Ozone chemistry, modeling stratospheric

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. 

The model tropopause is defined by a PV level of 3.5 pvu poleward of 20° latitude, and by a -2 K km 1 temperature lapse rate equatorward of 20° latitude. Consequently, in this study the troposphere is defined as the volume between the surface and the simulated tropopause. Because the model does not consider typical stratospheric chemical reactions explicitly, ozone concentrations are prescribed from 1-2 levels above the model tropopause up to the top of the model domain at 10 hPa. In both hemispheres we apply monthly and zonally averaged distributions from a 2D stratospheric chemistry model [31]. In the present version of the model, we use the simulated PV and the regression analysis of the MOZAIC data (Section 2) to prescribe ozone in the NH extratropical lower stratosphere, which improves the representation of ozone distributions influenced by synoptic scale disturbances [32, 33]. Furthermore, the present model contains updated reaction rates and photodissociation data [34]. 

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. 

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

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). 

Although there has been some controversy over whether there is indeed a true ozone deficit problem (e.g., Crutzen et al., 1995), a combination of measured concentrations of OH, HOz, and CIO with photochemical modeling seems to indicate that it may, indeed, exist (Osterman et al., 1997 Crtuzen, 1997), although the source of the discrepancy remains unclear. Measurements of CIO in the upper stratosphere have found concentrations that are much smaller (by a factor of 2) than predicted by the models (e.g., Dessler et al., 1996 Michelsen et al., 1996). Because of the chlorine chemistry discussed later, model overestimates of CIO will also result in larger predicted losses of 03 and hence smaller concentrations. 

As a result, increasing NCI, emissions does not have a significant direct effect at lower altitudes as it does at higher ones but rather has indirect effects on the halogen and HOx cycles, which reduce the ozone destruction due to these species. The net result, then, is interference in these other ozone-destroying cycles, leading to an increase in ozone at these altitudes as seen in the model predictions in Fig. 12.7. (In the very low stratosphere, NOx can also produce 03 through the VOC-NO,. chemistry discussed in Chapter 6.)... 

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). 

Several potential peroxy radical measurement techniques exist in the realm of atmospheric chemistry studies, although most have been used only in the laboratory. The techniques are summarized in Table I. Possibly, some laboratory methods could be applied to atmospheric measurements. The database for ambient peroxy radical concentrations in the troposphere and stratosphere is meager. Much of the available stratospheric data yield concentrations of H02 higher than those calculated with computer models. The reasons for this systematic difference are not known. In the troposphere, more measurements are called for in conjunction with other related species such as ozone, NO, NOjNo2 andjcv It wiH also t>e appropriate to develop multiple methods, and, when they have reached maturity, to perform intercomparison studies. 

Laboratory measurements (17) next showed that reaction R19 proceeded about 40 times faster than determined earlier, strongly promoting ozone production and increasing HO concentrations with major consequences for tropospheric and stratospheric chemistry. Table I presents an ozone budget calculated with a three-dimensional chemistry transport model of the troposphere which takes into account the afore mentioned reactions. 

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. 

The difference between the concentration of 03 and 03s is a measure of ozone that originates from photochemistry in the troposphere, referred to as 03t. The modeled 03, 03s and 03t fields from the three simulations are used to calculate budgets of chemistry and of transports within the troposphere and between the stratosphere and the troposphere, and to estimate the impact of STE on tropospheric 03 levels. 

We have addressed several aspects of STE of ozone and the impact on tropospheric ozone levels. Using ozone observations in the upper troposphere and lower stratosphere from MOZAIC, we have examined the rdation between ozone and PV in the lower stratosphere. A distinct seasonality in the ratio between ozone and PV is evident, with a maximum in spring and minimum in fall associated with the seasonality of downward transport in the meridional circulation and of the ozone concentrations in the lower stratosphere. The ozone-PV ratio is applied in our tropospheric chemistry-climate model to improve the boundary conditions for ozone above the tropopause, to improve the representativity of simulated ozone distributions near synoptic disturbances and realistically simulate cross-tropopause ozone transports. It is expected that the results will further improve when the model is applied in a finer horizontal and vertical resolution. 

One clear limitation of most of the models is that they have little or no representation of explicit stratospheric chemistry. This could also clearly limit the models ability to predict the full atmospheric impact on ozone of aircraft emissions since it is predicted by the models that approximately 1/3 of the ozone perturbations occur in the lower stratosphere. 

---