Ozone loss models

Second, the northern polar vortex is much less stable and hence less isolated from mixing with external air masses compared to the Antarctic case events in January and February in which there was substantial mixing of air from midlatitudes into the vortex have been reported (e.g., see Browell et al., 1993 Plumb et al., 1994). This makes it particularly important to make both measurements and model predictions with sufficient resolution (Edouard et al., 1996). In addition, the Arctic polar vortex tends to break up earlier than the Southern Hemisphere polar vortex since ozone destruction is determined to a large degree by the extent of exposure to sunlight, the earlier breakup and mixing with air external to the vortex cuts the ozone loss short. 

Figure 7. Calculated ozone production and loss rates for two different conditions from the AER two-dimensional model. Production and loss rates above 20 km are diurnally averaged loss rates for the spring equinox at 30°N. Midday loss rates are approximately two times larger. Production and loss rates for midday below 20 km are calculated for the chemically perturbed region over Antarctica on September 16,1987. The catalytic cycles responsible for the loss are explained in the text. Although ozone loss occurs at higher altitudes over Antarctica, in situ observations extend only to 19 km.

The chemical processes involved in depletion of lower stratospheric ozone are now fairly well understood [8]. However, 3-dimensional chemical transport models still under-predict ozone loss in the Arctic, where the winterly polar vortex is less stable compared to its Antarctic counterpart, temperatures in the lower... 

Such processes are always accompanied by a DP loss, either by electrophilic attack of ozone, by an ozone-catalyzed cleavage of the glycosidic bond or by attack of secondary radical species [15]. Residual lignin also plays a crucial role in ozone bleaching. Model studies showed that lignin with free phenolic hydroxyl groups accelerated carbohydrate oxidation, probably by activation of oxygen via phenoxyl radicals, whereas etherified phenolic model compounds had a protective effect [16,17]. 

According to these researchers, this could decrease the lower stratospheric temperature at the polar vortex by about 0.2°C, which in turn could trigger additional polar ozone losses of up to 8% (polar ozone depletion is very sensitive to small temperature changes). Another model, however, showed a much weaker effect on stratospheric temperatures and ozone loss. As discussed above, hydrogen levels are more likely to increase by 20% than by 400% in the coming decades (Tromp et al., 2003). 

Clearly, there is a balance between photochemical ozone production and ozone loss dependent on the concentrations of and NO - Figure 11 shows the dependence of the production of ozone on taken from a numerical model. There are distinct regions in terms of N(Oj) V5. [NOJ... 

Vogt R., Sander R., von Glasow R., and Crutzen P. (1999) Iodine chemistry and its role in halogen activation and ozone loss in the marine boundary layer a model study. J. Atmos. Chem. 32, 375-395. 

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). 

Based on CIO observations (Brune et al., 1990) and related model calculations, observed and calculated rates of ozone loss in February 1989, were shown to be of the order of 20 ppbv/day near 20 km (Schoeberl et al, 1990 Salawitch et al., 1990 McKenna et al., 1990). Further, the BrO observations of Toohey et al. (1990) revealed that the ClO-BrO catalytic cycle was probably of particular importance for the Arctic, since CIO enhancements were generally smaller than in the Antarctic and hence the efficiency of the CIO dimer cycle was reduced note that the rate of the latter depends on the square of CIO density, e.g., Salawitch et al., 1990 1993). However, the early warming observed, for example, in February 1989, as illustrated in Figure 6.19, prevented extensive total ozone loss in that year. Some early studies suggested that the less extensive denitrification of the Arctic would limit ozone losses... 

Arctic ozone losses and test photochemical understanding. These models have succeeded in explaining much of the observed ozone depletion, documenting its connections to chemical processes, and even reproducing much of the observed variability seen from one year to another (see e.g., Chipperfield et al., 4994 4996 Chipperfield and Pyle, 1998 Deniel et al., 1998 Douglass et al., 1995). 

Becker, G., R. Muller, D.S. McKenna, M. Rex, and K.S. Carslaw, Ozone loss rates in the Arctic stratosphere in the winter 1991/92 Model calculations compared with match results. Geophys Res Lett 23, 4325, 1998. 

Lutman, E.R., R. Toumi, R.L. Jones, D.J. Lary, and J.A. Pyle, Box model studies of C10x deactivation and ozone loss during the 1991/92 northern hemisphere winter. Geophys Res Lett 21, 1415, 1994a. 

An entirely new level of sophistication—not only in experiments but also in modeling—will be required for particles, aerosols, and the associated radiation field sets. New mid-IR laser-based instrumentation and use of long-duration balloons have helped make major advances in observations. The balloons can sit in the upper stratosphere and then be lowered to the lower stratosphere with power from fuel cells and solar panels. The modeling elements are equally important it is necessary to test the model and its validity, and the model must link the measurements. The observations must be linked to trajectories, the trajectories must be initialized, and sources of specific chemicals must be identified along with the positions of those sources. Considerable progress has been made on observations and refinement of models to help explain low ozone loss at the mid-altitudes, the increase in UV dosage, the appearance of water vapor in the stratosphere, and possibly, of climate changes 50 million years ago. 

Vogt, R Sander, R. von Glasow, R. Cmtzen, P.J., 1999 Iodine Chemisty and its Role in Halogen Activation and Ozone Loss in the Marine Boundary Layer A Model Study , in The Journal of Atmospheric Chemistry, 32 375-395. 

The calculated amount of ozone destroyed during the second day of sea-salt processing is 0.14 nmol moF. This compares to about 2.5 and 1.2 nmol moF d of ozone destruction through photolysis (O3 —> 0( D) 0( D) + H2O 20H) and chemical reactions (OH + O3 —> HO2 + O2, HO2 + O3 —> OH + 2O2) at O3 concentrations of 40 or 20 nmoF, respectively. Although the Br-catalysed ozone destruction during the second model day is only 5-10 % of the total ozone loss, we calculate at steady state a destruction of20-40 %, if recycling of HBr and HCl on sulphate aerosol is also considered. 

Mario Molina and Sherwood Rowland used Crutzen s work and other data in 1974 to build a model of the stratosphere that explained how chlorofluorocarbons could threaten the ozone layer. In 1985, ozone levels over Antarctica were indeed found to be decreasing and had dropped to the lowest ever observed by the year 2000, the hole had reached Chile. These losses are now known to be global in extent and it has been postulated that they may be contributing to global warming in the Southern Hemisphere. 

The detailed model was constructed as described by Carslaw et al. (1999, 2002). Briefly, measurements of NMHCs, CO and CH4 were used to define a reactivity index with OH, in order to determine which NMHCs, along with CO and CH4, to include in the overall mechanism. The product of the concentration of each hydrocarbon (and CO) measured on each day during the campaign and its rate coefficient for the reaction with OH was calculated. All NMHCs that are responsible for at least 0.1% of the OH loss due to total hydrocarbons and CO on any day during the campaign are included in the mechanism (Table 2). Reactions of OH with the secondary species formed in the hydrocarbon oxidation processes, as well as oxidation by the nitrate radical (NO3) and ozone are also included in the... 

Many wastewater flows in industry can not be treated by standard aerobic or anaerobic treatment methods due to the presence of relatively low concentration of toxic pollutants. Ozone can be used as a pretreatment step for the selective oxidation of these toxic pollutants. Due to the high costs of ozone it is important to minimise the loss of ozone due to reaction of ozone with non-toxic easily biodegradable compounds, ozone decay and discharge of ozone with the effluent from the ozone reactor. By means of a mathematical model, set up for a plug flow reactor and a continuos flow stirred tank reactor, it is possible to calculate more quantitatively the efficiency of the ozone use, independent of reaction kinetics, mass transfer rates of ozone and reactor type. The model predicts that the oxidation process is most efficiently realised by application of a plug flow reactor instead of a continuous flow stirred tank reactor. 

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. 

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