Null cycle

Abstract: A latitudinally averaged two-dimensional model has been used to study the distributions, budgets, and trends of trace gases in the atmosphere from pole to pole and from the surface to 24 km. The chemical mechanism used contains 56 chemical species, including 12 hydrocarbons and 125 chemical and photochemical reactions, as well as wet removal processes and dry deposition. Apart from the stratospheric sources of ozone and nitrogen oxides the model chemistry is driven completely by the time-dependent photolytic processes and the emission of 17 chemical species distributed according to 10 different source categories. Each source category is parameterized as a function of latitude and time of the year. The model results are generally in good agreement with observations, and the model reproduces the observed temporal and spatial variation in the mixing ratios of methane, carbon monoxide, hydrocarbons, and ozone. The global average concentration of the hydroxyl radical in the troposphere is 8.3×105 molecules cm−3, in agreement with recent calculations based on budgets and trends for CH3CCl3. Model budgets for NOx, CO, CH4, H2 and O3 are presented. These show that although the stratospheric source and dry deposition terms for ozone almost balance, the global annual turnover of ozone below 24 km, excluding the O3/NO/NO2 null cycle, is four times greater than the stratospheric source strength. These budgets also suggest that the observed increase in the mixing ratio of methane may be due not only to an increasing source strength but also to a downward perturbation in the abundance of the hydroxyl radical.

Abstract: Three simultaneous observations of atomic chlorine (Cl) and the chlorine monoxide radical (ClO) are reported which encompass the altitude interval between 25 and 45 kilometers. Together, Cl and ClO form a gas-phase catalytic cycle potentially capable of depleting stratospheric ozone. Observed Cl and C1O densities, although variable, imply that chlorine compounds constitute an important part of the stratospheric ozone budget. The results are compared with recent models of stratospheric photochemistry which have been used as a basis for predicting ozone depletion resulting from fluorocarbon release.

Abstract: . The role of halogen species (e.g., Br, Cl) in the troposphere of polar regions has been investigated since the discovery of their importance for boundary layer ozone destruction in the polar spring about 25 years ago. Halogen species take part in an auto-catalytic chemical reaction cycle, which releases Br2 and BrCl from the sea salt aerosols, fresh sea ice or snowpack, leading to ozone depletion. In this study, three different chemical reaction schemes are investigated: a bromine-only reaction scheme, which then is subsequently extended to include nitrogen-containing compounds and chlorine species and corresponding chemical reactions. The importance of specific reactions and their rate constants is identified by a sensitivity analysis. The heterogeneous reaction rates are parameterized by considering the aerodynamic resistance, a reactive surface ratio, β, i.e., the ratio of reactive surface area to total ground surface area, and the boundary layer height, Lmix. It is found that for β = 1, a substantial ozone decrease occurs after five days and ozone depletion lasts for 40 h for Lmix = 200 m. For about β ≥ 20, the time required for major ozone depletion ([O3] In polar regions, a small amount of NOx may exist, which stems from nitrate contained in the snow, and may have a strong impact on the ozone depletion. Therefore, the role of nitrogen-containing species on the ozone depletion rate is studied. The results show that the NOx concentrations are influenced by different chemical reactions over different time periods. During ozone depletion, the reaction cycle involving the BrONO2 hydrolysis is dominant. A critical value of 0.0004 of the uptake coefficient of the BrONO2 hydrolysis reaction at the aerosol and saline surfaces is identified, beyond which the existence of NOx species accelerates the ozone depletion event, whereas for lower values, deceleration occurs.

Abstract: The recognition of the roles of chlorine and bromine compounds in ozone depletion has led to the regulation or their source gases. Some source gases are expected to be more damaging to the ozone layer than others, so that scientific guidance regarding their relative impacts is needed for regulatory purposes. Parameters used for this purpose include the steady-state and time-dependent chlorine loading potential (CLP) and the ozone depletion potential (ODP). Chlorine loading potentials depend upon the estimated value and accuracy of atmospheric lifetimes and are subject to significant (approximately 20-50 percent) uncertainties for many gases. Ozone depletion potentials depend on the same factors, as well as the evaluation of the release of reactive chlorine and bromine from each source gas and corresponding ozone destruction within the stratosphere.