High-altitude ozone destruction

Halogenated solvents are an example of a class of chemicals that with a few exceptions (such as methyl chloride, CH3Cl, produced by some marine algae) are not naturally present in the atmosphere. Currently, such solvents are released in large amounts (Table 4-4). Many are degraded in the lower atmosphere (Section 4.6). Some, however, such as the CFCs, pose a unique set of problems because their low reactivity allows them to escape destruction in the troposphere therefore, they can enter the stratosphere, where they take part in a catalytic cycle that destroys beneficial high-altitude ozone (Section 4.6.4). 

For example, Wennberg et al. (1997) used high-resolution spectra taken from the Kitt Peak National Solar Observatory to search for evidence of IO. Combined with simulations using assumed IO chemistry, they conclude that the total stratospheric iodine is 0.2 ppt, with an upper limit of 0.3 ppt. Similarly, Pundt et al. (1998) conclude there must be <0.2 ppt iodine at altitudes <20 km, based on solar spectra obrained using balloon platforms. If these small concentrations based on a few measurements are typical, iodine will not be responsible for significant ozone destruction. 

The major chemical cycles describing catalytic ozone destruction by HOx, NOx and ClOx are now reasonably well characterized. The development of sophisticated computational models has facilitated the unification of field studies with laboratory experiments. At high altitudes where the... 

What actually happens is that NO, destroys ozone at high altitude but forms it at low altitude. As a result, whether there is a net increase or decrease in total ozone depends on the altitude at which NO, is injected. The transition from ozone formation to destruction appears to occur somewhat below 20 km, the precise level depending on details of the mathematical model (Johnston and Podolske, 1978). Thus the flight altitude of a supersonic transport is critical NO, from sources at the Earth s surface always seems to increase ozone (Turco et ai, 1978). 

Another group of compounds that can destroy stratospheric ozone are the nitrogen oxides, generally denoted as NO. (Examples of NO are NO, NO2, N2O, and N2O5.) These compounds come from the exhausts of high-altitude supersonic aircraft and from human and natural activities on Earth. Solar radiation decomposes a substantial amount of the other nitrogen oxides to nitric oxide (NO), which participates in the destruction of ozone as follows ... 

Following the discovery of the Antarctic ozone hole in 1985, atmospheric chemist Susan Solomon led the first expedition in 1986 specifically intended to make chemical measurements of the Antarctic atmosphere by using high-altitude balloons and ground-based spectroscopy. The expedition discovered that ozone depletion occurred after polar sunrise and that the concentration of chemically active chlorine in the stratosphere was l(X) times greater than that predicted from gas-phase chemistry. Solomon s group identified chlorine as the culprit in ozone destruction and polar stratospheric clouds as the catalytic surface for the release of so much chlorine. 

The research of Paul Crutzen, the third recipient of the Nobel Prize for Chemistry in 1995, involved the effect of nitric oxide (NO) on the destruction of stratospheric ozone. Unlike CFCs, which may take 50 to 100 years to diffuse into the upper atmosphere, nitric oxide is introduced directly to the stratosphere in the exhaust of high-altitude aircraft. Early in the 1970s, the United States considered construction of a large fleet of supersonic transport airplanes (SSTs), similar to the Concorde. Environmentalists argued, based in part on the work of Paul Crutzen, that to do so would significantly endanger the ozone layer. 

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