The abundance of ozone at a particular point in the stratosphere, the column abundance of ozone above a given geographical location, and the total amount of ozone in the stratosphere are controlled by a combination of production, destruction, and transport (of ozone and other chemicals into and out of the region of interest). The major mechanism for the production of ozone in the stratosphere is the breaking up of molecular oxygen (O2) by solar UV of wavelengths less than 242 nanometers (photolysis) to make oxygen atoms (O), followed by the reaction of oxygen atoms with molecular oxygen to make ozone. The destruction of ozone occurs via the reactions of oxygen atoms (O) with ozone (O3) (the Chapman Mechanism), as well as through cyclic chemical reactions involving naturally occurring species such the odd-hydrogen radicals (HOx: OH and HO2), nitrogen oxide radicals (NOx: mostly NO and NO2), and/or halogen radicals. The radicals are produced in the stratosphere by photolysis and oxidation of source gases (N2O, H2O, CH4, and a variety of chlorine- and bromine-containing compounds). In the absence of interference from the human emissions influencing the abundance of catalysts, there is a natural balance and this balance determines the ozone abundance in a location, the column amount over a region, and the total amount of ozone in the stratosphere. The natural amounts vary on a variety of timescales: daily variations in the ozone column are driven by meteorological variability ("weather"); seasonal variations are driven by changes in stratospheric temperature and winds; multiannual variations are driven by changes in solar input, by natural variations in the emissions of the source gases, and by interannual variability in stratospheric winds.
The natural abundance of stratospheric ozone can be changed by human influence. This change can be brought about by changes in production, destruction, and transport. The ozone abundance arises from a balance between these terms. Human emissions, for the most part, have led to an enhancement in the destruction term, shifting the balance to lower ozone abundance. Thus, any human emission of chemicals (gases or particles) that contributes to the enhancement of the ozone destruction term in the balance leads to a lowering of ozone, i.e., ozone layer depletion, and is evidenced by changes in the amount at a location, in the column amount above a location, or the total amount in the stratosphere. Because the destruction occurs through catalytic cycles that regenerate the ozone-destroying radicals multiple times, small changes in the source gases (and hence in radical concentrations) can have a large impact on ozone.
If there is an increase in concentrations of any of these source gases that contribute nitrogen, hydrogen, or halogen radicals to the stratosphere, there will be an increase in ozone-destroying radicals and hence in stratospheric ozone destruction. Changes in the sources gases could occur either naturally (e.g., by biogenic processes at the surface) or anthropogenically (by increased industrial emissions); some source gases are emitted both naturally and anthropogenically. The response of the stratosphere does not depend on whether the changes are natural or anthropogenic; the stratosphere does not "care." However, scientists and policymakers do care and in some circumstances it is useful to have a terminology that distinguishes the different origins of the source compounds. Therefore, ozone-depleting substances (ODSs) are those whose emissions come from human activities.
It will be important in the Assessment also to consider specifically gases that have been regulated (and which traditionally we have called ODSs). Thus, the Montreal Protocol has controlled the production (and hence their emissions into the atmosphere) of certain chemicals that are listed as controlled substances in Annexes A, B, C, and E of the Protocol. We will continue to call the controlled substances of the Montreal Protocol as ozone-depleting substances, or ODSs for short. This definition keeps the continuity in usage and will be clear to the Parties to the Montreal Protocol.
The above description yields a few key points. First, the ozone abundance can be changed not only by destruction but also via influence on production, transport, and stratospheric climate. There are long-standing examples of such production enhancements by hydrocarbons, in particular methane, via what is usually called smog chemistry (i.e., the chemistry that leads to the tropospheric pollutant ozone production). Second, ozone abundances can be changed by both changes in the concentrations of active agents, as well as by changes in the rates at which these chemical reactions occur. The most noteworthy way is by changes in the stratospheric climate (i.e., temperature), such as that caused by the enhancements in carbon dioxide in the atmosphere. Third, the ozone abundance can be influenced by changes in transport, such as that arising from a changing climate. And fourth, while the Montreal Protocol controls many substances that deplete ozone, not all such substances are currently controlled and, for clarity, they are not called ODSs here. Reference to such substances are clearly noted in this Assessment.