Box 3.1 How Much Ozone Is There in the Stratosphere?

If all the ozone in the atmosphere were brought down to sea level, it would be merely 2.5 to 4.5 mm thick. However, this amount of ozone can absorb most of the harmful UV radiation entering Earth's atmosphere. Atmospheric scientists use the Dobson Unit (DU) to measure the amount of ozone overhead, i.e., from the ground to outside the atmosphere. 1 DU is equal to 1 millicentimeter of ozone at sea-level pressure (0°C). So, in general, the amount of column ozone is between 250 and 450 DU. In Antarctica, the ozone column has been measured to be as low as 125 DU, i.e., a reduction of ozone by a factor of two or three.

The potential for ozone depletion in the stratosphere became an important topic when the first commercial supersonic transport aircraft were proposed in the late 1960s.1 At that time, scientists noted that nitrogen oxide (NOx) emissions from engine exhaust associated with aircraft operating in the stratosphere could be involved in a catalytic cycle leading to ozone loss.2 This realization helped lead to the U.S. withdrawal from this potential market. In 1974, Rowland and Molina3 proposed that chlorofluorocarbons (CFCs) emitted by human activities at Earth's surface could pass though the troposphere to the stratosphere. In the stratosphere, UV radiation from the sun is absorbed by the C-Cl bonds in CFCs, leading to rupture of the bonds and liberation of Cl atoms. The Cl atoms can participate in a catalytic cycle that leads to conversion of ozone to oxygen molecules. Later laboratory studies showed that bromine and iodine atoms also participate in similar reactions and can also affect ozone.4 The recognition that anthropogenic emissions of gas could lead to destruction of ozone in the stratosphere led to the Nobel Prize in chemistry being awarded in 1995 to P. Crutzen, M. Molina, and F.S. Rowland.

Although the amounts of CFCs and halons released into the atmosphere are small in terms of the total amount of gas there, they have a great impact on the global ozone balance for three reasons: (1) Ozone is in a constant state of "flux"—it is made and destroyed by natural processes that define a delicate balance (Box 3.2). (2) The production of ozone is controlled by solar input that does not undergo dramatic fluctuations. Removal of ozone from the atmosphere is controlled by catalytic processes, set in motion by small concentrations of natural and synthetic chemicals that can destroy a large number of ozone molecules without being destroyed in the process. Depletion is accelerated when the halogen atoms chlorine, bromine, and iodine are present; thus changes in the "balance" lead to a lower level of ozone. (3) A large fraction of the anthropogenic (and natural) reagents released at Earth's surface can be transported to the stratosphere if they are chemically stable in the troposphere. Because halons and CFCs are very stable in the troposphere, a large fraction of the released amounts reach the stratosphere, where they are quickly broken apart to release halogens that are active in destroying ozone. The parameter that defines how much of the released amounts reach the stratosphere is the atmospheric lifetime (Box 3.3).

Initially, ozone depletion was just a hypothesis based on laboratory data. It was unclear whether other trace gases in the atmosphere would interfere with the ozone removal cycles. Starting in the early 1970s, numerical models were developed to simulate the interactions of trace gases under atmospheric conditions, and the models showed that ozone depletion should occur. The trace gas concentrations simulated by these models compare favorably with measured concentrations, which lends credence to the models. However, direct comparison of model-calculated change in ozone with observed change is more difficult. Because the expected globally averaged ozone decrease is small, on the order of a few percent over a decade, detecting such change in the atmosphere requires extracting this long-term trend from large seasonal cycles (~30%) and interannual variations (~10%). It was not until the early 1980s that ground-based and satellite data were sufficient to determine a clear trend.

Direct measurements now show that stratospheric ozone depletion has occurred during the past two decades.5 In fact, the extent of ozone depletion is larger than predicted by the models based on gasphase chemistry. It has been shown in laboratory experiments that heterogeneous (gas-particle) chemistry on cold particles can occur such that a Cl atom can destroy even more ozone molecules than



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