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Abstract The Arctic is warming faster than anywhere else on Earth, prompting glacial melt, permafrost thaw, and sea ice decline. These severe consequences induce feedbacks that contribute to amplified warming, affecting weather and climate globally. Aerosols and clouds play a critical role in regulating radiation reaching the Arctic surface. However, the magnitude of their effects is not adequately quantified, especially in the central Arctic where they impact the energy balance over the sea ice. Specifically, aerosols called ice nucleating particles (INPs) remain understudied yet are necessary for cloud ice production and subsequent changes in cloud lifetime, radiative effects, and precipitation. Here, we report observations of INPs in the central Arctic over a full year, spanning the entire sea ice growth and decline cycle. Further, these observations are size-resolved, affording valuable information on INP sources. Our results reveal a strong seasonality of INPs, with lower concentrations in the winter and spring controlled by transport from lower latitudes, to enhanced concentrations of INPs during the summer melt, likely from marine biological production in local open waters. This comprehensive characterization of INPs will ultimately help inform cloud parameterizations in models of all scales. 

Abstract. We present a framework for estimating concentrations of episodicallyelevated high-temperature marine ice nucleating particles (INPs) in the seasurface microlayer and their subsequent emission into the atmosphericboundary layer. These episodic INPs have been observed in multipleship-based and coastal field campaigns, but the processes controlling theirocean concentrations and transfer to the atmosphere are not yet fullyunderstood. We use a combination of empirical constraints and simulationoutputs from an Earth system model to explore different hypotheses forexplaining the variability of INP concentrations, and the occurrence ofepisodic INPs, in the marine atmosphere. In our calculations, we examine the following two proposed oceanic sources of high-temperature INPs: heterotrophic bacteria and marine biopolymer aggregates (MBPAs). Furthermore, we assume that the emission of these INPs is determined by the production of supermicron sea spray aerosol formed from jet drops, with an entrainment probability that is described by Poisson statistics. The concentration of jet drops is derived from the number concentration of supermicron sea spray aerosol calculated from model runs. We then derive the resulting number concentrations of marine high-temperature INPs (at 253 K) in the atmospheric boundary layer and compare their variability to atmospheric observations of INP variability. Specifically, we compare against concentrations of episodically occurring high-temperature INPs observed during field campaigns in the Southern Ocean, the Equatorial Pacific, and the North Atlantic. In this case study, we evaluate our framework at 253 K because reliable observational data at this temperature are available across three different ocean regions, but suitable data are sparse at higher temperatures. We find that heterotrophic bacteria and MBPAs acting as INPs provide only apartial explanation for the observed high INP concentrations. We note,however, that there are still substantial knowledge gaps, particularlyconcerning the identity of the oceanic INPs contributing most frequently toepisodic high-temperature INPs, their specific ice nucleation activity, andthe enrichment of their concentrations during the sea–air transfer process. Therefore, targeted measurements investigating the composition of these marine INPs and drivers for their emissions are needed, ideally incombination with modeling studies focused on the potential cloud impacts ofthese high-temperature INPs. 

Abstract The secondary ice process (SIP) is a major microphysical process, which can result in rapid enhancement of ice particle concentration in the presence of preexisting ice. SPICULE was conducted to further investigate the effect of collision–coalescence on the rate of the fragmentation of freezing drop (FFD) SIP mechanism in cumulus congestus clouds. Measurements were conducted over the Great Plains and central United States from two coordinated aircraft, the NSF Gulfstream V (GV) and SPEC Learjet 35A, both equipped with state-of-the-art microphysical instrumentation and vertically pointing W- and Ka-band radars, respectively. The GV primarily targeted measurements of subcloud aerosols with subsequent sampling in warm cloud. Simultaneously, the Learjet performed multiple penetrations of the ascending cumulus congestus (CuCg) cloud top. First primary ice was typically detected at temperatures colder than −10°C, consistent with measured ice nucleating particles. Subsequent production of ice via FFD SIP was strongly related to the concentration of supercooled large drops (SLDs), with diameters from about 0.2 to a few millimeters. The concentration of SLDs is directly linked to the rate of collision–coalescence, which depends primarily on the subcloud aerosol size distribution and cloud-base temperature. SPICULE supports previous observational results showing that FFD SIP efficiency could be deduced from the product of cloud-base temperature and maximum diameter of drops measured ∼300 m above cloud base. However, new measurements with higher concentrations of aerosol and total cloud-base drop concentrations show an attenuating effect on the rate of coalescence. The SPICULE dataset provides rich material for validation of numerical schemes of collision–coalescence and SIP to improve weather prediction simulations 

Microorganisms are ubiquitous and highly diverse in the atmosphere. Despite the potential impacts of airborne bacteria found in the lower atmosphere over the Southern Ocean (SO) on the ecology of Antarctica and on marine cloud phase, no previous region-wide assessment of bioaerosols over the SO has been reported. We conducted bacterial profiling of boundary layer shipboard aerosol samples obtained during an Austral summer research voyage, spanning 42.8 to 66.5°S. Contrary to findings over global subtropical regions and the Northern Hemisphere, where transport of microorganisms from continents often controls airborne communities, the great majority of the bacteria detected in our samples were marine, based on taxonomy, back trajectories, and source tracking analysis. Further, the beta diversity of airborne bacterial communities varied with latitude and temperature, but not with other meteorological variables. Limited meridional airborne transport restricts southward community dispersal, isolating Antarctica and inhibiting microorganism and nutrient deposition from lower latitudes to these same regions. A consequence and implication for this region’s marine boundary layer and the clouds that overtop it is that it is truly pristine, free from continental and anthropogenic influences, with the ocean as the dominant source controlling low-level concentrations of cloud condensation nuclei and ice nucleating particles. 

Abstract. Ice-nucleating particles (INPs) represent a rare subset of aerosol particlesthat initiate cloud droplet freezing at temperatures above the homogenousfreezing point of water (−38 ∘C). Considering that the oceancovers 71 % of the Earth's surface and represents a large potential sourceof INPs, it is imperative that the identities, properties and relativeemissions of ocean INPs become better understood. However, the specificunderlying drivers of marine INP emissions remain largely unknown due tolimited observations and the challenges associated with isolating rare INPs. Bygenerating isolated nascent sea spray aerosol (SSA) over a range ofbiological conditions, mesocosm studies have shown that marine microbes cancontribute to INPs. Here, we identify 14 (30 %) cultivable halotolerantice-nucleating microbes and fungi among 47 total isolates recovered fromprecipitation and aerosol samples collected in coastal air in southernCalifornia. Ice-nucleating (IN) isolates collected in coastal air were nucleated ice fromextremely warm to moderate freezing temperatures (−2.3 to −18 ∘C). While some Gammaproteobacteria and fungi are known to nucleate ice attemperatures as high as −2 ∘C, Brevibacterium sp. is the first Actinobacteriafound to be capable of ice nucleation at a relatively high freezingtemperature (−2.3 ∘C). Air mass trajectory analysis demonstratesthat marine aerosol sources were dominant during all sampling periods, andphylogenetic analysis indicates that at least 2 of the 14 IN isolates areclosely related to marine taxa. Moreover, results from cell-washingexperiments demonstrate that most IN isolates maintained freezing activityin the absence of nutrients and cell growth media. This study supportsprevious studies that implicated microbes as a potential source of marineINPs, and it additionally demonstrates links between precipitation, marineaerosol and IN microbes. 

Abstract. Ice-nucleating particles (INPs) are efficiently removed fromclouds through precipitation, a convenience of nature for the study of thesevery rare particles that influence multiple climate-relevant cloudproperties including ice crystal concentrations, size distributions andphase-partitioning processes. INPs suspended in precipitation can be used toestimate in-cloud INP concentrations and to infer their originalcomposition. Offline droplet assays are commonly used to measure INPconcentrations in precipitation samples. Heat and filtration treatmentsare also used to probe INP composition and size ranges. Many previousstudies report storing samples prior to INP analyses, but little is knownabout the effects of storage on INP concentration or their sensitivity totreatments. Here, through a study of 15 precipitation samples collected at acoastal location in La Jolla, CA, USA, we found INP concentration changes upto > 1 order of magnitude caused by storage to concentrations ofINPs with warm to moderate freezing temperatures (−7 to−19 ∘C). We compared four conditions: (1) storage at roomtemperature (+21–23 ∘C), (2) storage at +4 ∘C, (3) storage at −20 ∘C and (4) flash-freezing samples with liquid nitrogen prior to storage at −20 ∘C. Results demonstrate that storage can lead to bothenhancements and losses of greater than 1 order of magnitude, withnon-heat-labile INPs being generally less sensitive to storage regime, butsignificant losses of INPs smaller than 0.45 µm in all tested storageprotocols. Correlations between total storage time (1–166 d) and changesin INP concentrations were weak across sampling protocols, with theexception of INPs with freezing temperatures ≥ −9 ∘C in samples stored at room temperature. We provide thefollowing recommendations for preservation of precipitation samples fromcoastal or marine environments intended for INP analysis: that samples bestored at −20 ∘C to minimize storage artifacts, thatchanges due to storage are likely an additional uncertainty in INPconcentrations, and that filtration treatments be applied only to freshsamples. At the freezing temperature −11 ∘C, average INPconcentration losses of 51 %, 74 %, 16 % and 41 % were observed foruntreated samples stored using the room temperature, +4, −20 ∘C, and flash-frozen protocols, respectively.Finally, the estimated uncertainties associated with the four storage protocolsare provided for untreated, heat-treated and filtered samples for INPsbetween −9 and −17 ∘C.