Showing papers by "A. R. Ravishankara published in 1994"

On the role of iodine in ozone depletion

Abstract: Ozone depletions in the lower stratosphere outside of polar regions are difficult to explain using only local chlorine and bromine chemistry. We speculate that iodine chemistry in combination with trends in anthropogenic chlorine and bromine may also be a factor in determining the widespread current depletion of lower stratospheric ozone. We also speculate on a related role for iodine in the sudden springtime surface ozone loss observed in the Arctic.

Heterogeneous reactions in sulfuric acid aerosols: A framework for model calculations

Abstract: A framework for applying rates of heterogeneous chemical reactions measured in the laboratory to small sulfuric acid aerosols found in the stratosphere is presented. The procedure for calculating the applicable reactive uptake coefficients using laboratory-measured parameters is developed, the necessary laboratory-measured quantities are discussed, and a set of equations for use in models are presented. This approach is demonstrated to be essential for obtaining uptake coefficients for the HOCl + HCl and ClONO2 + HCl reactions applicable to the stratosphere. In these cases the laboratory-measured uptake coefficients have to be substantially corrected for the small size of the atmospheric aerosol droplets. The measured uptake coefficients for N2O5 + H2O and ClONO2 + H2O as well as those for other heterogeneous reactions are discussed in the context of this model. Finally, the derived uptake coefficients were incorporated in a two-dimensional dynamical and photochemical model thus for the first time the HCl reactions in sulfuric acid have been included. Substantial direct chlorine activation and consequent ozone destruction is shown to occur due to heterogeneous reactions involving HCl for volcanically perturbed aerosol conditions at high latitudes. Smaller but significant chlorine activation also is predicted for background aerosol loadings at extreme high latitudes, suggesting chlorine activation can occur on background sulfuric acid aerosol in these regions. The coupling between homogeneous and heterogeneous chemistry is shown to lead to important changes in the concentrations of various reactive species. The basic physical and chemical quantities needed to better constrain the model input parameters are identified.

Atmospheric lifetime, its application and its determination: CFC-substitutes as a case study

Abstract: The concept of atmospheric lifetime, its application in atmospheric chemistry, and its use in defining environmental acceptability indices such as the ozone depletion potential and the global warming potential are described. The determination of the atmospheric lifetime from laboratory measured chemical kinetic and photochemical parameters is highlighted. A brief description of the laboratory methods used to determine kinetic parameters and the difficulties encountered in measuring them are given. In all these descriptions and discussions, chlorofluorocarbons (CFCs) and their substitutes are used as examples. The environmental acceptability of the currently proposed CFC substitutes, the hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are discussed. Lastly, the question is raised: Should atmospheric lifetime be used as an index of acceptability?

Kinetics of the reactions of CF3Ox radicals with NO, O3, and O2

Abstract: The technique of pulsed laser photolysis/pulsed laser-induced fluorescence detection of CF[sub 3]O was used to study the stratospherically important reactions of CF[sub 3]O radicals with NO (k[sub 5]), O[sub 3] (k[sub 4]), and O[sub 2] (k[sub 6]) and that of CF[sub 3]-OO with NO (k[sub 2]) and O[sub 3](k[sub 3]). Over the temperature range 233-360, K, k[sub 5](T) = (3.34 [+-] 0.68) x 10[sup [minus]11] exp[(160 [+-] 45)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1]. At 298 K, k[sub 2] = (1.57 [+-] 0.38) x 10[sup [minus]11] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1] in agreement with past work. The reactions of CF[sub 3]O and CF[sub 3]OO with O[sub 3] were found to be slow with rate coefficients at 298 K of k[sub 4] = (2.5[sub [minus]1.5][sup 0.7]) x 10[sup [minus]14] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1] and k[sub 3] [le] 7 x 10[sup [minus]15] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], respectively. No reaction of CF[sub 3]O with O[sub 2] was observed at 298 or 373 K, leading to an upper limit of k[sub 6] [le] 4 x 10[sup [minus]17] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1] at 373 K. The implications of these results to the chemistry of CF[sub 3]O[sub x] radicalsmore » and O[sub 3] in the stratosphere are discussed. 32 refs., 7 figs., 1 tab.« less

Kinetics of the Reactions of the CF3O Radical with Alkanes

Abstract: Utilizing the technique of pulsed laser photolysis/pulsed laser induced fluorescence, we have investigated the atmospherically important reactions of the (trifluoromethyl)peroxy radical, CF[sub 3]O, with several alkanes. The reaction rate coefficients for CF[sub 3]O + CH[sub 4] (k[sub 3]), C[sub 2]H[sub 6] (k[sub 4]), C[sub 3]H[sub 8] (k[sub 5]), (CH[sub 3])[sub 3]CH (k[sub 6]), and CD[sub 4] (k[sub 7]) were measured, as functions of temperature, to be k[sub 3] = (1.92 [+-] 0.33) x 10[sup [minus]12] exp[[minus](1370 [+-] 85)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], k[sub 4] = (4.84 [+-] 1.11) x 10[sup [minus]12] exp[-(400 [+-] 70)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], k[sub 5] = (5.12 [+-] 1.12) x 10[sup [minus]12] exp[-(15 [+-] 30)/T] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], k[sub 6] = (4.32 [+-] 0.42) x 10[sup [minus]12] exp[(135 [+-] 30)/T] cm[sup 3] molecule[sup 1] s[sup [minus]1], and k[sub 7] = (0.91 [+-] 0.31) x 10[sup [minus]12] exp[-(1540 [+-] 100)] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1], respectively. These kinetic data are compared with results from previous studies. The atmospheric implications of these findings are discussed. 28 refs., 7 figs., 4 tabs.