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

Sea spray aerosol (SSA) is a widely recognized important source of ice-nucleating particles (INPs) in the atmosphere. However, composition-specific identification, nucleation processes, and ice nucleation rates of SSA-INPs have not been well constrained. Microspectroscopic characterization of ambient and laboratory-generated SSA confirms that water-borne exudates from planktonic microorganisms composed of a mixture of proteinaceous and polysaccharidic compounds act as ice-nucleating agents (INAs). These data and data from previously published mesocosm and wave channel studies are subsequently used to further develop the stochastic freezing model (SFM) producing ice nucleation rate coefficients for SSA-INPs. The SFM simultaneously predicts immersion freezing and deposition and homogeneous ice nucleation by SSA particles under tropospheric conditions. Predicted INP concentrations agree with ambient and laboratory measurements. In addition, this holistic freezing model is independent of the source and exact composition of the SSA particles, making it well suited for implementation in cloud and climate models.

Imaging features of anorectal proctitis in monkeypox infection.

The monkeypox outbreak of 2022 saw the first community-sustained transmission of the monkeypox virus outside of Africa, and rapidly developed into multi-country spread. A common presenting sign of monkeypox infection during this outbreak has been rectal pain due to proctitis. Proctitis with large hypoattenuated anorectal ulcers on CT scan should invoke consideration for monkeypox infection in young homosexual or bisexual men with associated skin eruptions.

Does liquid-liquid phase separation impact ice nucleation in mixed polyethylene glycol and ammonium sulfate droplets?

Particles can undergo different phase transitions in the atmosphere including deliquescence, liquid-liquid phase separation (LLPS), melting, and freezing. In this study, phase transitions of particles/droplets containing polyethylene glycol with a molar mass of 400 g mol-1 (PEG400) and ammonium sulfate (AS), i.e., PEG400-AS particles/droplets, were investigated at different organic-to-inorganic dry mass ratios (OIRs) under typical tropospheric temperatures and water activities (aw). The investigated droplets (60-100 µm) with or without LLPS in the closed system froze through homogeneous ice nucleation. At temperatures lower than 200 K, multiple ice nucleation events were observed within the same individual droplets at low aw. Droplets with and without LLPS shared similar lambda values at the same OIR according to the lambda approach indicating they form ice through the same mechanism. A parameterization of lambda values was provided which can be used to predict freezing temperature of aqueous PEG400-AS droplets. We found that adding AS reduces the temperature dependence of aw in aqueous PEG400 droplets. Assuming incorrectly that aw is temperature-independent for a constant droplet composition leads to a deviation between the experimental determined ice nucleation rate coefficients for droplets at OIR > 1 and the predicted values by the water-activity-based ice nucleation theory. We proposed a parameterization of temperature dependence of aw to minimize the deviations of the measured melting temperatures and nucleation rate coefficients from the corresponding predictions for aqueous PEG400-AS system.

Ice nucleation imaged with X-ray spectro-microscopy.

Ice nucleation is one of the most uncertain microphysical processes, as it occurs in various ways and on many types of particles. To overcome this challenge, we present a heterogeneous ice nucleation study on deposition ice nucleation and immersion freezing in a novel cryogenic X-ray experiment with the capability to spectroscopically probe individual ice nucleating and non-ice nucleating particles. Mineral dust type particles composed of either ferrihydrite or feldspar were used and mixed with organic matter of either citric acid or xanthan gum. We observed in situ ice nucleation using scanning transmission X-ray microscopy (STXM) and identified unique organic carbon functionalities and iron oxidation state using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the new in situ environmental ice cell, termed the ice nucleation X-ray cell (INXCell). Deposition ice nucleation of ferrihydrite occurred at a relative humidity with respect to ice, RH i, between ∼120-138% and temperatures, T ∼ 232 K. However, we also observed water uptake on ferrihydrite at the same T when deposition ice nucleation did not occur. Although, immersion freezing of ferrihydrite both in pure water droplets and in aqueous citric acid occurred at or slightly below conditions for homogeneous freezing, i.e. the effect of ferrihydrite particles acting as a heterogeneous ice nucleus for immersion freezing was small. Microcline K-rich feldspar mixed with xanthan gum was also used in INXCell experiments. Deposition ice nucleation occurred at conditions when xanthan gum was expected to be highly viscous (glassy). At less viscous conditions, immersion freezing was observed. We extended a model for heterogeneous and homogeneous ice nucleation, named the stochastic freezing model (SFM). It was used to quantify heterogeneous ice nucleation rate coefficients, mimic the competition between homogeneous ice nucleation; water uptake; deposition ice nucleation and immersion freezing, and predict the T and RH i at which ice was observed. The importance of ferrihydrite to act as a heterogeneous ice nucleating particle in the atmosphere using the SFM is discussed.

Amplification of light within aerosol particles accelerates in-particle photochemistry.

Optical confinement (OC) structures the optical field and amplifies light intensity inside atmospheric aerosol particles, with major consequences for sunlight-driven aerosol chemistry. Although theorized, the OC-induced spatial structuring has so far defied experimental observation. Here, x-ray spectromicroscopic imaging complemented by modeling provides direct evidence for OC-induced patterning inside photoactive particles. Single iron(III)-citrate particles were probed using the iron oxidation state as a photochemical marker. Based on these results, we predict an overall acceleration of photochemical reactions by a factor of two to three for most classes of atmospheric aerosol particles. Rotation of free aerosol particles and intraparticle molecular transport generally accelerate the photochemistry. Given the prevalence of OC effects, their influence on aerosol particle photochemistry should be considered by atmospheric models.

Photolytic radical persistence due to anoxia in viscous aerosol particles.

In viscous, organic-rich aerosol particles containing iron, sunlight may induce anoxic conditions that stabilize reactive oxygen species (ROS) and carbon-centered radicals (CCRs). In laboratory experiments, we show mass loss, iron oxidation and radical formation and release from photoactive organic particles containing iron. Our results reveal a range of temperature and relative humidity, including ambient conditions, that control ROS build up and CCR persistence in photochemically active, viscous organic particles. We find that radicals can attain high concentrations, altering aerosol chemistry and exacerbating health hazards of aerosol exposure. Our physicochemical kinetic model confirmed these results, implying that oxygen does not penetrate such particles due to the combined effects of fast reaction and slow diffusion near the particle surface, allowing photochemically-produced radicals to be effectively trapped in an anoxic organic matrix.

Physiological integration can increase competitive ability in clonal plants if competition is patchy.

Physiological integration of connected plants of the same clone, or ramets, often increases clonal fitness when ramets differ in resource supply. However, review of the literature found that no study has directly tested the hypothesis that integration can increase the ability of clones to compete against other species. To test this, we grew two-ramet clonal fragments of the stoloniferous, perennial herb Fragaria chiloensis in which none, one, or both of the ramets had neighbors of a naturally co-occurring, dominant grass, Bromus carinatus, and connections between ramets were either severed to prevent integration or left intact. We also grew four-ramet fragments in which all ramets had neighbors and connections were severed or intact. Severance decreased the final leaf mass and area of two-ramet fragments by 25% and their final total mass by 15% when just one ramet was grown with B. carinatus. Severance had no significant effect on the total mass of fragments when none or all of the ramets were grown with the grass. This provides the first direct evidence that physiological integration can increase the competitive ability of clonal plant species, though only when competition is spatially heterogeneous. Integration may thus enable plant clones to grow into plant communities and to compete within communities with fine-scale disturbance. However, integration may not increase the competitive ability of clonal plants within uniformly dense communities of taller species.

Parasitism induces negative effects of physiological integration in a clonal plant.

Clonal integration often increases fitness of clonal plants, but it may decrease it when some but not all connected plants (ramets) within a clone are parasitized. This hypothesis was synthesized in a conceptual model and tested by growing pairs of connected ramets of two congeneric clonal plants, Sphagneticola trilobata and Sphagneticola calendulacea, with and without parasitizing one ramet with Cuscuta australis and with and without severing the connection (allowing or preventing integration). Consistent with the model, integration in S. calendulacea did not affect biomass of the parasitized ramet, decreased biomass of its connected, unparasitized ramet by 60% and of the clone by 40%, and increased biomass of the parasite by 50%. By contrast, integration in S. trilobata did not affect biomass of the clone or the parasite. The parasite increased export of nitrogen-15 from the connected, unparasitized ramet seven-fold in S. calendulacea but did not affect export in S. trilobata. Parasitism can cause clonal integration to negatively affect fitness in clonal plants because parasites can import resources from connected, unparasitized ramets, possibly partly through signaling. This is the first experimental demonstration that clonal integration can decrease fitness in plants induced by parasitism and may help explain community-level effects of parasites.

Effects of fragmentation of clones compound over vegetative generations in the floating plant Pistia stratiotes.

BACKGROUND AND AIMS: Clonal plants dominate many plant communities, especially in aquatic systems, and clonality appears to promote invasiveness and to affect how diversity changes in response to disturbance and resource availability. Understanding how the special physiological and morphological properties of clonal growth lead to these ecological effects depends upon studying the long-term consequences of clonal growth properties across vegetative generations, but this has rarely been done. This study aimed to show how a key clonal property, physiological integration between connected ramets within clones, affects the response of clones to disturbance and resources in an aquatic, invasive, dominant species across multiple generations. METHODS: Single, parental ramets of the floating stoloniferous plant Pistia stratiotes were grown for 3 weeks, during which they produced two or three generations of offspring; connections between new ramets were cut or left intact. Individual offspring were then used as parents in a second 3-week iteration that crossed fragmentation with previous fragmentation in the first iteration. A third iteration yielded eight treatment combinations, zero to three rounds of fragmentation at different times in the past. The experiment was run once at a high and once at a low level of nutrients. RESULTS: In each iteration, fragmentation increased biomass of the parental ramet, decreased biomass of the offspring and increased number of offspring. These effects persisted and compounded from one iteration to another, though more recent fragmentation had stronger effects, and were stronger at the low than at the high nutrient level. Fragmentation did not affect net accumulation of mass by groups after one iteration but increased it after two iterations at low nutrients, and after three iterations at both nutrient levels. CONCLUSIONS: Both the positive and negative effects of fragmentation on clonal performance can compound and persist over time and can be stronger when resource levels are lower. Even when fragmentation has no short-term net effect on clonal performance, it can have a longer-term effect. In some cases, fragmentation may increase total accumulation of mass by a clone. The results provide the first demonstration of how physiological integration in clonal plants can affect fitness across generations and suggest that increased disturbance may promote invasion of introduced clonal species via effects on integration, perhaps especially at lower nutrient levels.

Stochastic nucleation processes and substrate abundance explain time-dependent freezing in supercooled droplets.

Atmospheric immersion freezing (IF), a heterogeneous ice nucleation process where an ice nucleating particle (INP) is immersed in supercooled water, is a dominant ice formation pathway impacting the hydrological cycle and climate. Implementation of IF derived from field and laboratory data in cloud and climate models is difficult due to the high variability in spatio-temporal scales, INP composition, and morphological complexity. We demonstrate that IF can be consistently described by a stochastic nucleation process accounting for uncertainties in the INP surface area. This approach accounts for time-dependent freezing, a wide range of surface areas and challenges phenomenological descriptions typically used to interpret IF. The results have an immediate impact on the current description, interpretation, and experiments of IF and its implementation in models. The findings are in accord with nucleation theory, and thus should hold for any supercooled liquid material that nucleates in contact with a substrate.

Capacity for clonal integration in introduced versus native clones of the invasive plant Hydrocotyle vulgaris.

Clonal plants can make up a disproportionately high number of the introduced, invasive plant species in a region. Physiological integration of connected ramets within clones is a key ecological advantage of clonal growth. To ask whether clonal integration underlies the invasiveness of clonal plants, we tested the hypothesis that introduced clones of an invasive species will show higher capacity for integration than native clones of the same species. We conduct a greenhouse experiment on the widespread, perennial herb Hydrocotyle vulgaris. Clonal fragments consisting of pairs of connected ramets from seven sites in northwestern Spain where the species is native and seven sites in southeastern China where the species is introduced and invasive were grown for 79 days with the younger, apical ramet shaded to 30% of ambient light and the connection between ramets either severed or left intact. Severance decreased the final dry mass and ramet number of the apical ramet and its offspring in nearly all clones and increased the mass or ramet number of the basal portion of the fragment in about half of the clones, but these effects did not differ consistently between native and introduced clones. Severance did affect allocation more in introduced than in native clones, decreasing root/total mass more in apical portions and increasing it more in basal portions. Maintaining the connection between ramets caused introduced, but not native, clonal fragments to produce more leaf and less root mass and thus to lower allocation to roots. Regardless of severance, introduced clones accumulated about twice as much mass as native clones. Results suggest that introduced clones of a species can show greater effects of integration on allocation than native clones. In species such as H. vulgaris, this might increase competitiveness for light.

Clonal integration in Phagmites australis mitigates effects of oil pollution on greenhouse gas emissions in a coastal wetland.

Clonal integration, i.e., resource sharing within clones, enables clonal plants to maintain biomass production when ramets (asexual individuals) under stress are connected to those not under stress. Oil pollution can strongly reduce biomass production, and connected ramets within clones may experience different levels of oil pollution. Therefore, clonal integration may help plants maintain biomass production despite oil pollution. Because biomass production is often negatively correlated with greenhouse gas emissions, we hypothesized that oil pollution would increase greenhouse gas emissions and that clonal integration would reduce such an effect. We tested these hypotheses in a coastal wetland dominated by the rhizomatous grass Phragmites australis near a major site of oil production in the Yellow River Delta in China. We applied 0, 5, or 10 mm crude oil per year for two years in plots within stands of P. australis and tested effects of severing rhizomes connecting ramets inside and outside a plot (i.e. preventing clonal integration) on biomass production, soil chemistry and greenhouse gas emissions. When severed, ramets inside plots with no added oil produced about 220 g aboveground biomass m--2 over the second growing season, and plots absorbed about 500 g total CO2 equivalents m-2. Adding 10 mm oil per year reduced aboveground biomass by about 30%, and caused plots to emit about 800 g CO2 equivalents m-2. Leaving ramets connected to those outside plots eliminated the negative effects of oil pollution on biomass production, and caused plots given 10 mm oil per year to emit about 50% fewer total CO2 equivalents. We conclude that oil pollution can increase greenhouse gas emissions and clonal integration can reduce the effect of oil pollution on biomass production and greenhouse gas emissions. Our study provides the first experimental evidence that clonal integration in plants can reduce greenhouse gas emissions.

Aging induced changes in ice nucleation activity of combustion aerosol as determined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy.

Fresh soot particles are generally hydrophobic, however, particle hydrophilicity can be increased through atmospheric aging processes. At present little is known on how particle chemical composition and hydrophilicity change upon atmospheric aging and associated uncertainties governing the ice cloud formation potential of soot. Here we sampled two propane flame soots referred to as brown and black soot, characterized as organic carbon rich and poor, respectively. We investigated how the ice nucleation activity of these particles changed through aging in water and aqueous acidic solutions, using a continuous flow diffusion chamber operated at cirrus cloud temperatures (T ≤ 233 K). Single aggregates of both unaged and aged soot were chemically characterized by scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (STXM/NEXAFS) measurements. Particle wettability was determined through water sorption measurements. Unaged black and brown soot particles exhibited significantly different ice nucleation activities. Our experiments revealed significantly enhanced ice nucleation activity of the aged soot particles compared to the fresh samples, lowering the required relative humidities at which ice formation can take place at T = 218 K by up to 15% with respect to water (ΔRHi ≈ 25%). We observed an enhanced water uptake capacity for the aged compared to the unaged samples, which was more pronounced for the black soot. From these measurements we concluded that there is a change in ice nucleation mechanism when aging brown soot. Comparison of the NEXAFS spectra of unaged soot samples revealed a unique spectral feature around 287.5 eV in the case of black soot that was absent for the brown soot, indicative of carbon with hydroxyl functionalities. Comparison of the NEXAFS spectra of unaged and aged soot particles indicates changes in organic functional groups, and the aged spectra were found to be largely similar across soot types, with the exception of the water aged brown soot. Overall, we conclude that atmospheric aging is important to representatively assess the ice cloud formation activity of soot particles.

Modification by earthworms of effects of soil heterogeneity and root foraging in eight species of grass.

Spatial heterogeneity of soil nutrients and earthworm activity can each increase the performance of plant species, but their interactive effects have been little studied. The ability of plants to forage for nutrients by concentrating roots where nutrients are concentrated can partly explain the positive effects of nutrient heterogeneity, but whether root foraging can help explain the positive effects of earthworm activity is untested. We conducted a greenhouse experiment in which we grew eight species of Poaceae in homogeneous and heterogeneous soils with or without the earthworms Eisenia fetida and Metaphire guillelmi and measured net accumulation of plant mass and tillers. Effects of heterogeneity and earthworms on plant performance were positive in most species. The presence of earthworms reduced the directly measured effect of heterogeneity on total mass in some grass species. Most species showed root foraging ability. Ability showed no relationship to effects of heterogeneity or earthworms on final total dry mass. However, earthworms reduced foraging in some species, possibly by lessening heterogeneity. Earthworm activity in heterogeneous soil may thus reduce the benefits of root foraging for nutrients in plants.

Visualizing reaction and diffusion in xanthan gum aerosol particles exposed to ozone.

Atmospheric aerosol particles with a high viscosity may become inhomogeneously mixed during chemical processing. Models have predicted gradients in condensed phase reactant concentration throughout particles as the result of diffusion and chemical reaction limitations, termed chemical gradients. However, these have never been directly observed for atmospherically relevant particle diameters. We investigated the reaction between ozone and aerosol particles composed of xanthan gum and FeCl2 and observed the in situ chemical reaction that oxidized Fe2+ to Fe3+ using X-ray spectromicroscopy. Iron oxidation state of particles as small as 0.2 µm in diameter were imaged over time with a spatial resolution of tens of nanometers. We found that the loss off Fe2+ accelerated with increasing ozone concentration and relative humidity, RH. Concentric 2-D column integrated profiles of the Fe2+ fraction, α, out of the total iron were derived and demonstrated that particle surfaces became oxidized while particle cores remained unreacted at RH = 0-20%. At higher RH, chemical gradients evolved over time, extended deeper from the particle surface, and Fe2+ became more homogeneously distributed. We used the kinetic multi-layer model for aerosol surface and bulk chemistry (KM-SUB) to simulate ozone reaction constrained with our observations and inferred key parameters as a function of RH including Henry's Law constant for ozone, HO3, and diffusion coefficients for ozone and iron, DO3 and DFe, respectively. We found that HO3 is higher in our xanthan gum/FeCl2 particles than for water and increases when RH decreased from about 80% to dry conditions. This coincided with a decrease in both DO3 and DFe. In order to reproduce observed chemical gradients, our model predicted that ozone could not be present further than a few nanometers from a particle surface indicating near surface reactions were driving changes in iron oxidation state. However, the observed chemical gradients in α observed over hundreds of nanometers must have been the result of iron transport from the particle interior to the surface where ozone oxidation occurred. In the context of our results, we examine the applicability of the reacto-diffusive framework and discuss diffusion limitations for other reactive gas-aerosol systems of atmospheric importance.

Carbon Dioxide Diffusivity in Single, Levitated Organic Aerosol Particles.

The diffusivity of molecules relevant to condensed-phase chemistry within viscous secondary organic aerosol (SOA) remains highly uncertain. Whereas there has been an effort to characterize water diffusivity as well as the diffusivity of larger compounds, data are lacking almost entirely for small molecules, such as carbon dioxide (CO2). Here we use photochemically generated CO2 in single particles of aqueous citric acid as a SOA proxy, levitated in an electrodynamic balance, to deduce CO2 diffusivity in the particle with unprecedented accuracy. For medium viscosities at intermediate relative humidities (∼25-40% RH), we find CO2 diffusivities DCO2 ≈ 10-14 m2 s-1, agreeing with the Stokes-Einstein relationship based on current viscosity data but 10 times lower than that for water. Conversely, under dry high-viscosity conditions, we find that DCO2 ≈ 10-16 m2 s-1, which is 10 times higher than for water. We infer that the chemical degradation of atmospheric SOA particles will likely not be limited by CO2 diffusivity.

Particle-Phase Photosensitized Radical Production and Aerosol Aging.

Atmospheric aerosol particles may contain light absorbing (brown carbon, BrC), triplet forming organic compounds that can sustain catalytic radical reactions and thus contribute to oxidative aerosol aging. We quantify UVA induced radical production initiated by imidazole-2-carboxaldehyde (IC), benzophenone (BPh). and 4-benzoylbenzoic acid (BBA) in the presence of the nonabsorbing organics citric acid (CA), shikimic acid (SA), and syringol (Syr) at varying mixing ratios. We observed a maximum HO2 release of 1013 molecules min-1 cm-2 at a mole ratio XBPh < 0.02 for BPh in CA. Mixtures of either IC or BBA with CA resulted in 1011-1012 molecules min-1 cm-2 of HO2 at mole ratios ( XIC and XBBA) between 0.01 and 0.15. HO2 release was affected by relative humidity ( RH) and film thickness suggesting coupled photochemical reaction and diffusion processes. Quantum yields of HO2 formed per absorbed photon for IC, BBA and BPh were between 10-7 and 5 × 10-5. The nonphotoactive organics, Syr and SA, increased HO2 production due to the reaction with the triplet excited species ensuing ketyl radical production. Rate coefficients of the triplet of IC with Syr and SA measured by laser flash photolysis experiments were kSyr = (9.4 ± 0.3) × 108 M-1 s-1 and kSA = (2.7 ± 0.5) × 107 M-1 s-1. A simple kinetic model was used to assess total HO2 and organic radical production in the condensed phase and to upscale to ambient aerosol, indicating that BrC induced radical production may amount to an upper limit of 20 and 200 M day-1 of HO2 and organic radical respectively, which is greater or in the same order of magnitude as the internal radical production from other processes, previously estimated to be around 15 M per day.

Facilitation of amphibious habit by physiological integration in the clonal, perennial, climbing herb Ipomoea aquatica.

Physiological integration of connected ramets of clonal plants can increase clonal performance when ramets grow in contrasting microenvironments within a habitat. In amphibious clonal species, integration of ramets in different habitats, terrestrial and aquatic, is possible. This may increase performance of amphibious clones, especially under eutrophic conditions. To test this, clonal fragments consisting of two ramets of the amphibious, perennial, climbing herb Ipomoea aquatica connected by a stem were placed such that the proximal ramet was rooted in a simulated riparian community of four other species, while the distal ramet extended into a simulated aquatic habitat with open water and sediment. The connection between ramets was either left intact or severed, and 0, 5, or 25mg N L-1 was added to the aquatic habitat to simulate different degrees of eutrophication. Without added N, fragments in which the original ramets were left connected accumulated two times more total mass than fragments in which the ramets were disconnected from one another. The positive effect of connection increased two-fold with increasing N. These results were consistent with the hypotheses that physiological integration between connected terrestrial and aquatic ramets can increase clonal performance in plants and that this effect can be greater when the aquatic ramet is richer in nutrients. Connection reduced root to shoot ratio in terrestrial ramets, but increased it in aquatic ones, suggesting that physiological integration induced a division of labor in which terrestrial ramets specialized for light acquisition and aquatic ramets specialized for acquisition of nutrients. This provides the first report of increase in clonal performance and induction of division of labor due to physiological integration between ramets in different habitats.

Fatty Acid Surfactant Photochemistry Results in New Particle Formation.

Organic interfaces that exist at the sea surface microlayer or as surfactant coatings on cloud droplets are highly concentrated and chemically distinct from the underlying bulk or overlying gas phase. Therefore, they may be potentially unique locations for chemical or photochemical reactions. Recently, photochemical production of volatile organic compounds (VOCs) was reported at a nonanoic acid interface however, subsequent secondary organic aerosol (SOA) particle production was incapable of being observed. We investigated SOA particle formation due to photochemical reactions occurring at an air-water interface in presence of model saturated long chain fatty acid and alcohol surfactants, nonanoic acid and nonanol, respectively. Ozonolysis of the gas phase photochemical products in the dark or under continued UV irradiation both resulted in nucleation and growth of SOA particles. Irradiation of nonanol did not yield detectable VOC or SOA production. Organic carbon functionalities of the SOA were probed using X-ray microspectroscopy and compared with other laboratory generated and field collected particles. Carbon-carbon double bonds were identified in the condensed phase which survived ozonolysis during new particle formation and growth. The implications of photochemical processes occurring at organic coated surfaces are discussed in the context of marine SOA particle atmospheric fluxes.