Heterogeneous Atmospheric Chemistry of Alternative Halocarbon Oxidation Intermediates

Abstract

Alternative halocarbons designed to substitute for chlorofluorocarbons and halons banned by the Montreal Protocol are designed to undergo oxidative degradation in the troposphere.Intermediate partially oxygenated products from currently used alternatives include carbonyl halides, haloacetyl halides and haloacetic acids. These intermediates are expected to undergo heterogeneous reactions with aqueous cloud droplets and surface waters. The chemical and physical parameters which govern the heterogeneous uptake of a range of these species by liquid water have been measured in our laboratories using droplet train/flow tube and/or bubble column techniques. The results of these experiments will be presented and compared with available results from other laboratories. Their impact on our ability to predict the atmospheric fate of halocarbon oxidation intermediates will be discussed. Atmospheric Fate of Alternative Halocarbon Oxidation ProductsAlternative halocarbons planned as substitutes for chlorofluorocarbons and halons banned by the Montreal Protocol are designed to undergo oxidative degradation in the troposphere. Intermediate partially oxygenated products from currently used or proposed alternatives include carbonyl halides, haloacetyl halides and haloacetic acids. A wide variety of atmospheric processes could participate in the removal of relatively stable degradation intermediates, including: transport to the stratosphere followed by photolysis, uptake by both tropospheric and stratospheric aerosols, uptake by cloud droplets, rainout (wet deposition), and dry deposition to vegetation, soil and dew surfaces, and to the oceans. However, recent model analyses indicate that cloud droplet and ocean uptake are the most likely removal rate determining processes. The physicochemical processes which are important in determining the rate of trace atmospheric gas uptake by liquid surfaces include gas phase diffusion, mass accommodation, solvation (Henry's law solubility), liquid phase diffusion and liquid phase reaction. The most effective removal paths and their associated physicochemical parameters are displayed in Table I. Key parameters affecting atmospheric removal rates are the mass accommodation coefficient, a, the Henry's law constant, H, and the first order hydrolysis reaction rate coefficient, k hyd .

Oxidation Mechanisms of CF2Br2and CH2Br2Induced by Air Nonthermal Plasma

“…Its removal in the atmosphere is thus attributed to reaction with condensed atmospheric water . In the condensed phase (aqueous solution or rain droplets) hydrolysis is very fast (t 1/2 = 0.16 s) . The homogeneous gas-phase hydrolysis reaction has been examined in theoretical investigations which have shown that the reaction proceeds in a stepwise manner and that some of these steps have a high activation energy (126 – 151 kJ·mol –1 ). , normalF 2 normalC = normalO + normalH 2 normalO → CO 2 + 2 HF These barriers are however drastically reduced when one additional water molecule is considered in the calculations (it stabilizes and lowers the energy of some of the transition states acting as a proton relay), making the hydrolysis extremely fast .…”