High-latitude stratosphere

Figure 6.12. Observations of the cliff in NO2 reported by Noxon (1978). The solid and dashed lines represent Northern Hemisphere measurements while the filled circles and crosses show Southern Hemisphere evening and morning twilight data, respectively. The NO measurements of Fahey et al. (1989a) are shown for comparison. The two molecules interchange rapidly with one another in the sunlit atmosphere and hence provide a measure of NOx. Both datasets show very low NOx in the high latitude stratosphere. From Solomon (1999).

Similar heterogeneous reactions also can occur, but somewhat less efticientiy, in the lower stratosphere on global sulfate clouds (ie, aerosols of sulfuric acid), which are formed by oxidation of SO2 and COS from volcanic and biological activity, respectively (80). The effect is most pronounced in the colder regions of the stratosphere at high latitudes. Indeed, the sulfate aerosols resulting from emptions of El Chicon in 1982 and Mt. Pinatubo in 1991 have been impHcated in subsequent reduced ozone concentrations (85). 

Water vapor concentrations have also been used to show that stratospheric air in the midlatitudes cannot all have originated via the tropical pump, i.e., path I in Fig. 12.3. For example, Dessler et al. (1995b) have shown that water vapor concentrations in the lowermost stratosphere at 37.4°N, 122.1°W are higher than expected for an air mass that has passed through the cold tropical tropopause. Their data are consistent with path II, although as they point out, these measurements do not exclude path III, which represents convective transport from the troposphere to the stratosphere at mid and high latitudes. Lelieveld et al. (1997) report aircraft measurements of CO, 03, and HNO-, over western Europe that suggest that tropospheric air can be mixed into the lower stratosphere. 

Briihl, C P. J. Crutzen, and J.-U. GrooG, High-Latitude, Summertime N02. Activation and Seasonal Ozone Decline in the Lower Stratosphere Model Calculations Based on Observations by HALOE on UARS, J. Geophys. Res., 103, 3587-3597 (1998). 

Taalas, P J. Damski, and E. Kyro, Effect of Stratospheric Ozone Variations on UV Radiation and on Tropospheric Ozone at High Latitudes, J. Geophys. Res., 102, 1533-1539(1997). 

Sloan, L. C and D. Pollard, Polar Stratospheric Clouds A High Latitude Warming Mechanism in an Ancient Greenhouse World, Geophys. Res. Lett., 25, 3517-3520 (1998). 

The High-Latitude Lower Stratosphere in Winter and Spring... 

Brtthl, C., Crutzen, P.J., and Grooss, J.U. (1998) High-latitude, summertime NO, activation and seasonal ozone decline in the lower stratosphere Model calculations based on observations by HALOE on UARS, J. Geophys. Res. 103,3587-3597. 

This presentation focuses on the vertical distribution of halocarbons obtained by analysis of cryogenically collected whole-air samples. The balloon-borne cryogenic samplers developed and flown by the Max-Planck-Institut fur Aeronomie (MPAE) and the Kem-forschungsanlage Jiilich (KFA) are described by Fabian [17] and Schmidt [18]. Between 1977 and 1993, a total of 33 balloon ascents have been carried out by both institutions, 28 at northern midlatitudes (southern France, 44°N), 3 at high latitudes(Kiruna, 69°N) and 2 in the tropics (Hyderabad, 17,5°N). These stratospheric data are limited to balloon altitudes, i.e. up to about 35 km. Tropospheric data were obtained from balloon samples, samples collected aboard aircraft and at ground level. 

Figure 2 shows how the model temperature in northern high latitudes in the lower stratosphere compares with observations. It shows the average, maximum and minimum temperature at 50hPa and 85°N on each day of the year from the five simulation years under the 1979 ozone scenario (thin lines). For comparison are equivalent observed temperature fields (thick lines). In this case the Free University of Berlin stratospheric temperature dataset was used the data were averaged over the years 1972 to 1981. 

Hunt (163) have performed detailed transport calculations using ozone as a tracer and found that a tracer produced at low latitudes in the stratosphere is transported downward toward the poles. It then enters the troposphere through folds or breaks in the tropopause at middle and high latitudes and is transported downward back toward lower latitudes, where it is removed at the ground. 

Regener (202) first measured an ozone flux at the surface the flux has been confirmed by later experiments of Aldaz (3) and Gabally (68). The currently accepted view of sources and sinks of ozone is that it enters the troposphere from the stratosphere, principally at middle and high latitudes, and is destroyed at the earth s surface. Kroening and Ney (141) have suggested that sheets of high-density ozone found in the troposphere are due to direct transport of stable lamina ("ozone rivers") from the stratosphere. 

High latitude aerosol observations have been secured by the Stratospheric Aerosol Measurement (SAM II) satellite system these have shown that the aerosol extinction profiles measured within the northern polar vortex differ significantly above 18 km from those measured outside the vortex (Me Cormick et d., 1983). 

Ozone holes—Decreased concentrations of stratospheric ozone, occurring at high latitudes during the early springtime. Ozone holes are most apparent over Antarctica, where they develop under intensely cold conditions during September and November, allowing a greater penetration of deleterious solar ultraviolet radiation to Earth s surface. 

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