Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of ...knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air-ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed.
Molecular Simulations of Halomethanes at the Air/Ice Interface Habartová, A; Hormain, L; Pluhařová, E ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
10/2015, Letnik:
119, Številka:
39
Journal Article
Recenzirano
Halogenated organics are emitted into the atmosphere from a variety of sources of both natural and anthropogenic origin. Their uptake at the surface of aerosols can affect their reactivity, for ...example, in processes that take part in ozone destruction due to production of reactive chlorine, bromine, and iodine radicals. Classical molecular dynamics (MD) simulations are carried out to investigate the interaction of small halomethane molecules of atmospheric relevance with a crystalline ice surface. The following halomethanes were studied: CH3Cl, CH2Cl2, CHCl3, CH3Br, CH2Br2, and CHBr3. MD simulations provide an invaluable insight into the adsorption behavior of halomethanes species. The adsorption energy is increasing as the number of halogen atoms is increasing. Moreover, brominated methanes exhibit a stronger interaction with the ice than their chlorinated analogs. Implications for the atmospheric chemistry are discussed.
The reaction of ozone with aqueous sodium bromide particles is investigated with a combination of aerosol chamber experiments, kinetics modeling, and molecular dynamics simulations. The molecular ...bromine production in the chamber experiments is approximately an order of magnitude greater than that predicted by known chemistry in the gas and bulk aqueous phases with use of a comprehensive computer kinetics model. Molecular dynamics simulations indicate that ozone has significant residence time at the air−solution interface, while making frequent contacts with bromide ions for as long as 50 ps in the surface layer of a 6.1 M NaBr solution. The formation of a complex between ozone and bromide ion, O3···Br-, which can lead to production of Br2 by reaction at the air−water interface, is therefore feasible. Experimentally observed Br2 is well predicted by including an interface process with a reaction probability of 1.9 ± 0.8 × 10-6 (1 s) as the first step in a surface mechanism to produce additional gas-phase Br2. An estimate of the impact of this interface reaction on bromine formation in the marine boundary layer shows that several ppt of bromine could potentially be produced during the night from this proposed surface chemistry.
A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular ...dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass accommodation (α) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated toward the liquid surface when they are approximately 5 Å above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 to 140 ps, and the values of α for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atmospheric chemistry are discussed.
Collisions of the gaseous hydroxyl (OH) radical and ozone (O3) with the surfaces of sodium chloride or iodide solutions, as well as with the surface of neat water, were investigated by molecular ...dynamics simulations. The principal intent was to answer atmospherically relevant questions concerning the trapping and accommodation of the OH and O3 species at the surface and their uptake into the bulk solution. Although trapping is substantial for both species, OH adsorbs and absorbs significantly better than O3. Most of the trapped O3 molecules desorb from the surface within 50 ps, whereas a significant fraction of OH radicals remains at the interface for time intervals exceeding 100 ps. The aqueous surface has also an orientational effect on the OH species, favoring geometries with the H atom pointing toward the aqueous bulk. The effect of the dissolved salt on the trapping efficiency is minor; therefore, most likely, atomic ions solvated in aqueous aerosols do not act as strong scavengers of reactive gases in the atmosphere. However, frequent and relatively long contacts between the adsorbed molecules and halide anions do occur, allowing for heterogeneous atmospheric chemistry in the interfacial layer.
Hydroxyl Radical at the Air−Water Interface Roeselová, Martina; Vieceli, John; Dang, Liem X ...
Journal of the American Chemical Society,
12/2004, Letnik:
126, Številka:
50
Journal Article
Recenzirano
Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass ...accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air−water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (ΔG a) being about 1 kcal/mol more negative than the hydration free energy (ΔG s). The propensity of the hydroxyl radical for the air−water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.
We use molecular dynamics simulations to determine the melting point of ice Ih for the polarizable POL3 water force field (Dang, L. X. J. Chem. Phys.1992, 97, 2659). Simulations are performed on a ...slab of ice Ih with two free surfaces at several different temperatures. The analysis of the time evolution of the total energy in the course of the simulations at the set of temperatures yields the melting point of the POL3 model to be Tm = 180 ± 10 K. Moreover, the results of the simulations show that the degree of hydrogen-bond disorder occurring in the bulk of POL3 ice is larger (at the corresponding degree of undercooling) than in ice modeled by nonpolarizable water models. These results demonstrate that the POL3 water force field is rather a poor model for studying ice and ice−liquid or ice−vapor interfaces. While a number of polarizable water models have been developed over the past years, little is known about their performance in simulations of supercooled water and ice. This study thus highlights the need for testing of the existing polarizable water models over a broad range of temperatures, pressures, and phases, and developing a new polarizable water force field, reliable over larger areas of the phase diagram.
The accommodation of water vapor at the air/water interface is studied using classical molecular dynamics computer simulations. From 250 initial conditions of a gas phase water molecule approaching ...the surface of liquid water with a thermal impact velocity, the thermal and mass accommodation coefficients are determined to be 1.0 and 0.99, respectively, at 300 K. When the approaching water molecule is 5 Å above the liquid, it is accelerated towards the surface, which is followed by a slight rotational excitation upon impact. Within 4 ps of striking the surface, the total kinetic energy is equilibrated and the equilibrium number of water molecules in the first and second solvent shells is obtained.
The constrained molecular-dynamics technique was employed to investigate the transport of a nitrate ion across the water liquid/vapor interface. We developed a nitrate-ion-water polarizable potential ...that accurately reproduces the solvation properties of the hydrated nitrate ion. The computed free-energy profile for the transfer of the nitrate ion across the air/water interface increases monotonically as the nitrate ion approaches the Gibbs dividing surface from the bulk liquid side. The computed density profiles of 1M KNO(3) salt solution indicate that the nitrate and potassium ions are both found below the aqueous interface. Upon analyzing the results, we conclude that the probability of finding the nitrate anion at the aqueous interface is quite small.
Picosecond dynamics initiated by electron photodetachment in Cl-···H2O and Br-···H2O complexes is explored using classical Wigner trajectories which correctly map the initial quantum vibrational ...state of the systems. The three lowest potential energy surfaces of the neutral clusters reached after electron photodetachment are constructed by the ab initio Fock-Space Coupled Cluster Method and then quantitatively fitted to a Diatomics-in-Molecule Model which also allows for a simple inclusion of spin−orbit interactions. Because of large differences between the shapes of the anionic and neutral potential energy surfaces, and due to the presence of light hydrogen atoms, an unusual dynamical behavior is observed. Although the vertical photodetachment process places the systems above the dissociation threshold, the clusters do not dissociate directly. Instead, relatively long-lived vibrational resonances are observed. This is due to a strong excitation of the nondissociative (internal) water rotation and only a weak excitation of the dissociative intermolecular stretch upon photodetachment. Implications of the observed dynamics for the interpretation of experimental vibrationally resolved ZEKE spectra are discussed.
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