R. W. Sanders

Mesospheric ozone depletion during the Solar Proton Event of July 13, 1982 Part I Measurement

The near infrared spectrometer and the ultraviolet spectrometer on the Solar Mesosphere Explorer (SME) observed the ozone density as a function of latitude and altitude during the solar proton event of July 13, 1982. Airglow at 1.27 µm was observed at the earths limb. The altitude profiles of the emission were inverted providing ozone densities. The ozone densities observed showed a clear decrease during the event. The maximum depletion seen was 70%. The decrease was observed in the northern high latitudes at mesospheric altitudes. The decrease was very short lived, lasting less than a day.

Observations of the Nighttime Abundance of OClO in the Winter Stratosphere Above Thule, Greenland

Observations at Thule, Greenland, that made use of direct light from the moon on 2,3, 4,5, and 7 February 1988 revealed nighttime chlorine dioxide (OClO) abundances that were less than those obtained in Antarctica by about a factor of 5, but that exceeded model predictions based on homogeneous (gas-phase) photochemistry by about a factor of 10. The observed time scale for the formation of OClO after sunset strongly supports the current understanding of the diurnal chemistry of OClO. These data suggest that heterogeneous (surface) reactions due to polar stratospheric clouds can occur in the Arctic, providing a mechanism for possible Arctic ozone depletion.

Observations and interpretation of column OClO seasonal cycles at two polar sites

OClO and NO2 slant column abundances have been measured by ground-based spectrographs in both the Arctic (Kangerlussuaq, Greenland, at 67.0°N, 51.0°W) and the Antarctic (McMurdo Station, Antarctica, at 77.8°S, 166.6°E). A key process for stratospheric ozone depletion is the activation of chlorine by heterogeneous reactions. Sunlight is also an essential ingredient in the catalytic cycles that destroy ozone. This data set illustrates the seasonal timing of autumnal chlorine activation and the springtime disappearance of active chlorine at two polar sites. As such, the data provide constraints for model calculations. In the Northern Hemisphere during the winters of 1994–1995 and 1995–1996, the OClO data show the seasonal onset of stratospheric chlorine activation in mid-December and seasonal disappearance of active chlorine in mid-March. The data also show the seasonal decline and recovery (in mid-March) of nitrogen dioxide levels. In the Southern Hemisphere during the winters of 1991, 1993, and 1996, similar chemical processing occurs during late April to early May and late September to early October. The role of several factors in determining the seasonal cycles of column OClO are probed through model studies. Observed cold temperatures and reactions on liquid sulfate aerosols appear to play a key role in producing the timing of the observed OClO seasonal cycle in the Northern Hemisphere.

Observations of Stratospheric NO2 and O3 at Thule, Greenland

Scattered sunlight and direct light from the moon was used in two wavelength ranges to measure the total column abundances of stratospheric ozone(O3) and nitrogen dioxide (NO2) at Thule, Greenland (76.5�N), during the period from 29 January to 16 February 1988. The observed O3 column varied between about 325 and 400 Dobson units, and the lower values were observed when the center of the Arctic polar vortex was closest to Thule. This gradient probably indicates that O3 levels decrease due to dynamical processes near the center of the Arctic vortex and should be considered in attempts to derive trends in O3 levels. The observed NO2 levels were also lowest in the center of the Arctic vortex and were sometimes as low as 5 x 1014 molecules per square centimeter, which is even less than comparable values measured during Antarctic spring, suggesting that significant heterogeneous photochemistry takes place during the Arctic winter as it does in the Antarctic.