Ozone is a triatomic gas consisting of oxygen: O3 is its chemical symbol. It is only present in the atmosphere at very low concentrations, peaking at only 10 parts per million in the stratosphere, but it absorbs ultraviolet (UV) radiation from the sun, allowing life on Earth to flourish.
Humans have affected the Earth's ozone layer by emitting pollutants, especially chloroflurocarbons (CFCs) into the atmosphere. At first, these were thought to be relatively harmless, but their impacts became gradually know, most spectacularly with the formation of the Antarctic ozone hole in the early 1980s. Many objections to the scientific theories have been put forward. Here those objections are answered.
1. How can CFCs reach the upper atmosphere, if they are heavier than air?
It is true that CFCs are more dense than air. In the lower atmosphere (the troposphere) chemicals generally become well-mixed, despite being heavier than air. Once in the stratosphere CFCs and other chemicals are controlled by atmospheric circulation which transports the from the tropical troposphere to the upper stratosphere and then to high latitudes and down over the poles.
At altitudes greater than about 30 km (19 miles), the CFCs are broken down by the harsh UV from the sun, releasing chlorine atoms. Observations from the CLAES instrument on the Upper Atmosphere Research Satellite, shows the decay of CFCs with height. Other instruments on UARS and other satellites have shown the corresponding increase in active chlorine chemicals.
2. There are hardly any CFCs in the atmosphere. How can they affect ozone?
It is true that the CFCs supply chlorine at about one thousandth the concentration of ozone in the stratosphere. However, ozone destruction processes are catalytic: chlorine participates in the reactions which destroy ozone, but it is unaffected by the overall process:
Cl + O3 --> ClO + O2 ...................................................R1
ClO + O --> Cl + O2 ......................................................R2
Net: O + O3 --> 2O2
Hence, the Cl atom is present at the beginning of the 2 reactions.
3. How could the Antarctic zone hole form where the concentration of O atoms is very low?
When the ozone hole was first discovered, it was shown that ozone loss must have occurred in the lower stratosphere, where indeed the concentration of O atoms is too low to destroy ozone according to reactions R1 and R2. Following a set of measurements in Antarctica, it was discovered that high-altitude clouds contributed to the formation of ClO, and ozone loss is able to proceed via a different route.
4. Why don't we see a similar ozone hole in the Arctic?
In the antarctic, the temperature in winter and spring are always low enough to form clouds at the altitude needed to destroy ozone. In the Arctic, this happens infrequently. However, the 2010/2011 Arctic winter was one such winter, indicating that there may be a possible climate link. Ozone loss was particularly large in March 2011, comparable to the early stages of the Antarctic ozone hole.
5. CFC production is now banned. Why hasn't the ozone hole disappeared?
Measurements do indeed indicate that international agreements to limit CFC usage are being obeyed. However, CFCs have a multi-decadal timescale in the atmosphere, and it will not be for at least another 50 years that the antarctic ozone hole will disappear, according to current calculations.
Human Impacts on the Environment
The evolution of ozone in the atmosphere showed categorically that humans had the (unwitting) ability to influence the global atmosphere. On the positive side, the rapid agreement among nations, under the Montreal Protocol (1987) to limit and reverse the problem indicates what can happen when the political will is present.
Climate change is a similar issue facing mankind. In some ways, the Kyoto Protocol (1997) on climate Change took strength from the success of the Montreal Protocol. Unfortunately, climate change is a much tougher problem, and progress has been slow. In the meantime, one way of being an informed citizen is to understand the science of atmospheric ozone, which our lives depend on.
Further Reading
Fahey, D. and Hegglin, M., Twenty Questions and Answers About the Ozone Layer: 2010 Update
WMO/UNEP, Scientific assessment of Ozone Depletion, 2010
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