Volcanic aerosol and ozone depletion within the antarctic polar vortex ...

Volcanic aerosol and ozone depletion within the antarctic polar vortex during the austral spring of 1991 TERRY DESHLER

Department of Atmospheric Science University of Wyoming Laramie, Wyoming 82071

ALBERTO

ADRW',n

Istituto di Fisica deli' Atmosfera CNR, Frascati, Italy

Soon after the initial reports of ozone depletion over Antarctica and the implication of polar stratospheric clouds as crucial to the ozone destruction process, vertical profiles of ozone and aerosol have been measured during the austral spring from McMurdo Station, Antarctica (78'S) (e.g., Deshler et al. 1991; Johnson etal. 1992). In 1990 and 1991 these studies were additionally supported with lidar measurements (Gobbi 'etal. 1991). The period, 1986-1990, was volcanically quiescent; and the 1991 polar stratosphere was not expected to be different since the eruptions of Pinatubo (15' N, 13-15 June) and Cerro Hudson (46'S, 12-15 August) had both occurred after the 1991 polar vortex had formed. The 1991 balloon-borne observations of aerosol and ozone began 23 August and continued until 1 November; and the lidar measurements, from 26 August until 10 October. The lidar measurements during the period 11 September to 10 October are shown in figure 1 as profiles of scattering ratio, the ratio of back scattering resulting from aerosol and molecules to that produced only by molecules. Above 15 kilometers the stratosphere was very clean, and we observed only background stratospheric 0 5 18 1 1 SEP 12 SEP

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aerosols; however, on 11 and 12 September, we observed thin particle layers between 8 and 13 kilometers. These layers were inhomogeneous in both the vertical and horizontal. Lidar and aerosol profiles on 13 September indicated a very clean stratosphere both above and below 15 kilometers, with the exception of two thin layers at 10 and 12 kilometers. We obtained additional evidence of this early September appearance of volcanic aerosols with condensation nuclei profiles which showed a region of very high aerosol concentration between 9 and 13 kilometers on 8 September and every flight thereafter, indicating homogeneous nucleation of new aerosols. Fresh volcanic layers are the primary source for such high concentrations of new aerosols, but these high concentrations last only a short time because of coagulation. For the volcanic aerosols in Antarctica these observations of homogeneous nucleation, along with the altitude of the observations, help identify the source of the volcanic aerosols as Cerro Hudson (45' 573' W), which erupted 12-15 August. We completed optical particle counter flights on 27 September and 8 October. The vertical profile of aerosols, compared with lidar and temperature measurements, on 27 September is shown in figure 2. The combination of the persistent scattering ratios, low depolarization, and high aerosol concentration points to the fact that a great deal of volcanic aerosols were entrained into the vortex between 13 and 20 September and remained there rela-

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Figure 1. Lidar profiles of scattering ratio from 11 September 1991 to 10 October 1991 at McMurdo Station, Antarctica. Profiles from 29 August to 7 September (not shown) indicated only the stratospheric background aerosols.

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Figure 2(A), Lidar; (B), aerosol; and (C) temperature profiles on 27 September. The calculated scattering ratios, assuming different indices of refraction, are shown compared with the lidar measurements. The dashed lines on the aerosol and temperature profiles represent the background stratospheric conditions as measured on 13 September 1991. Note the thin volcanic layers observed at this time. The straight lines on the temperature profile represent existence temperatures for polar stratospheric clouds containing nitric acid trihydrate (Hanson and Mauersberger 1988) and water ide, assuming the vapor concentrations indicated.

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ozone is lost with more than 80 percent of the loss occurring between 12 and 20 kilometers. While above 20 kilometers ozone is quite variable because of polar vortex movements, the ozone at 12 kilometers and below has been very constant, except for occasional soundings with a higher than normal tropopause. The stability of this low altitude ozone changed in 1991. Figure 3 shows that ozone between 11 and 13 kilometers in 1991 is clearly lower than the previous four-year record. This change began when the volcanic aerosols first appeared and after one month ozone at these altitudes was reduced by 50 percent, compared with the previous four years. The half-life for ozone decay in this region was 30-40 days, one-half to two-thirds the rate observed for the decay of ozone at 18 kilometers resulting from chlorine processing by polar stratospheric clouds. Hofmann and Solomon (1989) have suggested that the presence of volcanic aerosols can lead to ozone depletion because of heterogeneous reactions on the volcanic aerosol surface. To our knowledge the measurements presented here, and more completely in Deshler et al. (1992), are the first direct in situ measurements confirming that volcanic aerosols can play a part in ozone destruction. Gratitude is extended to G. Di DonFrancesco, B. Johnson, L. Womack, and R. Thompson for help with the measurements in Antarctica. This research was supported by the National Science Foundation grant DPP 90-17805 and the Italian National Program for Antarctic Research under the FAADR grant.

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References

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Deshler, T., A. Adriani, D. J . Hofmann, and G. P. Gobbi. 1991. Evidence for denitrification in the 1990 antarctic spring stratosphere: II Lidar and aerosol measurements. Geophysical Research Letters, 18:1,9992,002. Deshler, T., A. Adriani, C. P. Gobbi, D. J . Hofmann, C. Di Donfrancesco, and B. J . Johnson. 1992. Volcanic aerosol and ozone depletion within the antarctic polar vortex during the austral spring of 1991. Geophysical Research Letters, 19:1,819-1,822. Gobbi, C. P., T. Deshler, A. Adriani, and D. J . Hofmann. 1991. Evidence for denitrification in the 1990 antarctic spring stratosphere: I Lidar and temperature measurements. Geophysical Research Letters, 18:1,9951,998. Hanson, D. R. and K. Mauersberger. 1988. Laboratory studies of the nitric acid trihydrate: Implications for the south polar stratosphere. Geophysical Research Letters, 15:855-858. Hofmann, D. J . and S. Solomon. 1989. Ozone destruction through heterogeneous chemistry following the eruption of El Chichon. Journal of Geophysical Research, 94:5,029-5,041. Johnson, B. J., T. Deshler, R. L. Thompson. 1992. Vertical profiles of ozone at McMurdo Station, Antarctica; spring 1991. Geophysical Research Letters, 19:1,105-1,108.

0.1 0.0 230 240 250 260 270 280 290 300 310 DAY NUMBER Figure 3. Temporal history of 0.5-kilometer averages of the 1991 ozone mixing ratio, 10-13 kilometers, compared with ozone measurements collected 1987 to 1990. lively undisturbed through the end of the measurements in October. During each antarctic spring since 1986 approximately 40 ozone profiles have been measured at McMurdo (Johnson et al. 1992). The results of these measurements have been quite consistent, indicating that during years of severe ozone depletion1987, 1989, 1990, 1991-approximately half the total column of

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