Ozone depletion occurs when the processes that destroy stratospheric ozone become more effective than the processes that produce and replenish it. Natural ozone destruction occurs continuously through photochemical reactions, so depletion does not mean that ozone destruction suddenly begins. Instead, the term generally refers to a sustained reduction in ozone concentrations caused by changes in atmospheric chemistry, circulation or other environmental conditions.
The most important human caused mechanism involves ozone depleting substances such as chlorofluorocarbons, halons, carbon tetrachloride, methyl chloroform and methyl bromide. These compounds are relatively stable in the lower atmosphere, allowing them to persist long enough to be transported into the stratosphere. There, energetic ultraviolet radiation can break their molecules apart and release chlorine or bromine atoms and reactive compounds. These halogen species can participate in catalytic cycles that destroy ozone far more efficiently than ordinary direct reactions would allow.
Chlorine and bromine are especially important because they can act catalytically. In a catalytic reaction cycle, a reactive species participates in a reaction that destroys ozone but is regenerated during the overall process, allowing the same atom to destroy many ozone molecules. A simplified chlorine cycle involves a chlorine atom reacting with ozone to form chlorine monoxide and molecular oxygen. Chlorine monoxide can then react through pathways that regenerate atomic chlorine while converting additional ozone into oxygen.
Similar processes involving bromine can be extremely effective at destroying ozone. The significance of catalysis is that relatively small concentrations of reactive halogens can cause substantial ozone loss. The chlorine and bromine atoms do not need to be consumed in a one to one reaction with every ozone molecule. Their regeneration allows them to function repeatedly, making the chemistry disproportionately powerful compared with their atmospheric abundance.
Polar ozone depletion is strongly enhanced by extremely cold conditions in the winter stratosphere. Over Antarctica, temperatures can become low enough for polar stratospheric clouds to form. Chemical reactions on the surfaces of particles in these clouds convert relatively inactive chlorine reservoir compounds into forms that can rapidly release reactive chlorine when sunlight returns. During the dark polar winter, chlorine can accumulate in chemically activated forms without immediately destroying large quantities of ozone because sunlight is limited.
In Antarctic spring, sunlight returns while temperatures can remain sufficiently low for rapid catalytic chemistry. Chlorine monoxide concentrations can become high, and ozone destruction can proceed rapidly over a large area. The Antarctic ozone hole is therefore produced by a combination of human caused halogen loading, very low temperatures, polar stratospheric clouds, sunlight and specialised polar atmospheric circulation.
The Arctic can also experience ozone depletion, but the circumstances differ from Antarctica. Arctic winters vary substantially in temperature and atmospheric circulation, and the polar vortex is generally less stable than the Antarctic counterpart. When the Arctic stratosphere becomes sufficiently cold for extensive polar stratospheric cloud formation, substantial ozone loss can occur. However, the magnitude varies considerably from year to year.
Natural atmospheric dynamics can transport ozone rich or ozone poor air into and out of polar regions, making the observed concentration dependent on both chemical destruction and circulation. This distinction is important because a low ozone measurement does not necessarily mean that all of the reduction was caused locally by chemical destruction. Scientists use observations and atmospheric models to separate chemical loss from transport effects and to determine the contribution of different processes.
Human produced halogenated chemicals are not the only factors affecting ozone. Volcanic eruptions can inject sulphur dioxide into the stratosphere, where it can form sulphate aerosols. These particles provide surfaces that can facilitate chemical reactions involving chlorine and bromine, potentially enhancing ozone depletion after major eruptions. Nitrogen oxides, hydrogen containing compounds, temperature changes and variations in atmospheric circulation can also influence ozone chemistry. Changes in climate can alter stratospheric temperatures and circulation, thereby modifying chemical reaction rates and transport. The relationship between climate change and ozone is therefore complex. Some greenhouse gases warm the lower atmosphere while cooling the stratosphere, and these temperature changes can influence ozone chemistry. Ozone itself also affects radiative heating and circulation, creating feedbacks between atmospheric composition and dynamics.
Natural processes also cause ozone variability. Solar ultraviolet output changes over the approximately eleven year solar cycle, altering the photochemical environment in which ozone forms and is destroyed. Atmospheric waves can modify stratospheric circulation and transport ozone between regions. Temperature fluctuations can change reaction rates and the formation of polar stratospheric clouds. These natural influences mean that ozone concentrations vary even in the absence of human caused depletion.
Scientists therefore distinguish short term natural variability from long term depletion caused by persistent ozone depleting substances. Long term monitoring using satellites, ground instruments, balloons and atmospheric models is essential for identifying trends. Without observations over many years, natural fluctuations could be mistaken for evidence of permanent recovery or deterioration.
International controls have substantially changed the chemical drivers of ozone depletion. The Montreal Protocol, adopted in 1987 and subsequently strengthened through amendments, established controls on the production and consumption of many ozone depleting substances. Because many of these chemicals have long atmospheric lifetimes, their effects do not disappear immediately after emissions decline.
Concentrations of several important ozone depleting substances have nevertheless responded to international controls, and the scientific assessment of ozone recovery indicates that the overall trajectory has improved compared with continued unrestricted emissions. The recovery process remains dependent on atmospheric chemistry and circulation, and unexpected emissions, changing climate conditions or other influences can affect regional behaviour. Continued compliance, monitoring and scientific assessment are therefore important components of ozone protection.
Ozone depletion is therefore caused by an imbalance in atmospheric ozone chemistry, with human released chlorine and bromine compounds being the principal cause of the major long term depletion observed since the twentieth century. The strongest seasonal depletion occurs when these compounds interact with cold polar conditions, polar stratospheric clouds and sunlight. Natural variability, volcanic aerosols, atmospheric circulation, solar changes and climate related effects modify the overall pattern.
Ozone depletion should also be distinguished from the ordinary daily destruction and formation of ozone, because natural ozone chemistry is continuous. The central scientific issue is the enhanced catalytic destruction produced by reactive halogens released from persistent human made compounds. Understanding this mechanism explains both the severity of historical ozone depletion and why controlling emissions of ozone depleting substances can allow the stratospheric ozone system to recover gradually over time.
