Photochemical modeling of the Antarctic stratosphere: Observational constraints from the airborne Antarctic ozone experiment and implications for ozone behavior

The rapid decrease in O3 column densities observed during Antarctic spring has been attributed to several chemical mechanisms involving nitrogen, bromine, or chlorine species, to dynamical mechanisms, or to a combination of the above. Chlorine-related theories, in particular, predict greatly elevated concentrations of ClO and OClO and suppressed abundances of NO2 below 22 km. The heterogeneous reactions and phase transitions proposed by these theories could also impact the concentrations of HCl, ClNO3 and HNO3 in this region. Observations of the above species have been carried out from the ground by the National Ozone Expedition (NOZE-I, 1986, and NOZE-II, 1987), and from aircrafts by the Airborne Antarctic Ozone Experiment (AAOE) during the austral spring of 1987. Observations of aerosol concentrations, size distribution and backscattering ratio from AAOE, and of aerosol extinction coefficients from the SAM-II satellite can also be used to deduce the altitude and temporal behavior of surfaces which catalyze heterogeneous mechanisms. All these observations provide important constraints on the photochemical processes suggested for the spring Antarctic stratosphere. Results are presented for the concentrations and time development of key trace gases in the Antarctic stratosphere, utilizing the AER photochemical model. This model includes complete gas-phase photochemistry, as well as heterogeneous reactions. Heterogeneous chemistry is parameterized in terms of surface concentrations of aerosols, collision frequencies between gas molecules and aerosol surfaces, concentrations of HCl/H2O in the frozen particles, and probability of reaction per collision (gamma). Values of gamma are taken from the latest laboratory measurements. The heterogeneous chemistry and phase transitions are assumed to occur between 12 and 22 km. The behavior of trace species at higher altitudes is calculated by the AER 2-D model without heterogeneous chemistry. Calculations are performed for solar illumination conditions typical of 60, 70, and 80 S, from July 15 to October 31

Photochemistry of the Venus Atmosphere

Carbon monoxide, produced in the Venus atmosphere by photolysis of CO_2, is removed mainly by reaction with OH. The radical OH is formed in part by photolysis of H_2O_2, in part by reaction of O with HO_2. Photolysis of HCl provides a major source of H radicals near the visible clouds of Venus and plays a major role in the overall photochemistry. The mixing ratio of O_2 is estimated to be approximately 10^(−7), about a factor of 10 less than a recent observational upper limit reported by Traub and Carleton. A detailed model, which accounts for the photochemical stability of Venus CO_2, is presented and discussed

Antarctic ozone decrease: Possible impact on the seasonal and latitudinal distribution of total ozone as simulated by a 2-D model

Satellite borne instruments, the Total Ozone Mapping Spectrometer (TOMS) and the Solar Backscatter Ultraviolet spectrometer (SBUV), show that total column ozone has decreased by more than 5 percent in the neighborhood of 60 S at all seasons since 1979. This is considerably larger than the decrease calculated by 2-D models which take into account solar flux variation and increases of trace gas concentrations over the same period. The meteorological conditions (warmer temperature and the apparent lack of polar stratospheric clouds) at these latitudes do not seem to favor heterogeneous chemistry as the direct cause for the observed ozone reduction. A mechanism involving the seasonal transport of ozone-poor air mass from within the polar vortex to lower latitudes (the so-called dilution effect) is proposed as a possible explanation for the observed year-round ozone reduction in regions away from the vortex

Bromine-Chlorine Coupling in the Antarctic Ozone Hole

The contribution from the chlorine and bromine species in the formation of the Antarctic ozone hole is evaluated. Since chlorine and bromine compounds are of different industrial origin, it is desirable, from a policy point of view, to be able to attribute chlorine-catalyzed loss of ozone with those reactions directly involving chlorine species, and likewise for bromine-catalyzed loss. In the stratosphere, however, most of the chemical families are highly coupled, and, for example, changes in the chlorine abundance will alter the partitioninig in other families and thus the rate of ozone loss. This modeling study examines formation of the Antarctic ozone hole for a wide range of bromine concentrations (5 - 25 pptv) and for chlorine concentrations typical of the last two decades (1.5, 2.5 and 3.5 ppbv). We follow the photochemical evolution of a single parcel of air, typical of the inner Antarctic vortex (50 mbar, 70 deg. S, NO(sub y) = 2 ppbv, with Polar Stratospheric Clouds(PSC)) from August 1 to November 1. For all of these ranges of chlorine and bromine loading, we would predict a substantial ozone hole (local depletion greater than 90%) within the de-nitrified, PSC- perturbed vortex. The contributions of the different catalytic cycles responsible for ozone loss are tabulated. The deep minimum in ozone is driven primarily by the chlorine abundance. As bromine levels decrease, the magnitude of the chlorine-catalyzed ozone loss increases to take up the slack. This is because bromine suppresses ClO by accelerating the conversion of ClO an Cl2O2 back to HCI. For this range of conditions, the local relative efficiency of ozone destruction per bromine atom to that per chlorine atom (alpha-factor) ranges from 33 to 55, decreasing with increase of bromine

Ozone depletion potential of CH_3Br

The ozone depletion potential (ODP) of methyl bromide (CH_3Br) can be determined by combining the model‐calculated bromine efficiency factor (BEF) for CH_3Br and its atmospheric lifetime. This paper examines how changes in several key kinetic data affect BEF. The key reactions highlighted in this study include the reaction of BrO + HO_2, the absorption cross section of HOBr, the absorption cross section and the photolysis products of BrONO_2, and the heterogeneous conversion of BrONO_2 to HOBr and HNO_3 on aerosol particles. By combining the calculated BEF with the latest estimate of 0.7 year for the atmospheric lifetime of CH_3Br, the likely value of ODP for CH_3Br is 0.39. The model‐calculated concentration of HBr (∼0.3 pptv) in the lower stratosphere is substantially smaller than the reported measured value of about 1 pptv. Recent publications suggested models can reproduce the measured value if one assumes a yield for HBr from the reaction of BrO + OH or from the reaction of BrO + HO_2. Although the DeMore et al. [1997] evaluation concluded any substantial yield of HBr from BrO + HO_2 is unlikely, for completeness, we calculate the effects of these assumed yields on BEF for CH_3Br. Our calculations show that the effects are minimal: practically no impact for an assumed 1.3% yield of HBr from BrO + OH and 10% smaller for an assumed 0.6% yield from BrO + HO_2

Model documentation, chapter 4

The modeling groups are listed along with a brief description of the respective models

Better protection of the ozone layer

How can we extend the Montreal Protocol to other ozone-depleting chemicals, such as fuel from the Space Shuttle and pharmaceuticals, when the life cycles of these compounds and the scales of the industries are different? © 1994 Nature Publishing Group

Global warming from chlorofluorocarbons and their alternatives: Time scales of chemistry and climate

The halocarbons (chloroflurocarbons, CFCs, and their replacement chemicals: the hydrochloroflurocarbons, HCFCs, and the hydrofluorocarbons, HFCs) are greenhouse gases. The atmospheric accumulation of these gases is expected to add to the global warming predicted for expected increases of CO2, CH4, N2O, tropospheric ozone and H2O. Over the next decades, production of CFCs is scheduled to be phased out, while emissions of their alternatives are expected to increase. A simple model is used to illustrate the methodology for determining the time variations of the radiative forcing and temperature changes attributable to the direct greenhouse effect from potential emissions of the halocarbons. Although there are uncertainties associated with the lifetimes of the greenhouse gases, CFCs and their substitutes, the future growth rates of these gases, and the parameters used to simulate the response of the Earth-climate system, the method serves to illustrate an important aspect of the greenhouse warming issue beyond what is provided by the various greenhouse warming indices. Our results show that for likely substitution scenarios, the warming due to halocarbons will correspond to 4–10% of the total expected greenhouse warming at the year 2100. However, uncontrolled growth of the substitutes could result in an eight-fold increase in halocarbon production and a doubling of the halocarbon contribution by 2100

Global warming from chlorofluorocarbons and their alternatives: Time scales of chemistry and climate

The halocarbons (chloroflurocarbons, CFCs, and their replacement chemicals: the hydrochloroflurocarbons, HCFCs, and the hydrofluorocarbons, HFCs) are greenhouse gases. The atmospheric accumulation of these gases is expected to add to the global warming predicted for expected increases of CO2, CH4, N2O, tropospheric ozone and H2O. Over the next decades, production of CFCs is scheduled to be phased out, while emissions of their alternatives are expected to increase. A simple model is used to illustrate the methodology for determining the time variations of the radiative forcing and temperature changes attributable to the direct greenhouse effect from potential emissions of the halocarbons. Although there are uncertainties associated with the lifetimes of the greenhouse gases, CFCs and their substitutes, the future growth rates of these gases, and the parameters used to simulate the response of the Earth-climate system, the method serves to illustrate an important aspect of the greenhouse warming issue beyond what is provided by the various greenhouse warming indices. Our results show that for likely substitution scenarios, the warming due to halocarbons will correspond to 4–10% of the total expected greenhouse warming at the year 2100. However, uncontrolled growth of the substitutes could result in an eight-fold increase in halocarbon production and a doubling of the halocarbon contribution by 2100