Model of Uptake of OH Radicals on Nonreactive Solids

Abstract

A model of adsorption and recombination of OH radicals was developed for nonreactive solid surfaces of atmospheric interest. A parametrization of this heterogeneous mechanism was carried out to determine the role of the catalytic properties of these solid surfaces, taking into account the adsorption energy, defects, surface diffusion, and chemical reactions in the gas-solid interface. The uptake process was simulated for diffusion-controlled chemical reactions on the surface on the basis of Langmuir-Hinshelwood and Eley-Rideal mechanisms. Using an analytical approach and the Monte Carlo technique, we show the dependencies of the uptake probability of the heterogeneous reactions on the OH concentration and adsorption energy. The model is employed in the analysis of the empirically derived uptake coefficient for water ice, Al(2)O(3), NaCl, NH(4)NO(3), NH(4)HSO(4), and (NH(4))(2)SO(4). We found the following values for the free energy of adsorption of OH radicals: E(ice) = 7.3-7.6 kcal/mol, E(Al)(2)(O)(3) = 11-11.7 kcal/mol, E(NH)(4)(NO)(3) = 10.2 kcal/mol, E(NaCl) = 10.2 kcal/mol, E(NH)(4)(HSO)(4) = 9.8 kcal/mol, and E((NH)(4))(2)(SO)(4) = 9.8 kcal/mol. The atmospheric implications of the catalytic reactions of OH with adsorbed reactive molecules are discussed. The results of the modeling of the uptake process showed that the heterogeneous decay rate can exceed the corresponding gas-phase reaction rate under atmospheric conditions.

Atom Tunneling in the Water Formation Reaction H2 + OH → H2O + H on an Ice Surface

“…The binding energy of this site is 32.1 kJ mol –1 (3860 K), lower than for the hollow and bridged binding sites. One of these values lies nicely in the range of experimentally determined desorption energies on silicate surfaces of 1656–4760 K and the desorption energies on water ice of 3670–3820 K …”

A New Approach to Determining Gas-Particle Reaction Probabilities and Application to the Heterogeneous Reaction of Deliquesced Sodium Chloride Particles with Gas-Phase Hydroxyl Radicals

“…Lower limits to the corresponding values of k II can be obtained from these values of a if it is assumed that k s ) 0, i.e., that there is no removal of OH by the substrate. Remerov and Bardwell 81 have developed a model for the uptake of OH on nonreactive surfaces where the OH is removed by reaction with a second OH on the surface. However, without knowledge of the energetics of interaction of OH with the TEM grid substrate material, we were not able to apply this model directly to calculate the loss of OH on the substrate surrounding the NaCl particles.…”

Kinetic and Product Yield Study of the Heterogeneous Gas−Surface Reaction of Anthracene and Ozone

“…Although the focus of past kinetic studies of this type has been on using the K O 3 to explain the partitioning of ozone to different substrates to account for the variability in the observed kinetics, these studies can also provide information about whether these surface reactions proceed in a diffusion-controlled manner or not. A study by Remorov and Bardwell modeled the recombination of OH on the surface of nonreactive salts using the Langmuir−Hinshelwood reaction mechanism. They suggest that the diffusion-limited rate constant ( k d ) is equal to approximately 10 -5 cm 2 s -1 .…”

“…We arrive at this range on the basis of the experimental values determined using eq 2 and assuming that the number of surface sites for ozone is between 10 10 and 10 14 molecules cm -2 . An upper limit for this reaction ( k upper ) can also be estimated using the known rate constants for the corresponding gas-phase reaction and the layer thickness …”