Ice-nucleating agents in sea spray aerosol identified and quantified with a holistic multimodal freezing model

Polysaccharides─Important Constituents of Ice-Nucleating Particles of Marine Origin

Remote marine regions are characterized by a high degree of cloud cover that greatly impacts Earth's radiative budget. It is highly relevant for climate projections to represent the ice formation in these clouds. Therefore, it is crucial to understand the sources of ice-nucleating particles (INPs) that enable primary ice formation. Here, we report polysaccharides produced by four different aquatic eukaryotic microorganisms (Thraustochytrium striatum, Tausonia pullulans, Naganishia diffluens, Penicillium chrysogenum) as responsible ice-nucleating macromolecules (INMs) in these samples originating from the marine biosphere. By deriving a classical nucleation theory-based parametrization of these polysaccharidic INMs and applying it to global model simulations, a comparison to currently available marine atmospheric INP observations demonstrates a 44% contribution of polysaccharides to the total INPs of marine origin within -15 to -20 °C. The results highlight the relevance of biological INMs as part of the INP population in remote marine regions.

A comprehensive characterisation of natural aerosol sources in the high Arctic during the onset of sea ice melt

The interactions between aerosols and clouds are still one of the largest sources of uncertainty in quantifying anthropogenic radiative forcing. To reduce this uncertainty, we must first determine the baseline natural aerosol loading for different environments. In the pristine and hardly accessible polar regions, the exact nature of local aerosol sources remains poorly understood. It is unclear how oceans, including sea ice, control the aerosol budget, influence cloud formation, and determine the cloud phase. One critical question relates to the abundance and characteristics of biological aerosol particles that are important for the formation and microphysical properties of Arctic mixed-phase clouds. Within this work, we conducted a comprehensive analysis of various potential local sources of natural aerosols in the high Arctic over the pack ice during the ARTofMELT expedition in May-June 2023. Samples of snow, sea ice, seawater, and the sea surface microlayer (SML) were analysed for their microphysical, chemical, and fluorescent properties immediately after collection. Accompanied analyses of ice nucleating properties and biological cell quantification were performed at a later stage. We found that increased biological activity in seawater and the SML during the late Arctic spring led to higher emissions of fluorescent primary biological aerosol particles (fPBAPs) and other highly fluorescent particles (OHFPs, here organic-coated sea salt particles). Surprisingly, the concentrations of ice nucleating particles (INPs) in the corresponding liquid samples did not follow this trend. Gradients in OHFPs, fPBAPs, and black carbon indicated an anthropogenic pollution signal in surface samples especially in snow but also in the top layer of the sea ice core and SML samples. Salinity did not affect the aerosolisation of fPBAPs or sample ice nucleating activity. Compared to seawater, INP and fPBAP concentrations were enriched in sea ice samples. All samples showed distinct differences in their biological, chemical, and physical properties, which can be used in future work for an improved source apportionment of natural Arctic aerosol to reduce uncertainties associated with their representation in models and impacts on Arctic mixed-phase clouds.

Microfluidics for the biological analysis of atmospheric ice-nucleating particles: Perspectives and challenges

Atmospheric ice-nucleating particles (INPs) make up a vanishingly small proportion of atmospheric aerosol but are key to triggering the freezing of supercooled liquid water droplets, altering the lifetime and radiative properties of clouds and having a substantial impact on weather and climate. However, INPs are notoriously difficult to model due to a lack of information on their global sources, sinks, concentrations, and activity, necessitating the development of new instrumentation for quantifying and characterizing INPs in a rapid and automated manner. Microfluidic technology has been increasingly adopted by ice nucleation research groups in recent years as a means of performing droplet freezing analysis of INPs, enabling the measurement of hundreds or thousands of droplets per experiment at temperatures down to the homogeneous freezing of water. The potential for microfluidics extends far beyond this, with an entire toolbox of bioanalytical separation and detection techniques developed over 30 years for medical applications. Such methods could easily be adapted to biological and biogenic INP analysis to revolutionize the field, for example, in the identification and quantification of ice-nucleating bacteria and fungi. Combined with miniaturized sampling techniques, we can envisage the development and deployment of microfluidic sample-to-answer platforms for automated, user-friendly sampling and analysis of biological INPs in the field that would enable a greater understanding of their global and seasonal activity. Here, we review the various components that such a platform would incorporate to highlight the feasibility, and the challenges, of such an endeavor, from sampling and droplet freezing assays to separations and bioanalysis.

Quantifying and Modeling the Impact of Phase State on the Ice Nucleation Abilities of 2-Methyltetrols as a Key Component of Secondary Organic Aerosol Derived from Isoprene Epoxydiols

Organic aerosols (OAs) may serve as ice-nucleating particles (INPs), impacting the formation and properties of cirrus clouds when their phase state and viscosity are in the semisolid to glassy range. However, there is a lack of direct parameterization between aerosol viscosity and their ice nucleation capabilities. In this study, we experimentally measured the ice nucleation rate of 2-methyltetrols (2-MT) aerosols, a key component of isoprene-epoxydiol-derived secondary organic aerosols (IEPOX-SOA), at different viscosities. These results demonstrate that the phase state has a significant impact on the ice nucleation abilities of OA under typical cirrus cloud conditions, with the ice nucleation rate increasing by 2 to 3 orders of magnitude when the phase state changes from liquid to semisolid. An innovative parametric model based on classical nucleation theory was developed to directly quantify the impact of viscosity on the heterogeneous nucleation rate. This model accurately represents our laboratory measurement and can be implemented into climate models due to its simple, equation-based form. Based on data collected from the ACRIDICON-CHUVA field campaign, our model predicts that the INP concentration from IEPOX-SOA can reach the magnitude of 1 to tens per liter in the cirrus cloud region impacted by the Amazon rainforest, consistent with recent field observations and estimations. This novel parameterization framework can also be applied in regional and global climate models to further improve representations of cirrus cloud formation and associated climate impacts.

Formation of Sodium Chloride on the Surface of Sulfate-Rich Gobi Desert Salt in Response to Water Adsorption

Dust storms in arid regions transport desert salts and dust, affecting geochemical processes, atmospheric chemistry, climate, and human health. This study examines how the gas-salt interface composition of desert salt changes with varying relative humidity (RH), using ambient pressure X-ray photoelectron spectroscopy (APXPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and molecular dynamics (MD) simulations. Ion chromatography analysis of desert salt indicates it is predominantly composed of sulfate, sodium, and magnesium ions, with traces of calcium, chloride, nitrate, and potassium ions. APXPS and NEXAFS surface analyses show that, at 0% RH, the gas-salt interface primarily features Na2SO4, with smaller amounts of MgSO4 and a trace of NaCl on the top layers. As humidity increases, the composition at the gas-salt interface changes, notably with Mg2+ binding to SO4 2- ions and a dominant NaCl formation throughout the studied surface depth. This shift indicates a transition from a sulfate- to a chloride-rich surface as humidity increases, contradicting MD simulations that predicted that on salt crystals covered by a submonolayer of water with electrolytes, chloride ions migrate toward the liquid-solid interface. This discrepancy indicates that other factors, like enhanced ionic mobility at grain boundaries, might drive the accumulation of chloride ions at the gas interface. The study emphasizes the crucial role of adsorbed water in ion migration and surface composition transformation of desert salts, affecting geochemical processes in arid regions.

Single particle characteristics and ice nucleation potential of particles collected during Asian dust storms in 2021

Dust storms have great impacts on air quality and climate. Dust can influence cloud microphysical properties and determine their radiative forcing and precipitation. Asian dust storms (ADS) are important sources of global aerosol. However, the physiochemical characteristics of dust from ADS at a single particle level are less understood, and the exact particles that can serve as ice nucleating particles (INPs) remain unclear. Here, we present the physicochemical properties and ice nucleation ability of dust particles collected in Beijing during two major ADS in March 2021. The particles from two ADS were classified into Illite, Kaolinite, Feldspar, Quartz, Chlorite, Mixed-dust, and Non-dust particles, which contributed 28.6 % ± 3.3 %, 20.0 % ± 3.9 %, 12.3 % ± 2.3 %, 11.1 % ± 2.8 %, 9.8 % ± 0.8 %, 13.7 % ± 1.8 %, and 4.4 % ± 1.7 % in number, respectively. On average, the ADS particles formed ice crystals via deposition ice nucleation from relative humidity with respect to ice (RHice) of 112 % ± 1 % at 250 K to 154 % ± 15 % RHice at 205 K. Part of the samples also formed ice via immersion freezing between 230 K and 250 K. Among the 149 identified INPs, Clay-like particles (Chlorite, Illite, and Kaolinite) contributed 71.1 % ± 6.2 % in number and followed by Mixed-dust-like particles (16.9 % ± 8.7 %) and Feldspar-like particles (10.4 % ± 6.3 %). Enrichment factor of each particle type in INPs is calculated as the ratio of its number fractions in INPs and the aerosol population. It ranges from 0.6 ± 0.7 to 1.3 ± 2.2. The contribution of each particle type to INP was correlated with its fraction in the population. These results imply that each particle type can serve as INP. Clay-like particles are the dominant INPs during the ADS. We conducted ice nucleation kinetic analysis and provided parameterizations of heterogeneous ice nucleation rate coefficient and contact angle for ADS. These parameterizations can be used in the modeling study to evaluate the impact of ADS in atmospheric ice crystal formation in clouds.

Is our understanding of aquatic ecosystems sufficient to quantify ecologically driven climate feedbacks?

The Earth functions as an integrated system-its current habitability to complex life is an emergent property dependent on interactions among biological, chemical, and physical components. As global warming affects ecosystem structure and function, so too will the biosphere affect climate by altering atmospheric gas composition and planetary albedo. Constraining these ecosystem-climate feedbacks is essential to accurately predict future change and develop mitigation strategies; however, the interplay among ecosystem processes complicates the assessment of their impact. Here, we explore the state-of-knowledge on how ecological and biological processes (e.g., competition, trophic interactions, metabolism, and adaptation) affect the directionality and magnitude of feedbacks between ecosystems and climate, using illustrative examples from the aquatic sphere. We argue that, despite ample evidence for the likely significance of many, our present understanding of the combinatorial effects of ecosystem dynamics precludes the robust quantification of most ecologically driven climate feedbacks. Constraining these effects must be prioritized within the ecological sciences for only by studying the biosphere as both subject and arbiter of global climate can we develop a sufficiently holistic view of the Earth system to accurately predict Earth's future and unravel its past.

Effect of Acidity on Ice Nucleation by Inorganic-Organic Mixed Droplets

Aerosol acidity significantly influences heterogeneous chemical reactions and human health. Additionally, acidity may play a role in cloud formation by modifying the ice nucleation properties of inorganic and organic aerosols. In this work, we combined our well-established ice nucleation technique with Raman microspectroscopy to study ice nucleation in representative inorganic and organic aerosols across a range of pH conditions (pH -0.1 to 5.5). Homogeneous nucleation was observed in systems containing ammonium sulfate, sulfuric acid, and sucrose. In contrast, droplets containing ammonium sulfate mixed with diethyl sebacate, poly(ethylene glycol) 400, and 1,2,6-hexanetriol were found to undergo liquid-liquid phase separation, exhibiting core-shell morphologies with observed initiation of heterogeneous freezing in the cores. Our experimental findings demonstrate that an increased acidity reduces the ice nucleation ability of droplets. Changes in the ratio of bisulfate to sulfate coincided with shifts in ice nucleation temperatures, suggesting that the presence of bisulfate may decrease the ice nucleation efficiency. We also report on how the morphology and viscosity impact ice nucleation properties. This study aims to enhance our fundamental understanding of acidity's effect on ice nucleation ability, providing context for the role of acidity in atmospheric ice cloud formation.