The observed long-term trend in mid-latitude Northern Hemisphere ozone in the lower stratosphere over the last two decades is the result of increased catalytic loss of ozone in the lower stratosphere by the rate limiting halogen radicals ClO, BrO, and IO. The observed variability of ozone in the lower stratosphere is the result of factors that modulate those halogen radicals, specifically water vapor, aerosol loading and temperature.
This hypothesis, the correctness of which will be tested in the course of this proposed research, addresses a central tenet in the latest WMO report: Scientific Assessment of Ozone Depletion: 2002 that has just been released. In the Scientific Summary for the key Chapter 4, under, “Attribution of Past Changes in Ozone,” on page 4.2, it is stated:
- “The vertical, latitudinal, and seasonal characteristics of changes in mid-latitude ozone are broadly consistent with the understanding that halogens are the primary cause of these changes, in line with similar conclusions from the 1998 Assessment.”
- “Assessment models forced by observed changes in halocarbons, source gases, and aerosols broadly reproduce the long-term changes observed in midlatitude total column ozone (35°N–60°N and 35°S–60°S) for 1980–2000, within the uncertainties of the observations and model range. However, the range of model results is large over SH midlatitudes, which is at least partly due to their treatment of the Antarctic ozone hole. In addition, models suggest that the chemical signal of ozone loss following the major eruption of the Mt. Pinatubo volcano in the early 1990s should have been symmetric between hemispheres, but observations show a large degree of interhemispheric asymmetry in midlatitudes.”