CIESIN Reproduced, with permission, from: Brune, W. H., R. Turco, W. A. Matthews, A. Douglass, M. Prendez, B. H. Subbaraya, R. A. Cox, G. Brasseur, X. Zhou, R. J. Zander, J. M. Rodriguez, and A. O'Neill. 1992. Stratospheric processes: Observations and interpretation. Chapter 4 in Scientific assessment of ozone: Depletion 1991. World Meteorological Organization Global Ozone Research and Monitoring Project--Report no. 25. Geneva: World Meteorological Organization.

Scientific assessment of ozone: Depletion 1991


Stratospheric Processes: Observations and Interpretation


W.H. Brune, R. Turco, W.A. Matthews, A. Douglass, M. Prendez, B.H. Subbaraya, R.A. Cox, G. Brasseur, X. Zhou, R.J. Zander, J.M. Rodriguez, A. O'Neill

Additional Contributors:

S. Solomon, A.F. Tuck, A.R. Ravishankara, M.K.W. Ko, M.J. Prather, J. Austin, M.R. Schoeberl

The polar vortex could be viewed as a flowing processor of air. The diabatic cooling that produces transport from the vortex exterior to the interior has been used to explain the observed distributions of N2O from the AASE mission (Proffitt et al., 1990). In addition, high-resolution analyses of potential vorticity from the European Center for Medium-Range Weather Forecasts (ECMWF), in combination with trajectory calculations, have been used to support the notion that air may be chemically processed within the Arctic polar vortex and then transported to the middle latitudes (Tuck et al., 1991) (Figure 4-6). Measurements of water vapor near the Antarctic and Arctic polar vortices may also indicate a substantial flow of air through the polar vortices (Kelly et al., 1990).

However, the polar vortex may also be viewed as a leaky containment vessel. Plumb (1990) contends that the large meridional flow proposed by Proffitt et al. (1990) is probably inconsistent with the angular momentum budget. Schoeberl et al. (1991) have combined data analyses, trajectory calculations, and radiative transfer computations to diagnose the residual polar circulation. They conclude that the polar vortex is essentially isolated from the middle latitudes. Their model of the vortex has rather rapid mixing outside the vortex, weak mixing across the vortex boundary, and modest mixing inside the vortex (Figure 4-7). However, rapid radiative cooling due to large PSCs, even if it occurs infrequently, may affect these conclusions. In support of the concept of weak mixing from inside to outside the vortex, three-dimensional model calculations using assimilated data fields (Rood et al., 1991) indicate that only a small amount of the chemically perturbed polar air passes from inside the vortex to the middle latitudes. This model has only modest spatial resolution, however, and so may not quantitatively represent the transport of the observed small-scale polar air parcels to the middle latitudes.


The ozone loss at the middle latitudes has well-defined characteristics that must be explained by any proposed stratospheric processes (see Chapter 2 for details). The decadal trend is greatest at high latitudes and in winter and spring, although significant trends occur as far south as 30degN and in summer. Further, the observed ozone decreases predominantly occur in the lower stratosphere below 25 km. Computer models containing only the currently understood gas phase chemistry cannot reproduce these characteristics of the observed ozone trends (see Chapter 8). As a result, some other chemistry or dynamics that are not properly represented in these models must be involved.

Isolating the causes of this decadal change in ozone at middle latitudes is considerably more difficult than establishing the link between stratospheric chlorine and the Antarctic ozone hole. While the ozone decreases in the Antarctic ozone hole are measured in percent per day, the ozone decreases at middle latitudes are measured in percent per decade. Subtle changes in either the dynamics or chemistry could possibly cause such ozone changes.

At present, the only proposed cause of the downward trends in total ozone at middle latitudes that can satisfy most current observations involves enhanced chlorine and bromine catalysis that is initiated by heterogeneous chemistry. These processes based on halogen photochemistry are appealing as the cause for a couple of reasons. First, they occur in the lower stratosphere where both PSCs and stratospheric aerosols are concentrated, precisely at the altitudes where the observed ozone trend is occurring. Second, the observed increases in stratospheric chlorine and bromine that have occurred over the last few decades could, by these processes, result in the observed ozone trend.

4.4.1 Transport of Polar Air to the Middle Latitudes

One reasonable postulate for the total ozone decrease at middle latitudes is that the chemically perturbed polar air is being transported to middle latitudes in the lower stratosphere. In the Southern Hemisphere, the mixing of the polar air, depleted of ozone, with the middle latitude air (the dilution effect) (Chipperfield and Pyle, 1988; Sze et al., 1989; Prather et al., 1990; Cariolle et al., 1990) would cause ozone loss at the middle latitudes at the breakup of the vortex in November or December. A second effect, which can occur in both hemispheres, is the transport to middle latitudes of air that has enhanced levels of reactive chlorine and depleted levels of reactive nitrogen (called chemical propagation) (Prather and Jaffe, 1990). In these air parcels, the reactive chlorine would be constantly depleting ozone at an accelerated pace until the ClO is converted to ClONO2 and HCl by gas phase chemistry. This conversion occurs over a period of weeks either by photolysis of any nitric acid remaining in the air parcel or by mixing of the polar air with middle latitude air.

The mixing of the polar air with the middle latitude air at the breakup of the vortices in the late winter and early spring can explain some of the ozone loss at the middle latitudes for these times. However, significant flow of air through the vortex, as proposed by Tuck et al. (1991) and Proffit et al (1990), is required if the ozone losses during the winter are to be explained mainly by this mechanism. Tuck et al (1991) estimate that 5 percent to 25 percent of the area north of 30degN consists of air that has come from the vortex. Conflicting results are reported from a three-dimensional model study capable of reproducing the observed ClO abundances at modest resolution (Douglass et al., 1991; Kaye et al., 1991). This study indicates that the transport of polar air masses contributes little to the ozone change at middle latitudes. In addition to any ozone loss that may be caused by transport of polar air, some ozone loss occurs in air parcels that come from the middle latitudes and pass through cold regions, in which PSCs form, that are outside the Arctic polar vortex (Jones et al., 1990; Lefevre et al, 1991). The amount of ozone that is being removed by all these processes at middle latitudes during winter and after the vortex breakup, perhaps even into summer, needs to be quantified.

The chemical signatures of air parcels from the vortices are enhanced levels of reactive chlorine, low levels of NOx, and possibly NOy, low levels of long-lived tracers such as N2O, and high values of the dynamical tracer, potential vorticity. In the Southern Hemisphere after mid-September, these air parcels would also exhibit low levels of ozone. Such air masses, with spatial extents of 10 to 100 km, have been observed outside of both polar vortices (Tuck et al., 1991, and references therein; Atkinson et al., 1989), as is shown for outside the Arctic polar vortex in 1989 (Figure 4-8). The observations of spikes of chemically perturbed air imply that some ozone is being lost at middle latitudes by the transport of polar air.

4.4.2 Photochemistry of the Sulfate Aerosol Layer

A newly proposed mechanism for explaining at least part of the observed ozone trends at middle latitudes involves heterogeneous reactions occurring on the global sulfate aerosols, as discussed in Chapter 3. In the current theory, the rapid reaction,


happens on the sulfate aerosol, and its reaction efficiency, gamma = ~0.1, is independent of temperature. The direct conversion of the reservoir chlorine species on sulfate aerosols by the reactions, ClONO2 + H2O --&gy; HOCl + HNO2 and ClONO2 + HCl 00 -->l; Cl2 + HNO3, is thought to be unimportant for most of the stratosphere (Watson et al., 1990; Mather and Brune, 1990; Rodriguez et al., 1991), except perhaps near the polar vortices where the temperatures are lower than 205 K (Wolff and Mulvaney, 1991; Drdla et al., 1991), or after large volcanic eruptions (Hofmann and Solomon, 1989; Brasseur et al., 1990; Pitari et al., 1991).

For a typical sulfate aerosol surface area of 0.5 mum2 cm[-3], the time constant for the conversion of N2O5 to nitric acid is ~5 days. The immediate result is that NOx is diminished, particularly at the high latitudes in seasons when the sunlight is too weak to photolyze the nitric acid significantly. Due to the coupling among the chlorine, nitrogen, and hydrogen families, concentrations of reactive chlorine and HOx increase. As a result, the negative ozone trends calculated by models that include the heterogeneous conversion of N2O5 on sulfate aerosols increase by almost a factor of two, not only for high latitudes in winter, but also at middle latitudes in summer (Rodriguez et al., 1991; see also Chapter 8). These models simulate the observed ozone trends substantially better than models with only gas phase chemistry (see the discussion in Chapter 8).

Preliminary evidence exists for the conversion of N2O5 to nitric acid on the global sulfate aerosols and the subsequent enhancement of ClO. First of all, from a limited set of measurements (King et al., 1991; Toohey et al., 1991), the observed abundances and latitudinal gradient of ClO are better simulated by models containing heterogeneous chemistry on sulfate aerosols than by models with only gas phase chemistry (Figure 4-9) (King et al., 1991; Toohey et al., 1991). This conclusion appears to be robust despite the inability of the models to simulate the downwelling of the stratosphere (Heidt et al., 1989; Loewenstein et al., 1990; Schmidt et al., 1991). Some observations of reactive nitrogen gases, when compared with model results, also indicate that these heterogeneous processes on sulfate aerosols are occurring. These observations include nitric acid measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) (Austin et al., 1986; Jackman et al., 1987; Rood et al., 1990) and Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) (Natarajan and Callis, 1991), N2O5 measurements (Natarjan and Callis, 1991; Evans et al, 1985), and the high-latitude column measurements of NO2 (Pommereau and Goutail, 1988; Solomon and Keys, 1991).

On the other hand, some discrepancies between model results and observations suggest that more work is required to understand all the processes that affect the abundances of reactive chlorine and nitrogen in the lower stratosphere. First, the observed abundances of ClO at low latitudes (<30degN) are better represented by the model with only gas phase chemistry (Figure 4-9). Second, the abundances of ClO obtained in summertime during only a few observations at 40degN latitude are smaller than predicted by models with heterogeneous chemistry on sulfate aerosols. Also, studies of in situ NO measurements (Considine et al., 1991; Kawa et al., 1991) indicate that the observed NO levels may be higher than predicted by models containing the N2O5 + H2O reaction but are smaller than predicted by models containing gas phase chemistry only.

4.4.3 Halogen Photochemistry as a Cause of the Observed Ozone Decline

Comparisons between observations and model results will become more definitive in establishing the role of heterogeneous chemistry on sulfate aerosols when more measurements are made and the models are improved. In addition, the impact of new laboratory data, such as the measurements of smaller, temperature-dependent absorption cross sections for HNO3 (Rattigan et al., 1991), must be carefully assessed. Nevertheless, the characteristics of the ozone trends and of the ClO and HNO3 abundances place tight constraints on any proposed mechanism. Thus, although proposed dynamical mechanisms might explain the observed ozone changes in the lower stratosphere, they would also have to simultaneously explain the observed abundances of ClO, NOx, and HNO3. The assertion that dynamics alone is probably not responsible for the observed ozone trend is supported by an analysis of total ozone in the Southern Hemisphere for July-September from 1963 to the present (Lehmann et al., 1991). In this study, the changes in the tropopause height and the transient eddy heat flux, used to represent dynamical influences, explain the interannual variability, but not the observed negative trend in total ozone over the last decade.

The proposed enhanced halogen catalysis that is initiated by heterogeneous chemistry currently appears to be the most likely cause of the declining ozone trend at middle latitudes. The summer trends and about one-half of the winter trends in ozone are simulated by models with heterogeneous chemistry on sulfate aerosols; the other half of the winter trend may result from transport of chemically-processed polar air to middle latitudes (see Chapter 8). If halogen chemistry is the cause, then as the atmospheric abundances of chlorine and bromine increase in the future, significant additional losses of ozone are expected not only in the Arctic, but also at middle latitudes.


This current assessment (in Chapter 6), as well as the last assessment (WMO, 1990), has pointed to the greater potential of bromine than chlorine per molecule to destroy stratospheric ozone. This greater ozone depletion potential for the lower stratosphere happens because ~50 percent of the available bromine is in the reactive forms (Br and BrO), compared with chlorine, which has only a few percent in the reactive ClO form. Moreover, BrO primarily reacts synergistically with ClO in the fast catalytic cycle already shown to destroy ozone, even in the absence of oxygen atoms. Finally, bromine catalysis is most efficient in the lower stratosphere where the ozone concentration is largest. Thus, as the abundances of chlorine increase throughout the stratosphere, ozone destruction by bromine catalysis will increase proportionally to the increases in the abundances of both stratospheric chlorine and bromine.

Reactive bromine has been detected directly in the stratosphere, particularly inside the Antarctic chemically perturbed region and the Arctic polar vortex (Toohey et al., 1990; Wahner et al., 1990a; Carroll et al., 1989). Direct measurements of the reaction of BrO with ClO in these polar regions come from the ground- and aircraft-based measurements of OClO (Solomon et al., 1988; Wahner et al., 1989; Pemer et al., 1991), for which the only known source is the reaction between BrO and ClO. Analyses of these measurements indicate that the BrO + ClO catalytic cycle is responsible for roughly 25 percent of the observed total ozone loss in the Antarctic ozone hole, with the Cl2O2 mechanism responsible for roughly 70 percent. Outside the polar vortices, or when the ClO abundances are only a few hundred pptv, the BrO + ClO catalytic cycle is more important, relative to the Cl2O2 cycle, than when ClO abundances exceed 1,000 pptv.


A large increase in the aerosol surface area caused by the injection of gas and debris from El Chichon in 1982, and the subsequent heterogeneous reactions may have lead to a decrease in the total ozone in the years that followed (Hofmann and Solomon, 1989; Brasseur et al., 1990). The ozone depletion resulted from chlorine catalysis that followed heterogeneous reactions on sulfate aerosols in the El Chichon cloud (see Chapter 3). Direct evidence for heterogeneous conversions--the shifts in the trace gas concentrations in the El Chichon cloud--has been collected (Arnold et al., 1991). Such eruptions could have a large impact in the future as the chlorine and bromine content of the stratosphere continue to increase.

Mt. Pinatubo in the Philippines erupted violently in June 1991, injecting a cloud of sulfur into the stratosphere. Analysis of early satellite observations suggest that Mt. Pinatubo injected two to three times as much sulfur into the stratosphere as El Chichon (Bluthetal, 1992). In the following few months, the aerosols from the eruption were observed as far north as Wyoming (Deshler et al., 1992, Sheridan et al., 1992), and a thick cloud encircled the Earth equatorward of 30deg latitude (McCormick and Veiga, 1992).

The potential impact of the Mt. Pinatubo eruption on total ozone has been modeled. The calculated decline in total ozone is predicted to be in the range of 3 to 12 percent in the northern middle latitudes between 30degN and 60degN, depending on which model was used (J.M. Rodriguez, private communication; G.P. Brasseur, private communication). At high latitudes (>70degN) in February, these models predict maximum ozone losses in the range of 8 to 24 percent. Ozone reductions of 2 percent or less result from chlorine chemistry that is initiated by the reaction of N2O5 + H2O on the sulfate aerosols. The largest impact appears to result from chlorine chemistry that is initiated by the reactions of ClONO2 + H2O on the volcanic sulfate aerosols. The rates of these latter processes depend heavily on the temperature and the water vapor mixing ratio and are thus somewhat uncertain. However, the assessment models do indicate that volcanic injections into the stratosphere could have a substantial effect on global ozone, at least for one or two years.

A positive aspect of the volcanic injection into the stratosphere is that it presents an opportunity to study these processes on sulfate aerosols. Sharp gradients in aerosol abundances that are present in the early stages of the spread of the volcanic plume are excellent for testing theories about chemical processes that occur on time scales of less than a few weeks. They also present an excellent opportunity to test theories about the motion and mixing of air parcels in the lower stratosphere. These studies will help shape our understanding of the heterogeneous processes that are occurring on sulfate aerosols and will give us greater predictive capability of the future trends of total ozone.


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