Models stratospheric ozone chemistry

For those more inclined to use environmental topics to enrich thermodynamics and kinetics parts of the physical chemistry curriculum, Modeling Stratospheric Ozone Chemistry and the Contrail projects are two examples. 

In view of this, it has been proposed that hydrated electrons generated on the surface of stratospheric ice crystals, via cosmic rays, could contribute to Cl formation via DEA of adsorbed CFCs. " Photodetachment of the chloride ions might then provide a mechanism to generate the Cl radicals that lead to ozone destruction. However, attempts to link these laboratory observations directly to stratospheric ozone chemistry have been strongly criticized, " although modeling does leave open the possibility that, at the very least, HCl destruction on ice crystals might be important for stratospheric chlorine chemistry. More work is evidently needed to resolve this controversy. 

By combining models of meteorology and ozone, Paul pioneered the field of atmospheric chemistry, and showed how local emissions can have a global effect, even though the substances in question occur in minute, i.e., trace amounts. With his work, that has had an impact well beyond his own field, he followed in the footsteps of pioneers in chemistry in the past centuries such as Scheele, Priestley, Lavoisier, and Laplace. Like Paul, they were also intrigued by the chemical composition of air, what controls it, and tried to unravel its importance for life on Earth. The central role of nitrogen oxides in stratospheric ozone chemistry was the first of Paul s impressive series of discoveries. 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

The chemistry of the stratospheric ozone will be sketched with a very broad brush in order to illustrate some of the characteristics of catalytic reactions. A model for the formation of ozone in the atmosphere was proposed by Chapman and may be represented by the following "oxygen only" mechanism (other aspects of... 

GrooB, J.-U., C. Bruhl, and T. Peter, Impact of Aircraft Emissions on Tropospheric and Stratospheric Ozone. Part I Chemistry and 2-D Model Results, Atmos. Em iron., 32, 3173-3184 (1998). 

As discussed in other chapters of this book and summarized in Chapter 16, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NC/) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to O, formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997). 

In this study we will present aspects of STE in relation with the budget and concentrations of ozone in the troposphere, specifically in the Northern Hemisphere. Firstly, we present ozone observations in the tropopause region from the measurement campaign MOZAIC, and discuss their correlation with potential vorticity. The results have been used to improve the parameterization of stratospheric ozone in a coupled tropospheric chemistry - general circulation model. We will show examples of the performance of the model regarding the simulation of ozone in the tropopause region, and present the simulated seasonality of cross-tropopause ozone transport in relation to other tropospheric ozone sources and sinks. Finally, we will examine and compare the influence of cross-tropopause transports to surface ozone concentrations for simulations with contemporary, pre-industrial, and future emission scenarios. 

Important alterations to the global distribution of stratospheric ozone are currently predicted by the best available models which synthesize the chemistry, radiation and dynamics of the middle atmosphere. While these predictions have fluctuated significantly since the first crude estimates were offered in the mid-1970s [31, progress in many fields has brought a growing realization that the stratosphere may well be the first natural system to submit to the scientific method. 

In parallel with their own research programs, the manufacturers, through the FPP also jointly fimded research to study the atmospheric chemistry of CFCs in order to assess the extent of any risk they might pose. Independent research workers of imiversities and research institutes worldwide were contracted to measure the rates of reactions, which were essential input data for the complex computer models needed to predict the rate of ozone depletion. This value could not be measured directly in the 1970s because the large daily and seasonal fluctuations in stratospheric ozone concentrations swamped the modest depletion expected from CFCs. 

Pitari, G.S., S. Palermi, G. Visconti, and R. Prinn, Ozone response to a CO2 doubling Results from a stratospheric circulation model with heterogeneous chemistry. J... 

Other processes that could contribute to upper stratospheric ozone changes include trends in methane, nitrous oxide, and water vapor. These source gases can, for example, lead to changes in HOx and NOx, which can in turn affect ozone loss rates and the competition between different catalytic cycles. However, the effect of these changes is considerably smaller than the dramatic impact of the five-fold enhancement in chlorine caused by human activities. By the turn of the 21st century, observations and modelling studies showed that chlorine chemistry dominated the trends found in upper stratospheric ozone (see Muller et al., in WMO/UNEP, 1999). 

Heterogeneous reactions. Knowledge of atmospherically relevant heterogeneous reactions is far from complete. Important reactions probably still remain to be identified and their rates and mechanisms determined. Just as ignorance of heterogeneous chemistry contributed to the failure of stratospheric ozone models to anticipate the formation of the antarctic ozone hole, much still is to be discovered and learned about the role of heterogeneous reactions in the troposphere. 

The role of reaction (21) in atmospheric chemistry is now well recognized, and general agreement upon the value of its rate constant, of critical importance for modelling calculations of the stratospheric ozone balance, now appears to be... 

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