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We do not know the exact pathways through which ice, liquid, cloud dynamics, and aerosols are interacting in clouds while forming snowfall but the involved processes can be identified by their fingerprints on snow particles. The general shape of individual crystals (dendritic, columns, plates) depends on the temperature and moisture conditions during growth from water vapor deposition. Aggregation can be identified when multiple individual particles are combined into a snowflake. Riming describes the freezing of cloud droplets onto the snow particle and can eventually form graupel. In order to exploit these unique fingerprints of cloud microphysical processes, optical in situ observations are required.The Video In Situ Snowfall Sensor (VISSS) was specifically developed for a campaign in the high Arctic (MOSAiC) to determine particle shape and particle size distributions. Different to other sensors, the VISSS minimizes uncertainties by using two-dimensional high-resolution images, a large measurement volume, and a design limiting the impact of wind. Tracking of particles over multiple frames allows determining fall speed and particle tumbling. The instrument design and software will be released as open-source. Here, we present the design of the instrument, show how particles are detected and tracked and introduce first results from campaigns in the high Arctic (MOSAiC), in the Colorado Rocky Mountains (SAIL), and in and Hyytiälä (Finland).

We present a framework for estimating concentrations of episodically elevated high-temperature marine ice nucleating particles (INPs) in the sea surface microlayer and their subsequent emission into the atmospheric boundary layer. These episodic INPs have been observed in multiple ship-based and coastal field campaigns, but the processes controlling their ocean concentrations and transfer to the atmosphere are not yet fully understood. We use a combination of empirical constraints and simulation outputs from an Earth System Model to explore different hypotheses for explaining the variability of INP concentrations, and the occurrence of episodic INPs, in the marine atmosphere. In our calculations, we examine 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 (≥ 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. We find that heterotrophic bacteria and MBPAs acting as INPs provide only a partial explanation for the observed high INP concentrations. We note, however, that there are still substantial knowledge gaps, particularly concerning the identity of the oceanic INPs contributing most frequently to episodic high-temperature INPs, their specific ice nucleation activity, and the enrichment of their concentrations during the sea-air transfer process. Therefore, targeted measurements investigating the composition of these marine INPs as well as drivers for their emission are needed, ideally in combination with modeling studies focused on the potential cloud impacts of these high-temperature INPs.

Sea spray emissions are an important source for ice nucleating particles (INPs) over remote ocean regions. Over the past years, our understanding of marine organic surfactants acting as INPs has advanced a lot. However, there are still significant knowledge gaps regarding the role of larger marine biogenic particles (e.g. polymers, diatom fragments, protists and bacteria) which are potentially the drivers of episodically observed high INP concentrations.In this study, we use a combination of ARM (Atmospheric Radiation Measurement) observations and output from E3SM (Energy Exascale Earth System Model) simulation runs to investigate the impact of larger marine biogenic particles acting as INPs. We use heterotrophic bacteria and nanogels (polymeric particles) as two hypothesized classes of marine INPs which can get transported across the sea-air interface. Based on the offline-calculated concentrations of these ice nucleating entities in the ocean surface layer, we conduct sensitivity studies to estimate INP concentration ranges, relying on current knowledge of enrichment factors and ice nucleation activities (e.g., ns values from McCluskey et al. (2018)). In comparison to observations of episodic high INP concentrations, our estimated concentrations are consistently lower. However, one of the main conclusions of our study is that large uncertainties regarding the links between ocean biology, organic matter in sea spray and ice nucleation properties, remain. Therefore, comprehensive observational datasets, including sea spray size distributions, aerosol and INP compositions, and ice nucleation efficiencies of individual marine species, are needed.

Seven different instruments and measurement methods were used to examine the immersion freezing of bacterial ice nuclei from Snomax® (hereafter Snomax), a product containing ice-active protein complexes from non-viable Pseudomonas syringae bacteria. The experimental conditions were kept as similar as possible for the different measurements. Of the participating instruments, some examined droplets which had been made from suspensions directly, and the others examined droplets activated on previously generated Snomax particles, with particle diameters of mostly a few hundred nanometers and up to a few micrometers in some cases. Data were obtained in the temperature range from −2 to −38 °C, and it was found that all ice-active protein complexes were already activated above −12 °C. Droplets with different Snomax mass concentrations covering 10 orders of magnitude were examined. Some instruments had very short ice nucleation times down to below 1 s, while others had comparably slow cooling rates around 1 K min−1. Displaying data from the different instruments in terms of numbers of ice-active protein complexes per dry mass of Snomax, nm, showed that within their uncertainty, the data agree well with each other as well as to previously reported literature results. Two parameterizations were taken from literature for a direct comparison to our results, and these were a time-dependent approach based on a contact angle distribution (Niedermeier et al., 2014) and a modification of the parameterization presented in Hartmann et al. (2013) representing a time-independent approach. The agreement between these and the measured data were good; i.e., they agreed within a temperature range of 0.6 K or equivalently a range in nm of a factor of 2. From the results presented herein, we propose that Snomax, at least when carefully shared and prepared, is a suitable material to test and compare different instruments for their accuracy of measuring immersion freezing.

During the eruption of the Eyjafjallajökull volcano in the south of Iceland in April/May 2010, about 40 Tg of ash mass were emitted into the atmosphere. It was unclear whether volcanic ash particles with d < 10 μm facilitate the glaciation of clouds. Thus, ice nucleation properties of volcanic ash particles were investigated in AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber experiments simulating atmospherically relevant conditions. The ash sample that was used for our experiments had been collected at a distance of 58 km from the Eyjafjallajökull during the eruption period in April 2010. The temperature range covered by our ice nucleation experiments extended from 219 to 264 K, and both ice nucleation via immersion freezing and deposition nucleation could be observed. Immersion freezing was first observed at 252 K, whereas the deposition nucleation onset lay at 242 K and RHice =126%. About 0.1% of the volcanic ash particles were active as immersion freezing nuclei at a temperature of 249 K. For deposition nucleation, an ice fraction of 0.1% was observed at around 233 K and RHice =116%. Taking ice-active surface site densities as a measure for the ice nucleation efficiency, volcanic ash particles are similarly efficient ice nuclei in immersion freezing mode (ns,imm ~ 109 m−2 at 247 K) compared to certain mineral dusts. For deposition nucleation, the observed ice-active surface site densities ns,dep were found to be 1011 m−2 at 224 K and RHice =116%. Thus, volcanic ash particles initiate deposition nucleation more efficiently than Asian and Saharan dust but appear to be poorer ice nuclei than ATD particles. Based on the experimental data, we have derived ice-active surface site densities as a function of temperature for immersion freezing and of relative humidity over ice and temperature for deposition nucleation.