Gas-Phase Flow and Diffusion Analysis of the Droplet-Train/Flow-Reactor Technique for the Mass-Accommodation Processes

Heterogeneous Uptake of 8−2 Fluorotelomer Alcohol on Liquid Water and 1-Octanol Droplets

The heterogeneous uptake of the 8-2 fluorotelomer alcohol, F(CF2)8CH2CH2OH, on liquid water surfaces over the temperature range 256-273 K and on 1-octanol surfaces over the temperature range 264-295 K has been investigated with a droplet train flow reactor. The uptake coefficient on water droplets is zero within the error of the measurement (+/-0.01) and is independent of droplet temperature. In contrast, significant uptake onto 1-octanol is observed. Measured uptake coefficients for 1-octanol showed a negative temperature dependence, varying from 0.034 +/- 0.005 (1sigma) at 295 K to 0.103 +/- 0.009 at 264 K. The measured uptake coefficients on 1-octanol were independent of gas-liquid contact time, for typical contact times varying between 3 and 15 ms, and independent of the 8-2 fluorotelomer alcohol gas-phase concentration, indicating that the uptake coefficients are equivalent to mass accommodation coefficients. The uptake coefficients on 1-octanol were also independent of relative humidity. These results show that the uptake of FTOHs on or into the aqueous component of cloud/fog droplets or aqueous aerosol particles is not likely to be an important atmospheric sink for these compounds. In these experiments, 1-octanol was used as a model compound for organic-containing atmospheric particles. The larger uptake coefficient measured for 1-octanol surfaces indicates that FTOH partitioning to organic-containing cloud/fog droplets and aerosol particles may be an atmospheric loss mechanism.

Gas-Phase Molecular Halogen Formation from NaCl and NaBr Aerosols: When Are Interface Reactions Important?

Unique interface reactions at the surface of sea-salt particles have been suggested as an important source of photolyzable gas-phase halogen species in the troposphere. Many factors influence the relative importance of interface chemistry compared to aqueous-phase chemistry. The Model of Aerosol, Gas, and Interfacial Chemistry (MAGIC 2.0) is used to study the influence of interface reactions on gas-phase molecular halogen production from pure NaCl and NaBr aerosols. The main focus is to identify the relative importance of bulk compared to interface chemistry and to determine when interface chemistry dominates. Results show that the interface process involving Cl-(surf) and OH(g) is the main source of Cl2(g). For the analogous oxidation of bromide by OH, gaseous Br2 is formed mainly in the bulk aqueous phase and transferred across the interface. However, the reaction of Br-(surf) with O3(g) at the interface is the primary source of Br2(g) under dark conditions. The effect of aerosol size is also studied. Potential atmospheric implications and effects of interface processes on aerosol pH are discussed.

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

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.

Mass Accommodation of H2SO4 and CH3SO3H on Water−Sulfuric Acid Solutions from 6% to 97% RH

The uptake of H2SO4 and CH3SO3H onto particles composed of water and sulfuric acid was studied in a laminar flow reactor at atmospheric pressure. Their first-order gas-phase loss rate coefficients were determined using a chemical ionization mass spectrometer. Relative humidity was varied from 6% to 97% at 295-297.5 K. The mass accommodation coefficient, alpha, was found to be close to unity for both species. These findings show that alpha does not limit particle growth rates resulting from H2SO4 and CH3SO3H uptake. Diffusion coefficients in N2 for these two species are also reported and a significant dependence upon relative humidity was seen for H2SO4 but not for CH3SO3H. Last, production of small particles was observed due to the presence of SO2 in particle chargers. Formation of these particles can be significantly reduced by adding an OH scavenger such as propane.

Hydroxyl radical at the air-water interface

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 (DeltaGa) being about 1 kcal/mol more negative than the hydration free energy (DeltaGs). 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.

MOLECULAR BEAM STUDIES OF GAS-LIQUID INTERFACES

Molecular beam scattering experiments provide a way to disentangle the elementary steps involved in energy transfer and chemical reactions between gases and liquids. After surveying the history and recent progress in this field, we review studies of the kinematics of gas-liquid collisions and proton exchange of HCl, DCl, and HBr with supercooled sulfuric acid and liquid glycerol. These experiments help to clarify the role of the surface region in controlling trapping and interfacial- and bulk-phase reactions.