Appendix B
Stability of Ozone

Biological processes influence both the generation and the destruction of ozone. Ozone formation is an endothermic process, one that requires energy to initiate the reaction:

where delta-H = the heat of reaction.

In other words, reactions involving ozone decomposition are thermodynamically favorable:

where O* is a reactive oxygen atom that reacts readily with water, methane, and nitrous oxide as follows:

where OH*, CH₃*, and NO* are hydroxyl, methyl, and nitric oxide radicals [82] [83], respectively. The enthalpy, that is, H, or the value of the heat of the reaction, demonstrates why nitrous oxide is a far better ozone scavenger than methane and water vapor. The rate of forward reaction, or ozone decomposition to a molecular oxygen and reactive oxygen atom, is several orders greater than the backward reaction, or ozone formation from molecular oxygen and reactive oxygen atoms [84].

Hydrocarbons and/or oxygenated hydrocarbons readily react with ozone in the troposphere, producing alkyl and hydroxyl radicals. It has recently been reported that airborne measurements showed very high concentrations of oxygenated hydrocarbons in the global troposphere. This surprisingly abundant presence of such oxygenated hydrocarbons (acetone, ethanol, methanol, and acetaldehyde), which may affect the “ozone cycle” may be due to land-use change, biomass burning, and alcohol-based biofuel use [85].

Halogenated hydrocarbons such as chlorofluorocarbons (CFCs) produce very reactive halogen atoms during photolysis. These halogen atoms are excellent ozone scavengers. Each CFC molecule can destroy 10 thousand molecules of ozone or more by chain reaction during its lifetime. Hydrochlorofluorocarbons (HCFCs) and perfluorocarbons (PFCs) are preferred alternatives to CFCs because they are roughly 10 times less destructive to ozone. According to a new study, CFC alternatives pose a potential risk of accumulating trifluoroacetic acid (CF₃COOH), a breakdown product, in certain wetlands [86].

Unlike CFCs, atmospheric methyl bromide (CH₃Br), another halogenated hydrocarbon, is not entirely anthropogenic in origin. About one-third comes from automobile emissions from leaded gasoline and fumigation. Atmospheric methyl bromide is the chief source of stratospheric bromine. Each bromine atom is roughly 50 times more destructive than a chlorine atom, which is responsible for 20 to 25 percent of the “Antarctic Ozone Hole” [87].

Proceed to Appendix C