Chemistry Of The Stratosphere

The stratosphere is often referred to as the "ozone layer," because of the relatively high concentrations produced by photochemical reactions in this region of the atmosphere. Ozone, derived from the Greek word meaning "to smell," was first discovered by Schonbein in 1839. It has a rather pungent smell, which is sometimes noticeable around copy machines and laser printers that use discharge processes.

As described earlier, in the stratosphere, a steady-state concentration of 03 is produced naturally by the Chapman cycle, reactions (l)-(4). Until about 1970, relatively little attention was paid to potential anthropogenic (i.e., man-made) perturbations of the stratosphere. At that time, Crutzen (1970) examined the potential role of NO and N02 formed in the stratosphere from reactions of NzO that was originally generated at the earth's surface. Because N20 is unre-active in the troposphere, it has a sufficiently long lifetime to end up in the stratosphere, where it can be converted into NO (see Chapter 12). Crutzen (1970) proposed that the NO and N02 formed from reactions of N20 can then participate in a chain reaction that destroys 03:

Subsequently, Johnston (1971) suggested that NOx emitted directly into the stratosphere from supersonic transport aircraft (SSTs) could decrease the steady-state concentration of 03 via reactions (8) and (32), leading to increased UV radiation at the earth's surface. Although the number of SSTs that ultimately were produced was far less than originally anticipated, the same issue of NOx destruction of 03 has again been revisited with respect to the possible future production and use of high-speed civil transport (HSCT) aircraft (see Chapter 12).

In 1974, Cicerone and Stolarski suggested that if there were sources of atomic chlorine in the stratosphere, the following catalytic ozone destruction cycle could occur:

Shortly thereafter, Molina and Rowland (1974) published their seminal paper in which they showed that, because of the lack of removal mechanisms for these inert chemicals in the troposphere, chlorofluorocarbons (CFCs) are expected to reach the stratosphere. Once in the stratosphere, the CFCs are exposed to UV radiation in the region in which they absorb, leading to the production of chlorine atoms:

From the outset, their hypothesis caused worldwide concern because UV-B can damage DNA, cause skin cancer, including malignant melanoma in humans, and have long-term effects on the eye, in addition to its impact on a variety of ecosystems.

This foundation of the interaction between tropo-spheric processes and emissions and stratospheric chemistry established by Crutzen, Molina, and Rowland led to their sharing the 1995 Nobel Prize in Chemistry.

Until the mid-1980's, estimates of the reduction in the steady-state concentration of stratospheric 03 were based largely on the application of large computer models that incorporated the known mechanisms and kinetics of individual chemical reactions. Because of natural variations in stratospheric ozone, unequivocal detection of the expected change is quite difficult. However, in 1985 a remarkable observation was made by a group of British researchers (Farman et al., 1985) who had been monitoring total column ozone in the Antarctic. Total column ozone is the integrated ozone measured in a vertical column throughout the atmosphere and is often expressed in Dobson units (DU), where 1 DU is equivalent to a column of 03 of 10"5 m height at STP (1 atm pressure and 273.15 K). Since approximately 85-90% of the total column ozone is in the stratosphere, changes in column ozone are particularly sensitive to stratospheric ozone levels.

Figure 1.7 shows the total column ozone measured in October at one Antarctic location, Halley Bay, as a function of year. This includes the original Farman et al. (1985) data, as well as more recent data up to 1994 (Jones and Shanklin, 1995). It is clear that starting in the late 1970's, there was a dramatic drop in total column ozone at the end of the polar winter when sunrise occurs. Observation of such a rapid change is unprecedented and quite remarkable.

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