Molecular Dynamics Simulations of Atmospheric Oxidants at the Air−Water Interface:  Solvation and Accommodation of OH and O₃

Abstract

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 (alpha) 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 angstroms 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 alpha 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.

Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface

“…Indeed, the comparison between the MD-computed and the experimental ∆G * is widely used to demonstrate the reliability of the interaction parameter used to describe physical and chemical systems in classical MD, especially for accurate modeling of gas uptake. 16,17,37,38,43,56 It is of interest to note that, even if for all the three water models we have a reasonable description of the free energy of hydration, there are some differences in the intensity of the surface minima and for the free energy cost of transferring one MA from the interface to the gas; these differences suggest that ∆G * could not be (in general) the only target for force field benchmark in MD simulations of gas uptake on liquid water. In Secs.…”

“…We determine four possible events for a molecule arriving at the surface, which are also schematically reported in Figure 2: scattering, adsorption, absorption, and desorption. The definitions above were inspired by similar studies of Veiceli et al 43 and Julin et al, 44 however, our criteria do present some notable differences, which are discussed in detail in the supplementary material. 84 There are two possible definitions of the mass accommodation coefficients.…”

“…We note that in this case the residence time can range broadly from 24 ps to 866 ps; this finding comes in spite of the fact that we employed both a larger number of simulations and longer timed simulations compared to similar studies of gas molecule uptakes present in the literature. [42][43][44] The statistics obtained for the residence time are insufficient to properly sketch a population distribution, and are therefore insufficient to draw any reliable conclusions concerning the average time of surface residence. Nevertheless, these results provide a range of values that can be used to better constrain nonreactive uptake coefficient in chemical transport models.…”

Technical note: Monte Carlo genetic algorithm (MCGA) for model analysis of multiphase chemical kinetics to determine transport and reaction rate coefficients using multiple experimental data sets

“…the upper limit of the accommodation coef- (2016) OH formation by secondary organic aerosol decomposition in water Lakey et al (2016a) Reactive oxygen species and PM 2.5 in lung lining fluid Lakey et al (2016b) Skin lipid (squalene) + O 3 ficient at unity), or by simulations (e.g. molecular dynamics simulations to estimate the surface accommodation coefficient and desorption lifetime as in Vieceli et al (2005) and Julin et al (2013)). Note that in the example given in Fig.…”

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

“…Note however that for a relevant surface reaction in kinetic flux models, it is necessary to use an effective desorption lifetime, τ d , in the millisecond to second time range Shiraiwa et al, 2010). This is many orders of magnitude longer than would be expected due to pure physisorption as estimated by molecular dynamic simulations (Vieceli et al, 2005), indicating that the adsorption process should involve chemisorption or formation of long-lived intermediates that would have the potential to extend these effective desorption lifetimes (Shiraiwa et al, 2011a;Berkemeier et al, 2016). Surface reactions in viscous aqueous organics are consistent with previous work by Gržinić et al (2015), , and Berkemeier et al (2016) for the uptake of N 2 O 5 to citric acid and the uptake of O 3 to shikimic acid over a range of relative humidities.…”