Ozone Recovery from Merged Observational Data and Model Analysis (OREGANO)

Stratospheric ozone (the “ozone layer”) protects the biosphere from harmful UltraViolet (UV) radiation. Ozone (O3) is expected to recover as a consequence of the Montreal Protocol signed in 1987 and its Amendments regulating the phase-out of ozone-depleting substances (ODS). The stratospheric halogen amount (mainly bromine and chlorine) released by ODSs reached its maximum abundance in the middle of the 1990s. Observations from satellites and the ground confirmed that the long-term decline of stratospheric ozone was successfully stopped. Future, stratospheric ozone levels do not only depend on changes in ODS but also on changes in greenhouse gases (GHG) and possibly stratospheric aerosols. The latter modifies both ozone chemistry and dynamics (transport, circulation) of ozone. The rate of ozone recovery thus depends on the geographic region and altitude. In some altitude domains like the lower tropical stratosphere, ozone will likely continue to decline according to the majority of chemistry-climate models. At middle latitudes, the current trends in lower stratospheric ozone remain highly uncertain in part due to larger uncertainties in observational data and larger year-to-year variability in ozone. The major goal of the OREGANO project is to advance our understanding of ozone recovery using a combination of observations and model analyses. The study topics in this project are: Long-term ozone column and profile trends from models and observations; Impact of atmospheric dynamics and chemistry on polar and extrapolar ozone; Role of tropospheric ozone in column ozone trends; Evaluation of the bromine monoxide – chlorine monoxide (BrO-ClO) cycle using nadir BrO and chlorine dioxide (OClO) observations; Impact of aerosol and GHG changes on stratospheric ozone trends (past and future). Recommendations for future satellite missions and programs will be made following the results of this study in support of continued ozone monitoring.