Ice Nucleating Particles Concentrations Research Articles

Molecular simulations reveal that heterogeneous ice nucleation occurs at higher temperatures in water under capillary tension

Abstract. Heterogeneous ice nucleation is thought to be the primary pathway for the formation of ice in mixed-phase clouds, with the number of active ice-nucleating particles (INPs) increasing rapidly with decreasing temperature. Here, molecular-dynamics simulations of heterogeneous ice nucleation demonstrate that the ice nucleation rate is also sensitive to pressure and that negative pressure within supercooled water shifts freezing temperatures to higher temperatures. Negative pressure, or tension, occurs naturally in water capillary bridges and pores and can also result from water agitation. Capillary bridge simulations presented in this study confirm that negative Laplace pressure within the water increases heterogeneous-freezing temperatures. The increase in freezing temperatures with negative pressure is approximately linear within the atmospherically relevant range of 1 to −1000 atm. An equation describing the slope depends on the latent heat of freezing and the molar volume difference between liquid water and ice. Results indicate that negative pressures of −500 atm, which correspond to nanometer-scale water surface curvatures, lead to a roughly 4 K increase in heterogeneous-freezing temperatures. In mixed-phase clouds, this would result in an increase of approximately 1 order of magnitude in active INP concentrations. The findings presented here indicate that any process leading to negative pressure in supercooled water may play a role in ice formation, consistent with experimental evidence of enhanced ice nucleation due to surface geometry or mechanical agitation of water droplets. This points towards the potential for dynamic processes such as contact nucleation and droplet collision or breakup to increase ice nucleation rates through pressure perturbations.

Physicochemical characterization and source apportionment of Arctic ice-nucleating particles observed in Ny-Ålesund in autumn 2019

Abstract. Ice-nucleating particles (INPs) initiate primary ice formation in Arctic mixed-phase clouds (MPCs), altering cloud radiative properties and modulating precipitation. For atmospheric INPs, the complexity of their spatiotemporal variations, heterogeneous sources, and evolution via intricate atmospheric interactions challenge the understanding of their impact on microphysical processes in Arctic MPCs and induce an uncertain representation in climate models. In this work, we performed a comprehensive analysis of atmospheric aerosols at the Arctic coastal site in Ny-Ålesund (Svalbard, Norway) from October to November 2019, including their ice nucleation ability, physicochemical properties, and potential sources. Overall, INP concentrations (NINP) during the observation season were approximately up to 3 orders of magnitude lower compared to the global average, with several samples showing degradation of NINP after heat treatment, implying the presence of proteinaceous INPs. Particle fluorescence was substantially associated with INP concentrations at warmer ice nucleation temperatures, indicating that in the far-reaching Arctic, aerosols of biogenic origin throughout the snow- and ice-free season may serve as important INP sources. In addition, case studies revealed the links between elevated NINP and heat lability, fluorescence, high wind speeds originating from the ocean, augmented concentration of coarse-mode particles, and abundant organics. Backward trajectory analysis demonstrated a potential connection between high-latitude dust sources and high INP concentrations, while prolonged air mass history over the ice pack was identified for most scant INP cases. The combination of the above analyses demonstrates that the abundance, physicochemical properties, and potential sources of INPs in the Arctic are highly variable despite its remote location.

Bioaerosols are the dominant source of warm-temperature immersion-mode INPs and drive uncertainties in INP predictability.

Ice-nucleating particles (INPs) are rare atmospheric aerosols that initiate primary ice formation, but accurately simulating their concentrations and variability in large-scale climate models remains a challenge. Doing so requires both simulating major particle sources and parameterizing their ice nucleation (IN) efficiency. Validating and improving model predictions of INP concentrations requires measuring their concentrations delineated by particle type. We present a method to speciate INP concentrations into contributions from dust, sea spray aerosol (SSA), and bioaerosol. Field campaign data from Bodega Bay, California, showed that bioaerosols were the primary source of INPs between -12° and -20°C, while dust was a minor source and SSA had little impact. We found that recent parameterizations for dust and SSA accurately predicted ambient INP concentrations. However, the model did not skillfully simulate bioaerosol INPs, suggesting a need for further research to identify major factors controlling their emissions and INP efficiency for improved representation in models.

Distinct secondary ice production processes observed in radar Doppler spectra: insights from a case study

Abstract. Secondary ice production (SIP) has an essential role in cloud and precipitation microphysics. In recent years, substantial insights were gained into SIP by combining experimental, modeling, and observational approaches. Remote sensing instruments, among them meteorological radars, offer the possibility of studying clouds and precipitation in extended areas over long time periods and are highly valuable to understand the spatiotemporal structure of microphysical processes. Multi-modal Doppler spectra measured by vertically pointing radars reveal the coexistence, within a radar resolution volume, of hydrometeor populations with distinct properties; as such, they can provide decisive insight into precipitation microphysics. This paper leverages polarimetric radar Doppler spectra as a tool to study the microphysical processes that took place during a snowfall event on 27 January 2021 in the Swiss Jura Mountains during the ICE GENESIS campaign. A multi-layered cloud system was present, with ice particles sedimenting through a supercooled liquid water (SLW) layer in a seeder–feeder configuration. Building on a Doppler peak detection algorithm, we implement a peak labeling procedure to identify the particle type(s) that may be present within a radar resolution volume. With this approach, we can visualize spatiotemporal features in the radar time series that point to the occurrence of distinct mechanisms during different stages of the event. By focusing on three 30 min phases of the case study and by using the detailed information contained in the Doppler spectra, together with dual-frequency radar measurements, aircraft in situ images, and simulated profiles of atmospheric variables, we narrow down the possible processes that could be responsible for the observed signatures. Depending on the availability of SLW and the droplet sizes, on the temperature range, and on the interaction between the liquid and ice particles, various SIP processes are identified as plausible, with distinct fingerprints in the radar Doppler spectra. A simple modeling approach suggests that the ice crystal number concentrations likely exceed typical concentrations of ice-nucleating particles by 1 to 4 orders of magnitude. While a robust proof of occurrence of a given SIP mechanism cannot be easily established, the multi-sensor data provide various independent elements each supporting the proposed interpretations.

Southern Alaska as a source of atmospheric mineral dust and ice-nucleating particles.

Ice-nucleating particles (INPs) influence cloud radiative properties and climate; however, INP sources and concentrations are poorly constrained, particularly in high-latitude regions. Southern Alaska is a known source of high-latitude dust, but its contribution to atmospheric mineral dust and INP concentrations has not been quantified. We show that glacial dust collected in southern Alaska is an effective ice-nucleating material under conditions relevant for mixed-phase clouds and is more active than low-latitude dust because of a biological component that enhances its activity. We use dispersion modeling to show that this source contributes to the regional INP population and that the dust emitted is transported over a broad area of North America, reaching altitudes where it could cause cloud glaciation. Our results highlight the importance of quantifying emissions and ice-nucleating characteristics of high-latitude dusts and suggest that the ice-nucleating ability of emitted dust in these regions should be represented in models using different parametrizations to low-latitude dust.

A bin microphysics parcel model investigation of secondary ice formation in an idealised shallow convective cloud

Abstract. We provide the first systematic study of ice formation in idealised shallow clouds from collisions of supercooled water drops with ice particles (mode 2). Using the University of Manchester bin microphysics parcel model, we investigated the sensitivity of ice formation due to mode 2 for a wide range of parameters, including aerosol particle size distribution, updraft speed, cloud-base temperature, cloud depth, ice-nucleating particle concentration, and freezing fraction of mode 2. We provide context to our results with other secondary ice production mechanisms as single mechanisms and combinations (rime splintering, spherical freezing fragmentation of drops (mode 1), and ice–ice collisions). There was a significant sensitivity to aerosol particle size distribution when updraft speeds were low (0.5 m s−1); secondary ice formation did not occur when the aerosol particle size distribution mimicked polluted environments. Where secondary ice formation did occur in simulated clouds, significant ice formation in the shallower clouds (1.3 km deep) was due to mode 2 or a combination which included mode 2. The deeper clouds (2.4 km deep) also had significant contributions from rime splintering or ice–ice collisional breakup secondary ice production (SIP) mechanisms. While simulations with cloud-base temperatures of 7 ∘C were relatively insensitive to ice-nucleating particle concentrations, there was a sensitivity in simulations with cloud-base temperatures of 0 ∘C. Increasing the ice-nucleating particle concentration delayed ice formation. Our results suggest that collisions of supercooled water drops with ice particles may be a significant ice formation mechanism within shallow convective clouds where rime splintering is not active.

An Evaluation of Phase, Aerosol‐Cloud Interactions and Microphysical Properties of Single‐ and Multi‐Layer Clouds Over the Southern Ocean Using in Situ Observations From SOCRATES

AbstractSingle‐ 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.

Ice nucleating abilities of deep waters from the Mexican Pacific ocean

Marine aerosol particles from biogenic origin are able to act as ice nucleating particles (INPs) and can play an important role in mixed-phase cloud formation. Although several field campaigns have been performed in the Atlantic, Arctic, and Southern oceans to evaluate the ice nucleating abilities of marine aerosol particles, this is not the case for the eastern Pacific Ocean, especially at the coasts of Mexico. The ice nucleating abilities via immersion freezing, of water samples collected during two oceanographic cruises in the Gulf of California (GoC) and off the west coast of the Baja California Peninsula (WBCP) at three different depths (i.e., 3–7 m, 10–72 m, and 500 m) were evaluated. The onset freezing temperature of the water samples from the GoC ranged between −10 °C and −22 °C, while it varied between −14.5 °C and −23 °C for the WBCP samples. The median freezing temperatures (i.e., the temperature at which 50% of the droplets freeze, T50) from both sets of samples was found to decrease with depth, indicating that superficial waters contain more ice active material. Although some meteorological variables (i.e., temperature, salinity, and chlorophyll-a), nutrients concentration, and primary production showed differences between the sampling points and depth, the obtained correlations with T50 were not statistically significant. The INP concentration of the GoC and the WBCP samples varied between 2 × 102 L−1 and 9 × 104 L−1 at a freezing temperature range from −10 °C to −36 °C. These concentrations are in agreement with those measured in the Gulf of Mexico; however, they are two orders of magnitude lower than those reported at higher latitudes. Even though it is clear that the ocean-atmosphere exchange takes place at the sea surface microlayer, it was found that deep waters also contain ice active material. Although the present results suggest that the Mexican Pacific Ocean is not a great source of INPs, in the absence of efficient INPs such as mineral dust or terrestrial bioparticles, the locally emitted marine aerosol particles can likely impact mixed-phase cloud formation.

Long-term variability in immersion-mode marine ice-nucleating particles from climate model simulations and observations

Abstract. Ice-nucleating particles (INPs) in the Southern Ocean (SO) atmosphere have significant impacts on cloud radiative and microphysical properties. Yet, INP prediction skill in climate models remains poorly understood, in part because of the lack of long-term measurements. Here we show, for the first time, how model-simulated INP concentrations compare with year-round INP measurements during the Macquarie Island Cloud Radiation Experiment (MICRE) campaign from 2017–2018. We simulate immersion-mode INP concentrations using the Energy Exascale Earth System Model version 1 (E3SMv1) by combining simulated aerosols with recently developed deterministic INP parameterizations and the native classical nucleation theory (CNT) for mineral dust in E3SMv1. Because MICRE did not collect aerosol measurements of super-micron particles, which are more effective ice nucleators, we evaluate the model's aerosol fields at other high-latitude sites using long-term in situ observations of dust and sea spray aerosol. We find that the model underestimates dust and overestimates sea spray aerosol concentrations by 1 to 2 orders of magnitude for most of the high-latitude sites in the Southern Hemisphere. We next compare predicted INP concentrations with concentrations of INPs collected on filter samples (typically for 2 or 3 d) and processed offline using the Colorado State University ice spectrometer (IS) in immersion freezing mode. We find that when deterministic parameterizations for both dust and sea spray INPs are used, simulated INPs are within a factor of 10 of observed INPs more than 60 % of the time during summer. Our results also indicate that the E3SM's current treatment of mineral dust immersion freezing in the SO is impacted by compensating biases – an underprediction of dust amount was compensated by an overprediction of its effectiveness as INPs. We also perform idealized droplet freezing experiments to quantify the implications of the time-dependent behavior assumed by the E3SM's CNT-parameterization and compare with the ice spectrometer observations. We find that the E3SM CNT 10 s diagnostic used in this study is a reasonable approximation of the exact formulation of CNT, when applied to ice spectrometer measurements in low-INP conditions similar to Macquarie Island. However, the linearized 10 s diagnostic underestimates the exact formula by an order of magnitude or more in places with high-INP conditions like the Sahara. Overall, our findings suggest that it is important to correct the biases in E3SM's simulated dust life cycle and update E3SM's INP parameterizations. INP prediction errors of 2 to 3 orders of magnitude can have considerable impacts on the simulated cloud and radiative properties in global climate models. On comparing INP concentrations during MICRE against ship-based campaigns, Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) and Antarctic Circumnavigation Expedition (ACE), we find that INPs from the latter are significantly higher only in regions closer to Macquarie Island. This alludes to the fact that physical, chemical and biological processes affecting INP concentrations as stimulated by the island could be partly responsible for the high INP concentrations observed at Macquarie Island during the MICRE campaign. Therefore, improvements to both aerosol simulation and INP parameterizations are required to adequately simulate INPs and their cloud impacts in E3SM. It will be helpful to include a parallel measurement of the size-resolved aerosol composition and explore opportunities for long-term measurement platforms in future field campaigns studying INP sources in remote marine regions.

Characteristics of atmospheric ice nucleating particles over East Antarctica retrieved from the surface snow

The concentration of ice nucleating particles (INPs) in the atmosphere is critical for understanding cloud microphysics and predicting the climate system. In this study, we collected surface snow samples along a traverse route from the coastal to the inland of East Antarctica to analyze INP concentrations and identify their spatial variations using a droplet freezing device. The overall concentration of INPs was found to be considerably low along the route, averaging at 0.8 ± 0.8 × 105 L−1 in water and 4.2 ± 4.8 × 10−3 L−1 in air at −20 °C. Although coastal areas had higher levels of sea salt species compared to inland regions, the concentration of INPs remained consistent along the route suggesting less important origination of INPs from the around ocean. Additionally, the heating experiment revealed the important contribution of proteinaceous INPs indicating the presence of biological INPs (bio-INPs). The fraction of bio-INPs was 0.52 on average at −20 °C and ranged from 0.1 to 0.7 from −30 °C to −15 °C. Finally, we parameterize the atmospheric INP concentrations as a function of freezing temperature which can be useful for modeling INP concentrations in this region.

The Relationship of Aerosol Properties and Ice‐Nucleating Particle Concentrations in Beijing

AbstractThis paper investigates immersion mode ice‐nucleating particle (INP) concentrations measured with a continuous flow diffusion chamber at temperatures of −20°C, −25°C, and −30°C in Beijing from 4 May to 4 June 2018. Aerosol conditions in the experiment period are classified into four types: clean, light PM2.5 pollution, heavy PM2.5 pollution, and dust. The PM mass concentrations and the aerosol size distributions are distinctly different in the four aerosol conditions. INP concentration increases as temperature decreases from −20°C to −30°C for all the four aerosol conditions. At a given temperature, INP concentrations progressively increase as the aerosol condition changes from clean to light PM2.5 pollution and to heavy PM2.5 pollution condition. At −25°C and −30°C, the dust condition has a much higher INP concentration than the other aerosol conditions. The INP concentration is closely related to the number concentration of aerosols with diameters of 1.0–2.5 μm (). Using a similar method as in the previous parameterization of INP concentrations for global and fairly clean aerosol conditions, a new parameterization of the INP concentration based on is provided for the complicated aerosol conditions with high aerosol number concentrations in Beijing.

Vertical Structure and Ice Production Processes of Shallow Convective Postfrontal Clouds over the Southern Ocean in MARCUS. Part I: Observational Study

Abstract A study of the vertical structure of postfrontal shallow clouds in the marine boundary layer over the Southern Ocean is presented. The central question of this two-part study regards cloud phase (liquid/ice) of precipitation, and the associated growth mechanisms. In this first part, data from the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) field campaign are analyzed, starting with a 75-h case with continuous sea surface-based thermal instability, modest surface heat fluxes, an open-cellular mesoscale organization, and very few ice nucleating particles (INPs). The clouds are mostly precipitating and shallow (tops mostly around 2 km above sea level), with weak up- and downdrafts, and with cloud-top temperatures generally around −18° to −10°C. The case study is extended to three other periods of postfrontal shallow clouds in MARCUS. While abundant supercooled liquid water is commonly present, an experimental cloud-phase algorithm classifies nearly two-thirds of clouds in the 0° to −5°C layer as containing ice (cloud ice, snow, or mixed phase), implying that much of the precipitation grows through cold-cloud processes. The best predictors of ice presence are cloud-top temperature, cloud depth, and INP concentration. Measures of convective activity and turbulence are found to be poor indicators of ice presence in the studied environment. The water-phase distribution in this cloud regime is explored through numerical simulations in Part II. Significance Statement Climate models generally predict a lower albedo than observed over the Southern Ocean, and this is largely attributed to a lack of cloudiness, especially in the postfrontal cold sector of midlatitude cyclones. This in turn may be due to an excess of ice in these simulated clouds, resulting in rapid precipitation fallout and an overly brief cloud lifespan. The objective of this study is to examine whether shallow postfrontal clouds over the Southern Ocean are dominated by supercooled drops, or by snow and ice, using data collected by a U.S. Department of Energy Atmospheric Radiation Measurement Mobile Facility deployed aboard an Australian Antarctic supply vessel. We find that these clouds contain much supercooled liquid, even though cloud-top temperatures generally are around −18° to −8°C, and that about two-thirds of the clouds just above the freezing level contain ice. Much of the precipitation appears to grow through cold-cloud processes above the freezing level, rather than drizzle/rain. Updrafts and/or turbulence in convection or in cloud-top generating cells do not initiate much ice, compared to observations elsewhere in a similar temperature range. This may be attributable to the extremely low concentration of ice nucleating particles in this environment. Ultimately, the deepest clouds with the coldest cloud tops are most likely to be ice dominated.

Dominant Role of Arctic Dust With High Ice Nucleating Ability in the Arctic Lower Troposphere

AbstractRecent observations show that dust emitted within the Arctic (Arctic dust) has a remarkably high ice nucleating ability, especially between −20°C and −5°C, but its impacts on the number concentrations of ice nucleating particles (INPs) and radiative balance in the Arctic are not well understood. Here we incorporate an observation‐based ice‐nucleation parameterization indicating the high ice nucleating ability of Arctic dust into a global aerosol‐climate model. A simulation using this parameterization better reproduces INP observations in the Arctic and estimates >100 times higher dust INP number concentrations with ∼100% contribution from Arctic dust in the Arctic lower troposphere (>60°N and >700 hPa) during summer and fall (June–November) than a simulation applying a standard ice‐nucleation parameterization suitable for desert dust to Arctic dust. Our results demonstrate the importance of considering an ice‐nucleation parameterization suitable for Arctic dust when simulating INPs and their effects on aerosol‐cloud interactions in the Arctic.

Ice-nucleating particles in northern Greenland: annual cycles, biological contribution and parameterizations

Abstract. Ice-nucleating particles (INPs) can initiate ice formation in clouds at temperatures above −38 ∘C through heterogeneous ice nucleation. As a result, INPs affect cloud microphysical and radiative properties, cloud lifetime, and precipitation behavior and thereby ultimately the Earth's climate. Yet, little is known regarding the sources, abundance and properties of INPs, especially in remote regions such as the Arctic. In this study, 2-year-long INP measurements (from July 2018 to September 2020) at Villum Research Station in northern Greenland are presented. A low-volume filter sampler was deployed to collect filter samples for offline INP analysis. An annual cycle of INP concentration (NINP) was observed, and the fraction of heat-labile INPs was found to be higher in months with low to no snow cover and lower in months when the surface was well covered in snow (> 0.8 m). Samples were categorized into three different types based only on the slope of their INP spectra, namely into summer, winter and mix type. For each of the types a temperature-dependent INP parameterization was derived, clearly different depending on the time of the year. Winter and summer types occurred only during their respective seasons and were seen 60 % of the time. The mixed type occurred in the remaining 40 % of the time throughout the year. April, May and November were found to be transition months. A case study comparing April 2019 and April 2020 was performed. The month of April was selected because a significant difference in NINP was observed during these two periods, with clearly higher NINP in April 2020. In parallel to the observed differences in NINP, also a higher cloud-ice fraction was observed in satellite data for April 2020, compared to April 2019. NINP in the case study period revealed no clear dependency on either meteorological parameters or different surface types which were passed by the collected air masses. Overall, the results suggest that the coastal regions of Greenland were the main sources of INPs in April 2019 and 2020, most likely including both local terrestrial and marine sources.

POLIPHON conversion factors for retrieving dust-related cloud condensation nuclei and ice-nucleating particle concentration profiles at oceanic sites

Abstract. Aerosol–cloud interactions (ACIs) are the largest contributor to the uncertainty in the global radiation budget. To improve the current consideration of ACIs in global circulation models, it is necessary to characterize the 3-D distribution of dust-related cloud condensation nuclei concentration (CCNC) and ice-nucleating particle concentration (INPC) globally. This can potentially be realized using the POlarization LIdar PHOtometer Networking (POLIPHON) method together with spaceborne lidar observations. However, dust-related conversion factors that convert bulk aerosol optical properties from lidar measurements to aerosol microphysical properties are still less constrained in many regions, which limits the applications of the POLIPHON method. Here we retrieve the essential dust-related conversion factors at remote oceanic and coastal sites using the historical AErosol RObotic NETwork (AERONET) database. Depolarization-ratio-based dust ratios Rd at 1020 nm are applied to identify the dust-occurring cases, thus enabling us to contain fine-mode dust-dominated cases (after the preferential removal of large-sized dust particles during transport), study the evolution of dust microphysical properties along the transoceanic pathway, and mitigate occasional interference of large-sized marine aerosols. The newly proposed scheme is proven to be valid and feasible by intercomparisons with previous studies at nine sites in/near deserts. The dust-related conversion factors are calculated at 20 oceanic and coastal sites using both pure dust (PD) and PD plus dust-dominated mixture (PD+DDM) datasets. At nearly half of the sites, the conversion factors are solely calculated using the PD datasets, while at the remaining sites, the participation of DDM datasets is required to ensure a sufficient number of data for the calculation. Evident variation trends in conversion factors are found for cv,d (extinction-to-volume concentration, gradually decreasing), c250,d (extinction-to-particle (with a radius > 250 nm) number concentration, gradually increasing), and cs,d (extinction-to-surface-area concentration, gradually decreasing) along both the transpacific and transatlantic dust transport pathways. The retrieved dust-related conversion factors are anticipated to inverse 3-D dust-related CCNC and INPC distributions globally, thereby improving the understanding of ACIs in atmospheric circulation models.

Measurements of Aerosol Particle Size Distributions and INPs Over the Southern Ocean in the Late Austral Summer of 2017 on Board the R/V Mirai: Importance of the Marine Boundary Layer Structure

AbstractThe Southern Ocean (SO) is a unique region with rich ecosystems and minimal anthropogenic and terrestrial natural impacts especially during austral summer. We conducted measurements and sampling of aerosol particles to investigate the characteristics and role of aerosol particles in cloud formation on Japanese research vessel Mirai over the SO in the late austral summer of 2017. Observations of air masses with minimal terrestrial impacts indicated that bimodal aerosol particle size distributions were mostly observed over the SO and relative abundances of two modes varied with the changes in the structure of the marine boundary layer (MBL) such as surface mixing layer. As the local minimum diameter between two modes (DH) is considered a critical size for cloud‐droplet activation in the MBL, we analyzed the aerosol particles with diameter larger than DH (CNAcc) as a proxy for cloud condensation nuclei. Variations in the concentrations of aerosol particles with diameter larger than 300 nm were accounted for by the sea‐spray source function, whereas it substantially underestimated those of CNAcc. This implies the significance of the nonsea‐spray sources of CNAcc in the MBL over the SO. The concentrations of ice nucleating particles (NINPs) active at −25°C were 20–100 m−3, consistent with recent observations over the summertime SO. The observational evidence of very low NINPs relative to the CNAcc concentrations (40–110 cm−3) over a wide area of the SO is important for predicting the effects of cloud radiative properties on the radiative balances over the SO using climate models.

Ice nucleation catalyzed by the photosynthesis enzyme RuBisCO and other abundant biomolecules

Atmospheric aerosol and the cloud droplets and ice crystals that grow on them remain major sources of uncertainty in global climate models. A subset of aerosol, ice nucleating particles, catalyze the freezing of water droplets at temperatures warmer than −38 °C. Here we show that RuBisCO, one of the most abundant proteins in plants and phytoplankton, is one of the most efficient known immersion ice nucleating particles with a mean freezing temperature of −7.9 ± 0.3 °C. Further, we demonstrate RuBisCO is present in ambient continental aerosol where it can serve as an ice nucleating particle. Other biogenic molecules act as immersion ice nucleating particles, in the range of −19 to −26 °C. In addition, our results indicate heat denaturation is not a universal indicator of the proteinaceous origin of ice nucleating particles, suggesting current studies may fail to accurately quantify biological ice nucleating particle concentrations and their global importance.

Persistence and Potential Atmospheric Ramifications of Ice-Nucleating Particles Released from Thawing Permafrost.

Permafrost underlies approximately a quarter of the Northern Hemisphere and is changing amidst a warming climate. Thawed permafrost can enter water bodies through top-down thaw, thermokarst erosion, and slumping. Recent work revealed that permafrost contains ice-nucleating particles (INPs) with concentrations comparable to midlatitude topsoil. These INPs may impact the surface energy budget of the Arctic by affecting mixed-phase clouds, if emitted into the atmosphere. In two 3-4-week experiments, we placed 30,000- and 1000-year-old ice-rich silt permafrost in a tank with artificial freshwater and monitored aerosol INP emissions and water INP concentrations as the water's salinity and temperature were varied to mimic aging and transport of thawed material into seawater. We also tracked aerosol and water INP composition through thermal treatments and peroxide digestions and bacterial community composition with DNA sequencing. We found that the older permafrost produced the highest and most stable airborne INP concentrations, with levels comparable to desert dust when normalized to particle surface area. Both samples showed that the transfer of INPs to air persisted during simulated transport to the ocean, demonstrating a potential to influence the Arctic INP budget. This suggests an urgent need for quantifying permafrost INP sources and airborne emission mechanisms in climate models.

On coarse patterns in the atmospheric concentration of ice nucleating particles

The atmospheric concentration of ice nucleating particles active at around −10 °C (INP−10) is very low. Nevertheless, these particles play a role in the development of cloud systems, so their spatial and temporal patterns merit attention. We collated available datasets on INP−10 to identify such patterns. Among the five low altitude observatories in northern Eurasia, median values throughout May to October were lowest in Scandinavia (4 and 6 m−3), somewhat higher in central Europe (11 m−3), substantially higher in the West Siberian Plain (69 m−3) and highest in the Central Yakutian Lowland (204 m−3), suggesting that the abundance of INP−10 in northern Eurasia may increase with continentality and from West to East. The range of values at the same observatories was narrower throughout November to April (2 to 27 m−3). On average, by an order of magnitude smaller values were reported for the four Arctic observatories. Consequently, increasing poleward transport of air masses from the midlatitudes likely raises the concentration of INP−10 in the Arctic, particularly when air masses had surface contact in eastern parts of northern Eurasia.

Role of K-feldspar and quartz in global ice nucleation by mineral dust in mixed-phase clouds

Abstract. Ice-nucleating particles (INPs) enable ice formation, profoundly affecting the microphysical and radiative properties, lifetimes, and precipitation rates of clouds. Mineral dust emitted from arid regions, particularly potassium-containing feldspar (K-feldspar), has been shown to be a very effective INP through immersion freezing in mixed-phase clouds. However, despite the fact that quartz has a significantly lower ice nucleation activity, it is more abundant than K-feldspar in atmospheric desert dust and therefore may be a significant source of INPs. In this contribution, we test this hypothesis by investigating the global and regional importance of quartz as a contributor to INPs in the atmosphere relative to K-feldspar. We have extended a global 3-D chemistry transport model (TM4-ECPL) to predict INP concentrations from both K-feldspar and quartz mineral dust particles with state-of-the-art parameterizations using the ice-active surface-site approach for immersion freezing. Our results show that, although K-feldspar remains the most important contributor to INP concentrations globally, affecting mid-level mixed-phase clouds, the contribution of quartz can also be significant. Quartz dominates the lowest and the highest altitudes of dust-derived INPs, affecting mainly low-level and high-level mixed-phase clouds. The consideration of quartz INPs also improves the comparison between simulations and observations at low temperatures. Our simulated INP concentrations predict ∼ 51 % of the observations gathered from different campaigns within 1 order of magnitude and ∼ 69 % within 1.5 orders of magnitude, despite the omission of other potentially important INP aerosol precursors like marine bioaerosols. Our findings support the inclusion of quartz in addition to K-feldspar as an INP in climate models and highlight the need for further constraining their abundance in arid soil surfaces along with their abundance, size distribution, and mixing state in the emitted dust atmospheric particles.