A surface-promoted sulfate-reducing ammonium oxidation reaction was discovered to spontaneously take place on common inorganic aerosol surfaces undergoing solvation. Several key intermediate ...species—including elemental sulfur (S
), bisulfide (HS
), nitrous acid (HONO), and aqueous ammonia NH
—were identified as reaction components associated with the solvation process. Depth profiles of relative species abundance showed the surface propensity of key species. The species assignments and depth profile features were supported by classical and first-principles molecular dynamics calculations, and a detailed mechanism was proposed to describe the processes that led to unexpected products during salt solvation. This discovery revealed chemistry that is distinctly linked to a solvating surface and has great potential to illuminate current puzzles within heterogeneous chemistry.
Ice-nucleating particles (INPs) trigger the formation of cloud ice crystals in the atmosphere. Therefore, they strongly influence cloud microphysical and optical properties and precipitation and the ...life cycle of clouds. Improving weather forecasting and climate projection requires an appropriate formulation of atmospheric INP concentrations. This remains challenging as the global INP distribution and variability depend on a variety of aerosol types and sources, and neither their short-term variability nor their long-term seasonal cycles are well covered by continuous measurements. Here, we provide the first year-long set of observations with a pronounced INP seasonal cycle in a boreal forest environment. Besides the observed seasonal cycle in INP concentrations with a minimum in wintertime and maxima in early and late summer, we also provide indications for a seasonal variation in the prevalent INP type. We show that the seasonal dependency of INP concentrations and prevalent INP types is most likely driven by the abundance of biogenic aerosol. As current parameterizations do not reproduce this variability, we suggest a new mechanistic description for boreal forest environments which considers the seasonal variation in INP concentrations. For this, we use the ambient air temperature measured close to the ground at 4.2 m height as a proxy for the season, which appears to affect the source strength of biogenic emissions and, thus, the INP abundance over the boreal forest. Furthermore, we provide new INP parameterizations based on the Ice Nucleation Active Surface Site (INAS) approach, which specifically describes the ice nucleation activity of boreal aerosols particles prevalent in different seasons. Our results characterize the boreal forest as an important but variable INP source and provide new perspectives to describe these new findings in atmospheric models.
The phase state of atmospheric particulate is important to atmospheric
processes, and aerosol radiative forcing remains a large uncertainty in
climate predictions. That said, precise atmospheric ...phase behavior is
difficult to quantify and observations have shown that “precondensation” of
water below predicted saturation values can occur. We propose a revised
approach to understanding the transition from solid soluble particles to
liquid droplets, typically described as cloud condensation nucleation – a
process that is traditionally captured by Köhler theory, which describes
a modified equilibrium saturation vapor pressure due to (i) mixing entropy
(Raoult's law) and (ii) droplet geometry (Kelvin effect). Given that
observations of precondensation are not predicted by Köhler theory, we
devise a more complete model that includes interfacial forces giving rise to
predeliquescence, i.e., the formation of a brine layer wetting a salt
particle at relative humidities well below the deliquescence point.
Ice-nucleating particle concentrations (INPCs) can spread over several orders of magnitude at any given temperature. However, this variability is rarely accounted for in heterogeneous ice-nucleation ...parameterizations. In this paper, we present an approach to incorporate the random variation in the INPC into the parameterization of immersion freezing and analyze this novel concept with various sensitivity tests. In the new scheme, the INPC is drawn from a relative frequency distribution of cumulative INPCs. At each temperature, this distribution describing the INPCs is expressed as a lognormal frequency distribution. The new parameterization scheme does not require aerosol information from the driving model to represent the heterogeneity of INPCs. The scheme's performance is tested in a large-eddy simulation of a relatively warm Arctic mixed-phase stratocumulus. We find that it leads to reasonable ice masses in the cloud, especially when compared to immersion freezing schemes that yield one fixed INPC per temperature and lead to almost no ice production in the simulated cloud. The scheme is sensitive to the median of the frequency distribution and highly sensitive to the standard deviation of the distribution, as well as to the frequency of drawing a new INPC and the resolution of the model. Generally, a higher probability of drawing large INPCs leads to substantially more ice in the simulated cloud. We expose inherent challenges to introducing such a parameterization and explore possible solutions and potential developments.
This paper presents experimental values for the coefficient of restitution (en) for millimeter-sized ice particles colliding with massive walls at different temperatures. Three different wall ...materials are tested: hardened glass, ice and Acrylonitrile butadiene styrene (ABS) polymer. The results show a high sensitivity to impact velocity Vi, where en decreases rapidly with increasing Vi. The results also show a decrease in en with increasing temperature T. A novel model that predicts en based on the assumption of collisional melting and viscous damping caused by an increased premelted liquid-layer, is proposed. The model predicts both the velocity and the temperature trends seen in the experiments. The difference obtained in experiments between wall materials is also captured by the new model. A generalized regime map for ice particle collisions is proposed to combine the new model with previous work.
Display omitted
•Experimental results for ice particles upon collision with massive walls.•Novel model for collisional damping due to a premelted liquid layer on ice.•General regime map for the coefficient of restitution of ice particles.
Ice particle activation and evolution have important atmospheric implications for cloud formation, initiation of precipitation and radiative interactions. The initial formation of atmospheric ice by ...heterogeneous ice nucleation requires the presence of a nucleating seed, an ice-nucleating particle (INP), to facilitate its first emergence. Unfortunately, only a few long-term measurements of INPs exist, and as a result, knowledge about geographic and seasonal variations of INP concentrations is sparse. Here we present data from nearly 2 years of INP measurements from four stations in different regions of the world: the Amazon (Brazil), the Caribbean (Martinique), central Europe (Germany) and the Arctic (Svalbard). The sites feature diverse geographical climates and ecosystems that are associated with dissimilar transport patterns, aerosol characteristics and levels of anthropogenic impact (ranging from near pristine to mostly rural). Interestingly, observed INP concentrations, which represent measurements in the deposition and condensation freezing modes, do not differ greatly from site to site but usually fall well within the same order of magnitude. Moreover, short-term variability overwhelms all long-term trends and/or seasonality in the INP concentration at all locations. An analysis of the frequency distributions of INP concentrations suggests that INPs tend to be well mixed and reflective of large-scale air mass movements. No universal physical or chemical parameter could be identified to be a causal link driving INP climatology, highlighting the complex nature of the ice nucleation process. Amazonian INP concentrations were mostly unaffected by the biomass burning season, even though aerosol concentrations increase by a factor of 10 from the wet to dry season. Caribbean INPs were positively correlated to parameters related to transported mineral dust, which is known to increase during the Northern Hemisphere summer. A wind sector analysis revealed the absence of an anthropogenic impact on average INP concentrations at the site in central Europe. Likewise, no Arctic haze influence was observed on INPs at the Arctic site, where low concentrations were generally measured. We consider the collected data to be a unique resource for the community that illustrates some of the challenges and knowledge gaps of the field in general, while specifically highlighting the need for more long-term observations of INPs worldwide.
The particle–gas interface in aerosol systems is of essential importance because it is here that many key atmospheric processes occur. In this study, we employ ambient pressure X-ray photoelectron ...spectroscopy (APXPS) to investigate the surface properties and processes of an atmospherically relevant carboxylic salt, sodium acetate, at subdeliquescence conditions. From the depth profiles of the elemental ratios of sodium, oxygen, and carbon, we find that after deliquescence–efflorescence cycles the salt surface is sodium-depleted. The mechanism of the observed depletion is proposed to be (i) the formation of neutral acetic acid in the solution due to the nature of the basic salt; (ii) the selective surface enhancement of neutral molecules under aqueous condition; and (iii) a hypothetical kinetic barrier to re-homogenization due to spatial separation and special local conditions on the surface, resulting in varied local surface composition. When the relative humidity gradually increases and approaches the deliquescence point, both reversible water uptake and reversible surface dissociation are confirmed by near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge and sodium K-edge, respectively. The solvation of sodium requires a higher relative humidity than needed for water adsorption, which suggests that water molecules are taken up by the surface, but the solvation of the salt surface begins only when sufficient water molecules are present, to facilitate the process. The sodium-depleted surface requires additional adsorbed water to affect and dissolve the sodium ions in deeper regions.
The angle of repose is a measure reflecting the internal friction and cohesion properties of a granular material. In this paper, we present an experimental setup and measurements for the angle of ...repose of snow for seven different snow samples over a large range of temperatures. The results show that the angle of repose is dependent on the fall height, the temperature, and the grain size of the snow. These variables are quantified, and their interdependencies are separately studied. With increased snow temperature, the angle of repose increases, and this can be explained by the presence of a liquid layer on ice that can be thermodynamically stable at temperatures below the melting point of water. With decreasing grain size the angle of repose also increases which is expected since the cohesive energy decreases more slowly than the grain mass. For increasing fall height, the snow grains generally accelerate to larger collisional velocities, yielding a smaller angle of repose. In general, the dimensionless cohesion number was found to largely reflect the dependencies of the variables and is therefore useful for understanding what affects the angle of repose. The results demonstrate that the drag force and collision dynamics of ice grains are important for understanding how snow accumulates on a surface, for example if one desires predicting snow accretion by simulating a dispersed cloud of snow.
Display omitted
•Experimental results for the angle of repose of snow with seven different snow samples.•Temperature dependency was concluded to follow an Arrhenius relationship.•Fall height was shown to be an important experimental parameter.•A size dependency was established where smaller grains yield higher angles.•The cohesion number captures the nonlinear dependencies observed.
Hydrogen peroxide is a primary atmospheric oxidant significant in terminating gas-phase chemistry and sulfate formation in the condensed phase. Laboratory experiments have shown an unexpected ...oxidation acceleration by hydrogen peroxide in grain boundaries. While grain boundaries are frequent in natural snow and ice and are known to host impurities, it remains unclear how and to which extent hydrogen peroxide enters this reservoir. We present the first experimental evidence for the diffusive uptake of hydrogen peroxide into grain boundaries directly from the gas phase. We have machined a novel flow reactor system featuring a drilled ice flow tube that allows us to discern the effect of the ice grain boundary content on the uptake. Further, adsorption to the ice surface for temperatures from 235 to 258 K was quantified. Disentangling the contribution of these two uptake processes shows that the transfer of hydrogen peroxide from the atmosphere to snow at temperatures relevant to polar environments is considerably more pronounced than previously thought. Further, diffusive uptake to grain boundaries appears to be a novel mechanism for non-acidic trace gases to fill the highly reactive impurity reservoirs in snow’s grain boundaries.
PDF
