Lower atmosphere studies Vertical profiles of ozone and aerosol ...
Lower atmosphere studies Vertical profiles of ozone and aerosol within, at the edge of, and outside of the antarctic polar vortex in the spring of 1988 T. DESF-I[ER, D.J. HOFMANN, and J.V. HEREFORD LI)cparf went of Physics and Atronoi,iii University of Wyoming Laraniie, Wioinin' 82071
Since the springtime ozone depletion over Antarctica was first discovered by Farman, Gardiner, and Shanklin (1985), there have been extensive measurements to understand the underlying causes and the temporal and spatial structure of the ozone hole. As part of this effort, ozone, temperature, and aerosol soundings were taken at McMurdo Station in 1986, 25 August to 3 November (Hofmann et al. 1987 and Hofmann, Rosen, and Harder 1988); and in 1987, 29 August to 9 November (Hofmann et al. 1989). Results from these measurements indicated that ozone depletion occurred primarily in September in periods of fewer than 10 days and was confined to the 12-20-kilometer layer. In 1987, ozone depletion proceeded at a faster rate, lasted longer, and extended to higher altitudes than in 1986. Aerosol measurements in both years indicated that upward motion was not occurring in the vortex, and that a condensation nuclei layer existed just above the ozone depletion region in mid-October. The time of appearance of this layer could not be determined by the 1986 or 1987 measurements, although it did not exist in late August in 1987. These measurements were repeated in 1988 to further substantiate the earlier observations, to determine the time of formation of the condensation nuclei layer, and to collect measurements in polar stratospheric clouds with a new aerosol counter designed for sensitivity to low concentrations of large particles (i.e., those with radii larger than 1.0 micrometer). Field operations. Ozone and temperature profiles were measured in 41 balloon flights at McMurdo from 24 August to 14 November 1988. Optical particle counters were included on four flights to measure sulfate layer particles (radii greater than 0.15 micrometer), four flights to measure condensation nuclei (radii greater than 0.01 micrometer), and two flights to measure larger particles. Because of the motion of the antarctic polar vortex, measurements were obtained within, at the edge of, and outside the vortex. Although the polar vortex did not remain over McMurdo as it did in 1986 and 1987, it was overhead long enough to es1989 REVIEW
tablish that ozone depletion was slower, less extensive, and ended earlier than in either 1986 or 1987 (Deshler, Hofmann, and Hereford in press). A comparison of the 18 ± 1 kilometer temperature and ozone mixing ratio for 1986, 1987, and 1988 is shown in figure 1. Averaged over September, the ozone mixing ratio at 18 kilometers decayed with a half-life of 37 days compared to 28 days in 1986 and 12 days in 1987. Note that in 1988 the 18-kilometer temperature began to warm substantially by mid-September, while in 1986 and 1987 it remained below - 70°C until late October. Figure 2 presents vertical ozone and temperature profiles in late August and at maximum depletion for the 3 years. While ozone partial pressure in the 1618-kilometer layer decayed to values as low as 10 nanobars in 1986 and 3 nanobars in 1987, in 1988 at maximum depletion ozone partial pressure was 60-70 nanobars in the depleted region, a reduction of only 30-50 percent. Even with these differences in degree of ozone depletion, there were similar-30 MCV1URD0 18 km
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Figure 2. Comparison of 1986, 1987, and 1988 ozone and temperature soundings in late August and during the height of ozone depletion. (km denotes kilometer. nb denotes nanobar. hPa denotes hectopascals.)
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ities to previous measurements. Ozone depletion was caused by a sink between 12 and 20 kilometers. Primary depletion was episodic and occurred in periods of fewer than 10 days. Measurements at the edge of the vortex displayed the ozone layering observed in 1986 and 1987 and suggest the exchange of ozone rich and poor air across the vortex wall in the 12-20kilometer layer. Outside the vortex, vertical profiles displayed a region of high ozone and constant temperature above 20 kilometers. Measurements of sulfate layer aerosol again indicated no upward motion within the vortex during ozone depletion. Measurements of condensation nuclei were made on 26 August and 5 September. On 26 August, ozone depletion had not begun and the condensation nuclei profile was normal, although there was an indication of the beginning of an enhanced layer at 26 kilometers. By 5 September, this enhanced layer was well established. It existed just above the layer of ozone depletion which was then most severe in the 20-22kilometer layer. The condensation nuclei layer was observed to persist through the final measurements on 25 October just above the region of ozone depletion. On one flight in early September with the aerosol counter designed to sample larger particles, layers of ozone depletion were found to be correlated with layers of enhanced aerosol concentration for aerosol with radii greater than or equal to 0.2 micrometer (Hofmann 1989). Although there were also layers of enhanced aerosol for aerosol greater than or equal to 1.0 micrometer, the surface area distribution was dominated by the smaller particle mode, radii
greater than or equal to 0.2 micrometer. These observations are consistent with the idea that heterogeneous chemistry is a controlling factor in ozone depletion. D.J. Hofmann, J.V. Hereford, and S. Gabriel were at McMurdo from 24 August to 14 November and T. Deshler from 3 October to 14 November. This work was supported by National Science Foundation grant DPP 87-15913.
Do gold, chromium oxide, and carbon-containing particles provide tracers of Mount Erebus emissions?
canoes (Chuan, Rose, and Woods 1987; Woods and Chuan, 1988). To use volcanic particles as tracers of the emissions of Mount Erebus, it is necessary to find particles that are peculiar to Mount Erebus. Chemically, Mount Erebus is known to be rich in fluorine and chlorine. These do not, however, provide good particle tracers because they appear either as gaseous compounds or as soluble salts which apparently do not survive very far from the source, because they have been found only in the immediate vicinity of the summit. Therefore, we must seek materials that are not necessarily abundant but are distinctive of the Mount Erebus emissions. After reviewing aerosol samples collected since 1986 at the summit of Mount Erebus, in the snow near the volcano and in the ambient atmosphere, we have identified the particles described here, which we believe are characteristic of the Mount Erebus emissions. Crystalline particles of elemental gold. We believe that these particles form in some vapor-phase reactions in the plume leaving Mount Erebus, with subsequent quenching of the vapors and nucleation into 10-micrometer size crystals (Meeker, Kyle, and Chuan 1987). These gold particles are numerous enough to be readily discernible in the analysis by scanning electron microscopy (figure, block A), and energy-dispersive X-ray measurements. Both the morphology and the elemental X-ray spectrum of gold are quite distinctive so that the identification of the crystalline elemental gold is quite easy. To our knowledge, such gold particles have not been reported from other volcanoes. Chromium-oxide particles. These are very small amorphous particles with a distinctive surface texture. While metal oxides
RAYMOND CHUAN and JULIE M. PALAIS
Glacier Research Group University of New Hampshire Durham, New Hampshire 03824
Since 1983, a largeaerosolparticle* sampling program has been in operation during the austral summer in the vicinity of Mount Erebus, Ross Island, Antarctica, in an attempt to assess the effects of the emissions from Mount Erebus on the composition of the atmosphere and the ice. Chuan et al. (1986) characterized many types of aerosol particles collected both in the plume of Mount Erebus and in the ambient atmosphere between Mount Erebus and the South Pole. Many of these particles have also been seen in the emissions from other vol-
* The term "large" aerosol particles is taken in the atmospheric aerosol sense, meaning they are 1 to 100 microns in size, whereas in the volcanological sense these would be considered "fine" particles (see for example Rose, Chuan, and Woods 1982). 1989 REVIEW
References Deshler, T., D.J. Hofmann, and J.V. Hereford. In press. Ozone profile measurements within, at the edge of, and outside the Antarctic polar vortex in the spring of 1988. Journal of Geophysical Research. Farman, J.C., B.C. Gardiner, and J.D. Shanklin. 1985. Large losses of total ozone in Antarctica reveal seasonal C1O \/NO\ interaction. Nature, 315, 207-210.
Hofmann, D.J., J.W. Harder, S.R. Rolfe, and J.M. Rosen. 1987. Balloonborne measurements of the development and vertical structure of the Antarctic ozone hole in 1986. Nature, 326, 59-62. Hofmann, D.J., J.M. Rosen, and J.W. Harder. 1988. Aerosol measurements in the winter/spring Antarctic stratosphere 1. Correlative measurements with ozone. Journal of Geophysical Research, 93, 665676.
Hofmann, D.J. 1989. Direct ozone depletion in springtime Antarctic lower stratospheric clouds. Nature, 337, 447-449. Hofmann, D.J., J.W. Harder, J.M. Rosen, J.V. Hereford, and J.R. Carpenter. 1989. Ozone profile measurements at McMurdo station Antarctica during the spring of 1987. Journal of Geophysical Research, 94, 16,527-16,536.
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