Stratosphere observations

FIGURE 14-30 Measured (—) and model-predicted ( ) change in monthly mean temperatures (a) at the earth s surface and (b) in the stratosphere (observations at 30 mbar and 10°S, model results for the 10- to 70-mbar layer from 8°S to f6°S) (adapted from Lacis and Mishchenko, 1995). 

The strategy for research in the stratosphere has been to develop computer simulations to predict trends in photochemistry and ozone change. Incorporated in these simulations are laboratory data on chemical kinetics and photolytic processes and a theoretical understanding of atmospheric motions. An important aspect of this approach is knowing if the computer models represent the conditions of the stratosphere accurately enough that their predictions are valid. These models are made credible by comparisons with stratospheric observations. 

Sandford S. A. and Bradley J. P. (1989) Interplanetary dust particles collected in the stratosphere observations of atmospheric heating and constraints on their inter-relationships and sources. Icarus 82, 146-166. 

HF. They estimated the magnitude of the halogen source of different types of volcanic eruptions and found that up to several hundred terragrams of HCl could be emitted into the atmosphere in the largest eruptions. Only a small fraction (down to — 0.01% Tabazadeh and Turco, 1993) of this reaches the stratosphere, most HCl being removed by precipitation in the emption plume. More passively degassing volcanoes could produce —(0.4-4) X 10 g HCl per year which predominantly remains in the troposphere. From stratospheric observations there is no indication of any growth in chlorine or bromine after the eruptions of Pinatubo or El Chichon. 

The description of dynamical processes is often facilitated by distinguishing between zonal mean motions and fluctuations about the zonal mean (called the eddies). As indicated in Section 3.4, these fluctuations can be of varied spatial scale — from a few meters to thousands of kilometers. In the stratosphere, observed wave structure indicates that most of the waves are of large scales, of zonal wavenumber 3 or less, as discussed earlier. Note that the use of the word eddy in the context of atmospheric dynamics is thus rather different than the idea of a whirlwind, which is probably a more common usage of the same word (Oxford English Dictionary). Any atmospheric quantity ip(X,ip,z) can be expressed as the sum of the zonal mean ... 

While many natural processes produce chlorine at ground level (including for example, sea salt and volcanic emissions of HC1), these compounds are efficiently removed in precipitation (rain and snow) due to high solubility. The removal of HC1 emitted, for example, in volcanic plumes (which contain a great deal of water and hence form rain) is extremely efficient (see, e.g., Tabazadeh and Turco, 1993). This renders even the most explosive volcanic plumes ineffective at providing significant inputs of chlorine to the stratosphere. Observations... 

Naudet, J.P., D. Hugenin, P. Rigaud, and D. Cariolle, Stratospheric observations of NO3 and its experimental and theoretical distribution between 20 and 40 km. Planet... 

Bregman, A., M. van den Broek, K.S. Carslaw, R. Muller, T. Peter, M.P. Scheele, and J. Leieveld, Ozone depletion in the late winter lower Arctic stratosphere Observations and model results. J Geophys Res 102, 10,815, 1997. 

Much of the observed structure of the stratosphere can be understood in terms of elementary wave propagation, momentum and heat transport by waves, and in situ forcing by radiatively active trace gases. The dynamical aspects of these interactions can often be described satisfactorily in analytical terms, although detailed calculations of the stratospheric circulation require the use of numerical (computer) methods. In what follows, a brief introduction to stratospheric dynamic meteorology is presented. The conceptual development will be oriented toward the interpretation of the stratospheric observations discussed in the previous sections. 

This relatively simple set of equations allows one to interpret the stratospheric observations described in Section III. 

Water is present in Jupiter s atmosphere both in the troposphere and in the stratosphere. Observations of tropospheric water were made with the Kuiper Airborne Telescope and the IRIS (Infrared Interferometer Spectrometer and Radiometer) Spectrometer on board Voyager 1 and Voyager 2 From these data the vertical distribution of water in the level from 16 bar in Jupiter s troposphere was derived. A thin H2O ice cloud would form at 2 bars, T = 200 K (Bjoraker, Larson and Kunde, 1986 [30]). 

This cycle accounts for 30—50% of the total photochemical ozone loss observed during spring in the lower stratosphere at mid-north latitudes (76). 

Halogen radicals account for about one-third of photochemical ozone loss observed in the spring in the lower stratosphere (below 21 km) at 15—60°N latitude (76). The following three cycles (4—6) are the most important. Rate constant data are given in Reference 11. 

Meridional circulation in two-dimensional stratospheric models has been specified based on observations or general circulation model calculations recendy efforts have been undertaken to calculate circulations from first principles, within the stratospheric models themselves. An important limitation of using models in which circulations are specified is that these caimot be used to study the feedbacks of changing atmospheric composition and temperature on transport, factors which may be important as atmospheric composition is increasingly perturbed. 

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). 

During the mid-1980s, each September scientists began to observe a decrease in ozone in the stratosphere over Antarctica. These observations are referred to as "ozone holes." In order to understand ozone holes, one needs to know how and why ozone is present in the earth s stratosphere. 

The discovery of ozone holes over Antarctica in the mid-1980s was strong observational evidence to support the Rowland and Molina hypothesis. The atmosphere over the south pole is complex because of the long periods of total darkness and sunlight and the presence of a polar vortex and polar stratospheric clouds. However, researchers have found evidence to support the role of CIO in the rapid depletion of stratospheric ozone over the south pole. Figure 11-3 shows the profile of ozone and CIO measured at an altitude of 18 km on an aircraft flight from southern Chile toward the south pole on September 21, 1987. One month earlier the ozone levels were fairly uniform around 2 ppm (vol). 

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 fundamental aspects of the problem are well established the measured concentrations of the CFCs indicate that they accumulate in the lower atmosphere and that they reach the stratosphere. As expected, chlorine atoms and CIO radicals are found in the stratosphere together with other species such as O, OH, HO2, NO, NO2, HCl, CIONO2, HOCl, etc. The observed concentrations are in reasonable agreement with the model predictions if the limitations of the models, as well as atmospheric variability, are taken into account. 

It is only recently that a decrease in stratospheric ozone levels attributable to the CFCs has been observed. In spite of the relatively large natural... 

A detailed analysis of the atmospheric measurements over Antarctica by Anderson et al. (19) indicates that the cycle comprising reactions 17 -19 (the chlorine peroxide cycle) accounts for about 75% of the observed ozone depletion, and reactions 21 - 23 account for the rest. While a clear overall picture of polar ozone depletion is emerging, much remains to be learned. For example, the physical chemistry of the acid ices that constitute polar stratospheric clouds needs to be better understood before reliable predictions can be made of future ozone depletion, particularly at northern latitudes, where the chemical changes are more subtle and occur over a larger geographical area. 

---