud layers have the highest ice‐containing sample frequencies, which decrease with increasing cloud layer height up to the third highest cloud layer. This suggests a prominent seeder‐feeder mechanism over the region. Ice nucleating particle (cloud condensation nuclei) concentrations are positively (negatively) correlated with ice‐containing sample frequencies in select cases. Differences in microphysical properties are observed for single‐ and multi‐layer clouds. Drop concentrations (size distributions) are greater (narrower) for single‐layer clouds compared with the lowest multi‐layer clouds. When differentiating cloud layers by top (single‐ and highest multi‐layer clouds) and non‐top layers (underlying multi‐layer clouds), total particle size distributions (including liquid and ice) are similarly broader for non‐top cloud layers. Additionally, drop concentrations in coupled environments are approximately double those in decoupled environments. Plain Language Summary: Weather and climate models continue to struggle simulating cloud microphysical properties over the Southern Ocean, including cloud phase occurrence frequencies. High resolution observations of Southern Ocean clouds are crucial toward improving model simulations. This paper uses in situ observations to compare and contrast microphysical properties and phase frequencies of single‐ and multi‐layer clouds, as well as relates phase frequencies with ice nucleating particle and cloud condensation nuclei number concentrations. In situ observations used in this study were acquired during the Southern Ocean Cloud‐Radiation Aerosol Transport Experimental Study. A suite of cloud probe instrumentation is used to classify 1 Hz cloud samples as either liquid, ice or mixed phase (i.e., liquid and ice particles in the same sample volume). The lowest occurrence frequency of liquid phase samples is observed in the lowest cloud layers of multi‐layer clouds, whereas the highest frequency is observed in single‐layer clouds. Ice nucleating particle number concentrations are negatively correlated with liquid phase occurrence frequencies in select cases. In contrast, cloud condensation nuclei number concentrations are positively correlated with liquid phase frequencies, although this is only observed for clouds above the boundary layer. Key Points: The occurrence frequency of ice is greater in multi‐layer clouds than in single‐layer cloudsDrop number size distributions are broader in multi‐layer clouds compared to single‐layer cloudsLiquid drop number concentrations are approximately double in environments coupled with the surface compared to decoupled environments [ABSTRACT FROM AUTHOR]

Sea spray aerosols (SSA) represent one of the most abundant aerosol types on a global scale and have been observed at all altitudes including the upper troposphere. SSA has been explored in recent years as a source of ice nucleating particles (INPs) in cirrus clouds due to the ubiquity of cirrus clouds and the uncertainties in their radiative forcing. This study expands upon previous works on low temperature ice nucleation of SSA by investigating the effects of atmospheric aging of SSA and the ice nucleating activity of newly formed secondary marine aerosols (SMA) using an oxidation flow reactor. Polydisperse aerosol distributions were generated from a Marine Aerosol Reference Tank (MART) filled with 120 L of real or artificial seawater and their subsequent freezing was measured using a Continuous Flow Diffusion Chamber (CFDC). Results show that for both primary SSA (pSSA), and the aged SSA and SMA (aSSA+SMA) at temperatures > 220 K, 1 % of the particles freeze via homogeneous nucleation. However, below 220 K, heterogeneous nucleation occurs for both pSSA and aSSA+SMA at much lower relative humidities (RHs), where up to 1 % of the aerosol population freezes between 75--80 % RH. Similarities between freezing behaviors of the pSSA and aSSA+SMA at all temperatures suggest atmospheric aging has little effect on the heterogeneous freezing behavior of SSA at these cirrus temperatures and remains dominated by the crystalline salts. Occurrence of 1 % frozen fraction of SMA, generated in the absence of primary SSA, was observed at/near water saturation below 220 K, suggesting it is not an effective INP at cirrus temperatures, similar to findings in the literature of other organic aerosols. Thus, any SMA coatings on the pSSA are also unlikely to modify the ice nucleation behavior of pSSA. These results demonstrate the ability of lofted primary sea spray particles to remain an effective ice nucleator at cirrus temperatures, even after atmospheric aging. [ABSTRACT FROM AUTHOR]

Mineral dust and sea spray aerosol represent important sources of ice nucleating particles (INPs), the minor fraction of aerosol particles able to trigger cloud ice crystal formation and, consequently, influence multiple 37 climate-relevant cloud properties including lifetime, reflectivity, and precipitation efficiency. Mineral dust is considered the dominant INP source in many parts of the world due to its ice nucleation efficiency and its sheer abundance, with global emission rates of up to 4700 Tg a-1. However, INPs emitted from the ocean surface in sea spray aerosol frequently dominate INP populations in remote marine environments, including 41 parts of the Southern Ocean where cloud-resolving model simulations have demonstrated that cloud reflectivity is likely strongly controlled by INPs. Here we report INP concentrations measured in aerosol and seawater samples during Air Quality and Climate Change in the Arabian BAsin (AQABA), a shipborne campaign that spanned the Red Sea, Gulf of Aden, Arabian Sea, Arabian Gulf, and part of the Mediterranean. In aerosol samples collected within a few hundred kilometers of the first and second ranked sources of dust globally, the Sahara and Arabian Peninsula, INP concentrations ranged from 0.2 to 11 L-1 at -20 °C with observed ice nucleation site densities (ns) 1-3 orders of magnitude below levels predicted by mineral dust INP parameterizations. Over half of the samples (at least of 26) were collected during dust storms with average dust mass concentrations between 150 and 490 μg m-3 (PM10). The impacts of heat and peroxide treatments indicate that organics were responsible for the observed ice nucleation (IN) -activity at temperatures ≥ -15 °C with proteinaceous (heat-labile) INPs frequently observed at higher freezing temperatures > -10 °C. Overall, results demonstrate that despite proximity to the Sahara and the Arabian Peninsula and the dominance of mineral dust in the aerosol sampled, existing mineral dust 54 parameterizations alone would not skillfully represent the near-surface ns in the observed temperature 55 regime (-6 to -25 °C). The decreased ns, and results demonstrating that organics dominated the observed IN activity ≥ -15 °C, indicate that the IN-active organic species are limited compared to the mineral IN components of dust. Future efforts to develop or improve representations of dust INPs at modest supercooling (≥ -15 °C) would benefit from a characterization of the specific organic species associated with dust INPs. More generally, an improved understanding of the organic species associated with 60 increased IN -activity and their variability across dust source regions would directly inform efforts to determine whether ns-based parameterizations are appropriate for faithful representation of dust INPs in this sensitive temperature regime, whether region-specific parameterizations are required, or whether an alternative to the ns approach is necessary. [ABSTRACT FROM AUTHOR]