Not until 1973 was chlorine found to be a catalytic agent in ozone destruction. Catalytic destruction of ozone removes the odd oxygen species [atomic oxygen (O) and ozone (O3)] while leaving chlorine unaffected. This process was known to be potentially damaging to the ozone layer, but conclusive evidence of stratospheric ozone loss was not discovered until 1984. Announcement of polar ozone depletion over Antarctica in March 1985 prompted scientific initiatives to discover the Ozone Depletion Processes, along with calls to freeze or diminish production of chlorinated fluorocarbons. A complex scenario of atmospheric dynamics, solar radiation, and chemical reactions was found to explain the anomalously low levels of ozone during the polar springtime. Recent expeditions to the Arctic regions show that similar processes can occur in the northern hemisphere, but to a somewhat lesser degree due to warmer temperatures and erratic dynamic patterns.
A primary objective for researchers in addressing this issue has been analysis of Measurements and Trends in Ozone and Chlorofluorocarbon Levels. Global monitoring of ozone levels from space by the Total Ozone Mapping Spectrometer (TOMS) instrument has shown statistically significant downward trends in ozone at all latitudes outside the tropics. Measurements at several ground-based stations have shown corresponding upward trends in CFCs in both the northern and southern hemisphere. Despite rapid phaseout of CFCs, ozone levels are expected to be lower than pre-depletion levels for several decades due to the long tropospheric lifetimes of CFCs. These compounds are carried into the stratosphere, where they can undergo hundreds of catalytic cycles involving ozone before being scavenged by other chemical species.
Replacement compounds for CFCs have also been evaluated for their Ozone Depletion Potential (ODP). Hydrochlorofluorocarbons (HCFCs) still contain chlorine atoms, but the presence of hydrogen makes them reactive with chemical species in the troposphere. This greatly reduces the prospects of the chlorine reaching the stratosphere, as chlorine will be removed by chemical processes in the lower atmosphere. Hydrofluorocarbons (HFCs), potential replacements for CFCs that contain no chlorine, have been evaluated for potential effects of fluorine compounds on ozone destruction. McFarland and Kaye give an overview of the CFC-ozone issue in the 1992 paper "Chlorofluorocarbons and Ozone."