Ambient concentrations of ice-forming particles measured during ship expeditions are collected and summarised with the aim of determining the spatial distribution and variability in ice nuclei in ...oceanic regions.
The presented data from literature and previously unpublished data from over 23 months of ship-based measurements stretch from the Arctic to the Southern Ocean and include a circumnavigation of Antarctica. In comparison to continental observations, ship-based measurements of ambient ice nuclei show 1 to 2 orders of magnitude lower mean concentrations. To quantify the geographical variability in oceanic areas, the concentration range of potential ice nuclei in different climate zones is analysed by meridionally dividing the expedition tracks into tropical, temperate and polar climate zones. We find that concentrations of ice nuclei in these meridional zones follow temperature spectra with similar slopes but vary in absolute concentration. Typically, the frequency with which specific concentrations of ice nuclei are observed at a certain temperature follows a log-normal distribution. A consequence of the log-normal distribution is that the mean concentration is higher than the most frequently measured concentration. Finally, the potential contribution of ship exhaust to the measured ice nuclei concentration on board research vessels is analysed as function of temperature. We find a sharp onset of the influence at approximately −36 ∘C but none at warmer temperatures that could bias ship-based measurements.
Atmospheric ice‐nucleating particles (INPs) play a critical role in cloud freezing processes, with important implications for precipitation formation and cloud radiative properties, and thus for ...weather and climate. Additionally, INP emissions respond to changes in the Earth System and climate, for example, desertification, agricultural practices, and fires, and therefore may introduce climate feedbacks that are still poorly understood. As knowledge of the nature and origins of INPs has advanced, regional and global weather, climate, and Earth system models have increasingly begun to link cloud ice processes to model‐simulated aerosol abundance and types. While these recent advances are exciting, coupling cloud processes to simulated aerosol also makes cloud physics simulations increasingly susceptible to uncertainties in simulation of INPs, which are still poorly constrained by observations. Advancing the predictability of INP abundance with reasonable spatiotemporal resolution will require an increased focus on research that bridges the measurement and modeling communities. This review summarizes the current state of knowledge and identifies critical knowledge gaps from both observational and modeling perspectives. In particular, we emphasize needs in two key areas: (a) observational closure between aerosol and INP quantities and (b) skillful simulation of INPs within existing weather and climate models. We discuss the state of knowledge on various INP particle types and briefly discuss the challenges faced in understanding the cloud impacts of INPs with present‐day models. Finally, we identify priority research directions for both observations and models to improve understanding of INPs and their interactions with the Earth System.
Plain Language Summary
Atmospheric ice‐nucleating particles (INPs) are rare particles that play a critical role in enabling ice crystals to form in clouds. Ice crystals in clouds act as seeds for most precipitation that reaches the Earth's surface, and impact climate by changing the amount of sunlight clouds reflect. A variety of naturally occurring particle sources contribute to atmospheric INPs, including wind‐blown dusts, sea spray particles, biological particles such as fungal spores, bacteria and pollen, and ash and other particles from forest fires. Human‐caused particulate pollution may also contribute to atmospheric INPs in some circumstances. A better understanding of these particles, including their sources to the atmosphere, their interactions with atmospheric processes, and their impacts on clouds, is therefore required to improve climate predictability in the coming century. While impressive progress has been achieved in recent years in process‐level understanding of INPs and their cloud impacts, progress in this area will require better integration between observational and modeling tools and perspectives. This review discusses the current state of knowledge regarding INPs, key observational and modeling gaps, and identifies priority research areas that emphasize a unified effort between measurement and modeling communities.
Key Points
Atmospheric ice‐nucleating particles (INPs) play a critical role in weather and climate by facilitating ice formation in clouds
This review summarizes current knowledge on observational constraints, modeling, and cloud impacts of INPs
Research priorities are identified to both advance fundamental understanding and bridge the observation‐model gap for INPs
Abstract
Emission rates and properties of ice nucleating particles (INPs) are required for proper representation of aerosol–cloud interactions in atmospheric models. Few investigations have ...quantified marine INP emissions, a potentially important INP source for remote oceanic regions. Previous studies have suggested INPs in sea spray aerosol (SSA) are linked to oceanic biological activity. This proposed link was explored in this study by measuring INP emissions from nascent SSA during phytoplankton blooms during two mesocosm experiments. In a Marine Aerosol Reference Tank (MART) experiment, a phytoplankton bloom was produced with chlorophyll-a (Chl a) concentrations reaching 39 μg L−1, while Chl a concentrations more representative of natural ocean conditions were obtained during the Investigation into Marine Particle Chemistry and Transfer Science (IMPACTS; peak Chl a of 5 μg L−1) campaign, conducted in the University of California, San Diego, wave flume. Dynamic trends in INP emissions occurred for INPs active at temperatures > −30°C. Increases in INPs active between −25° and −15°C lagged the peak in Chl a in both studies, suggesting a consistent population of INPs associated with the collapse of phytoplankton blooms. Trends in INP emissions were also compared to aerosol composition, abundances of microbes, and enzyme activity. In general, increases in INP concentrations corresponded to increases in organic species in SSA and the emissions of heterotrophic bacteria, suggesting that both microbes and biomolecules contribute to marine INP populations. INP trends were not directly correlated with a single biological marker in either study. Direct measurements of INP chemistry are needed to accurately identify particles types contributing to marine INP populations.
Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested ...in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 °C, averaging an order of magnitude increase per 5 °C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using “dry” geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.
Sea spray aerosol (SSA) generated by bubble bursting at the ocean surface is an important component of aerosol‐cloud interactions over remote oceans, providing the atmosphere with ice‐nucleating ...particles (INPs) or particles required for heterogeneous ice nucleation. Studies have shown that organic INPs are emitted during phytoplankton blooms, but changes in INP number concentrations (nINPs) due to ocean biological activity have not been directly demonstrated in natural SSA. In this study, a clean sector sampler was used to differentiate ice nucleation and composition of pristine SSA from terrestrial aerosol at the Mace Head Research Station in August 2015. Average nINPs active at −15 °C (nINPs,−15 °C) were 0.0011 L−1, and large variability (up to a factor of 200) was observed for INPs active warmer than −22 °C. Highest nINPs in the clean sector occurred during a period of elevated marine organic aerosol from offshore biological activity (M1, nINPs,−15 °C = 0.0077 L−1). A peak in nINPs was also observed in terrestrial organic aerosol (T1, nINPs,−15 °C = 0.0076 L−1). The impacts of heating and hydrogen peroxide digestion on nINPs indicate that INPs at Mace Head Research Station were largely organic and that INPs observed during M1 and T1 were biological (i.e., protein containing). Complexities of predicting increases in nINPs due to offshore biological activity are explored. A parameterization for pristine SSA INPs over the North Atlantic Ocean was developed, illustrating that SSA is associated with a factor of 1,000 fewer ice‐nucleating sites per surface area of aerosol compared to mineral dust.
Key Points
Pristine sea spray aerosol is a dominant regional source of ice‐nucleating particles at a remote North Atlantic coastal site
Ice‐nucleating particles at a remote North Atlantic coastal site were largely composed of organic carbon and/or heat labile material
Sea spray aerosol is associated with a factor of 1,000 fewer ice‐nucleating sites per surface area of aerosol compared to mineral dust
Abstract The abundance of atmospheric ice nucleating particles (INPs) is a source of uncertainty for numerical representation of ice-phase transitions in mixed-phase clouds. While sea spray aerosol ...(SSA) exhibits less ice nucleating (IN) ability than terrestrial aerosol, marine INP emissions are linked to oceanic biological activity and are potentially an important source of INPs over remote oceans. Inadequate knowledge of marine INP identity limits the ability to parameterize this complex INP source. A previous manuscript described abundances of marine INPs in relation to several aerosol composition and ocean biology observations during two laboratory mesocosm experiments. In this study, the abundances and chemical and physical properties of INPs found during the same mesocosm experiments were directly probed in SSA, seawater, and surface microlayer samples. Two unique marine INP populations were found: 1) dissolved organic carbon INPs are suggested to be composed of IN-active molecules, and 2) particulate organic carbon INPs are attributed as intact cells or IN-active microbe fragments. Both marine INP types are likely to be emitted into SSA following decay of phytoplankton biomass when 1) the surface microlayer is significantly enriched with exudates and cellular detritus and SSA particles are preferentially coated with IN-active molecules or 2) diatom fragments and bacteria are relatively abundant in seawater and therefore more likely transferred into SSA. These findings inform future efforts for incorporating marine INP emissions into numerical models and motivate future studies to quantify specific marine molecules and isolate phytoplankton, bacteria, and other species that contribute to these marine INP types.
The impact of aerosols on human health and climate is well-recognized, yet many studies have only focused on total PM2.5 or changes from anthropogenic activities. This study quantifies the health and ...climate effects of organic aerosols (OA) from anthropogenic, biomass burning, and biogenic sources. Using two atmospheric chemistry models, CAM-chem and GEOS-Chem, our findings reveal that anthropogenic primary OA (POA) has the highest efficiency for health effects but the lowest for direct radiative effects due to spatial and temporal variations associated with population and surface albedo. The treatment of POA as nonvolatile or semivolatile also influences these efficiencies through different chemical processes. Biogenic OA shows moderate efficiency for health effects and the highest for direct radiative effects but has the lowest efficiency for indirect effects due to the reduced high cloud, caused by stabilized temperature profiles from aerosol–radiation interactions in biogenic OA-rich regions. Biomass burning OA is important for cloud radiative effect changes in remote atmospheres due to its ability to be transported further than other OAs. This study highlights the importance of not only OA characteristics such as toxicity and refractive index but also atmospheric processes such as transport and chemistry in determining health and climate impact efficiencies.
Heterogeneous ice nucleation in the atmosphere regulates cloud properties, such as phase (ice versus liquid) and lifetime. Aerosol particles of marine origin are relevant ice nucleating particle ...sources when marine aerosol layers are lifted over mountainous terrain and in higher latitude ocean boundary layers, distant from terrestrial aerosol sources. Among many particle compositions associated with ice nucleation by sea spray aerosols are highly saturated fatty acids. Previous studies have not demonstrated their ability to freeze dilute water droplets. This study investigates ice nucleation by monolayers at the surface of supercooled droplets and as crystalline particles at temperatures exceeding the threshold for homogeneous freezing. Results show the poor efficiency of long chain fatty acid (C16, C18) monolayers in templating freezing of pure water droplets and seawater subphase to temperatures of at least -30 °C, consistent with theory. This contrasts with freezing of fatty alcohols (C22 used here) at nearly 20 °C warmer. Evaporation of μL-sized droplets to promote structural compression of a C19 acid monolayer did not favor warmer ice formation of drops. Heterogeneous ice nucleation occurred for nL-sized droplets condensed on 5 to 100 μm crystalline particles of fatty acid (C12 to C20) at a range of temperatures below -28 °C. These experiments suggest that fatty acids nucleate ice at warmer than -36 °C only when the crystalline phase is present. Rough estimates of ice active site densities are consistent with those of marine aerosols, but require knowledge of the proportion of surface area comprised of fatty acids for application.
Stratocumulus clouds over the Southern Ocean have fewer droplets and are more likely to exist in the predominately supercooled phase than clouds at similar temperatures over northern oceans. One ...likely reason is that this region has few continental and anthropogenic sources of cloud‐nucleating particles that can form droplets and ice. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds in this region. These measurements and others indicate that biogenic sulfur‐based particles >0.1 μm diameter contribute the majority of cloud condensation nuclei number concentrations in summer. Ice nucleating particles tend to have more organic components, likely from sea‐spray. Both types of cloud nucleating particles may increase in a warming climate likely to have less sea ice, more phytoplankton activity, and stronger winds over the Southern Ocean near Antarctica. Taken together, clouds over the Southern Ocean may become more reflective and partially counter the region's expected albedo decrease due to diminishing sea ice. However, detailed modeling studies are needed to test this hypothesis due to the complexity of ocean‐cloud‐climate feedbacks in the region.
Plain Language Summary
Clouds over the Southern Ocean tend to have less droplets and ice crystals than similar clouds over northern oceans due to fewer sources of cloud‐nucleating aerosol particles in the region. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds. These measurements indicate that while sea‐spray‐derived salts do provide cloud nuclei, the majority of aerosol particles that influence summertime clouds in this region are biogenic—that is, derived from ocean microorganisms, with the ocean region near Antarctica being a large summertime source. These cloud‐nucleating particles may increase in a warming climate likely to have less sea ice and more phytoplankton activity near Antarctica. These additional particles could make low clouds reflect more light and offset a portion of the warming expected due to diminishing sea ice in a future climate.
Key Points
Biogenic sulfate dominates the number concentration of 0.1–0.5 microns diameter particles and cloud condensation nuclei (CCN) over the summertime Southern Ocean
Biogenic organics are a key component of ice nucleating particles over the Southern Ocean
As Antarctic climate changes, increased biological activity could partially offset warming effects of sea‐ice loss via influences on CCN
Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation, and ...radiative processes, and their interactions. Projects between 2016 and 2018 used in situ probes, radar, lidar, and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN), and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF–NCAR G-V aircraft flying north–south gradients south of Tasmania, at Macquarie Island, and on the R/V Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons. Results show largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multilayered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of dynamics and turbulence that likely drive heterogeneity of cloud phase. Satellite retrievals confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.
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