Ozone is produced by the action of ultraviolet radiation on molecular oxygen (O2). That production is balanced by loss due to catalytic reactions of the oxides of hydrogen, nitrogen, chlorine, and bromine. In Chapter 5 on Stratospheric Ozone Photochemistry we saw that these oxides are produced by the destruction of longer lived "source" gases which are transported upwards from the troposphere. These source gases include methane (CH4) and water for hydrogen oxides; nitrous oxide (N2O) for nitrogen oxides; CFCs, HCFCs, and methyl chloride (CH3Cl) for chlorine oxides; and methyl bromide (CH3Br) and halons for bromine oxides. Figure 10.01 shows a schematic diagram of the chemical production and loss cycle for stratospheric ozone.
The source gases have the properties that they are relatively nonreactive in the troposphere, not water soluble, and don't absorb visible or near-ultraviolet light. This gives them long atmospheric residence times (from 1 year to hundreds of years) and allows at least a portion of them to be transported to the stratosphere where they can release their potential catalysts. The concentrations of many of these source gases are observed to be increasing. These increases can be directly linked to human activities. In the case of CFCs, which are entirely industrially produced, this link is well understood. In other cases, such as methane, the links are less well understood since they often involve interactions between human activities and natural ecosystems. This chapter will explore some of the origins and issues associated with the source gases that have an influence on stratospheric ozone levels.
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The concentration of a gas in the atmosphere is determined by a balance between the rate at which it is being introduced into the atmosphere (its source) and the rate at which it is being removed from the atmosphere (its sink). The source of virtually all gases to the atmosphere is the surface of the Earth. Evidence based on noble gases (e.g. argon, neon) indicates that the original atmospheric envelope, that began with the formation of Earth, was not retained. The existing atmosphere then accumulated over billions of years from the escape of gases from volcanic vents and fumaroles. Gases are being continually emitted from such vents as well as from vegetation, the oceans and wetlands, animal digestive processes, and industrial processes.
Gases that are emitted into the atmosphere reside there for a time, may be transformed by chemical reactions, and are eventually removed by some type of loss process. Loss processes include surface deposition, rainout of soluble materials, and oxidation.
Gases come into contact with Earth's surface and can be removed by undergoing chemical reaction or dissolving in surface waters or being taken up by plants and converted to another form. An example of a gas whose concentration is modified by deposition is carbon dioxide (CO2), which is taken up by plants during photosynthesis. The plant uses the carbon and emits oxygen (O2) gas back to the atmosphere. Animals, on the other hand, breathe in O2 and respire CO2 back to the atmosphere. Carbon dioxide also dissolves in ocean and lake waters where it can be converted to carbonate compounds such as seashells or limestone.
A further important removal mechanism for atmospheric gases is dissolving in rainwater. The dissolved form is rained to the ground and eventually washed away into streams to lakes or the ocean. Many of the noxious pollutants emitted in industrial processes are removed in this way from the atmosphere. Acid rain is the result of dissolution of nonmetal oxides in atmospheric water. Even in unpolluted air, rainfall will be slightly acidic (pH ~5.6) due to the solution of CO2 into the rainwater.
Atmospheric gases may also react with various oxidizing agents in the atmosphere. Earth's atmosphere is like a slow burning flame, in which hydrocarbons are oxidized to CO2 and H2O. The most important of these oxidizing agents is the hydroxyl radical (OH). OH has the important property that it can react with reduced gases like hydrocarbons, removing one of their hydrogen atoms to form water (H2O). The hydrocarbon becomes quite reactive when the first hydrogen has been removed and goes through a rapid chain reaction eventually resulting in CO2 and H2O. This is both good and bad. The oxidation removes the hydrocarbon but also leads to a number of intermediate products which are not particularly healthy to breathe.
Sources gases that ultimately affect ozone concentrations in the stratosphere can be controlled by biological, geological, and chemical processes. We can represent the processes that involve the movement of a particular compound (in its neutral, ionic, or radical forms) through the various Earth systems by means of a biogeochemical cycle. Biogeochemical cycles which can affect Earth's ozone balance are the nitrogen cycle, the carbon cycle, the hydrogen cycle, and the halogen cycles.
Gases are emitted into the atmosphere; they live there for a time; they may be converted to other gases; and they are finally removed, completing a portion of a biogeochemical cycle. An important property of a gas is its atmospheric lifetime (sometimes called residence time). Highly reactive gases are removed quickly, nonreactive gases more slowly. The major gases in the atmosphere, nitrogen (N2) and O2, have very long lifetimes. They are not water soluble; they don't react; and they don't absorb visible or near ultraviolet radiation. Their lifetimes in the atmosphere are measured in the millions of years. Other gases that are eventually transported into the stratosphere have similar properties. They are not water soluble, they are nonreactive in the troposphere, and they don't absorb the visible and near-ultraviolet radiation that penetrates into the troposphere (so that they are not easily photodissociated).
At the other extreme, the complex hydrocarbons (terpenes) emitted by trees have an atmospheric lifetime of hours. They are converted by chemical reactions into a multitude of oxidation products, some with short lifetimes, others with relatively longer lifetimes.
Water soluble gases, such as hydrochloric acid (HCl), nitric acid (HNO3), and hydrogen peroxide (H2O2) can be dissolved in rainwater and are removed from the atmosphere on a time scale of about 5-10 days. Hydrocarbons which react with the oxidant OH have a range of timescales from a few hours for terpenes to 10 years for methane (CH4).
Pollutants that are reactive are generally not very pleasant, but tend to be removed relatively quickly. Longer lived pollutants can be transported over significant distances and can lead to problems which are regional rather than local. A particularly important class of pollutants, for the issue of stratospheric ozone, are those which are very long lived. Pollutants which are nonreactive, insoluble, and don't absorb visible or near-UV radiation can exist in the atmosphere for decades to centuries. Their concentrations will build up in time if they continue to be emitted. Examples of these are the manmade chlorofluorocarbons.
Each year a small fraction of these long lived gases will be transported to the stratosphere where they can be subjected to ultraviolet radiation of much shorter wavelengths (radiation which is absorbed by the ozone layer and thus doesn't reach the ground). This process of absorption may lead to the breakdown of the gas into other, usually more reactive gases. This absorption of ultraviolet radiation and subsequent molecular breakdown process is termed photolysis. As an example, the chlorofluorocarbon-11 (CFC13) absorbs radiation of wavelengths less than 260 nanometers (nm) and dissociates into a reactive chlorine atom (Cl) and reactive CFCl2, which undergoes further rapid reactions to release the rest of the chlorine and fluorine.
Another process that is important in the stratosphere is the reaction with excited oxygen atoms (O(1D)). These excited atoms are produced when ozone (O3) absorbs ultraviolet light and is photodissociated into O2 and O(1D). The O(1D) reacts with a number of relatively long lived molecules, including nitrous oxide (N2O) and H2O. The reaction with N2O produces nitric oxide (NO) while the reaction with H2O produces OH.
The OH produced from the reaction of O(1D) with H2O is another important initiator of chemical reactions in the stratosphere. As in the troposphere, OH reacts with hydrocarbons (HCs) or hydrochlorofluorocarbons (HCFCs), removing a hydrogen atom to form water and a reactive radical that undergoes further rapid reaction.
Radicals are chemical species that contain an odd number of electrons. Although they do not possess a positive or negative charge like ions, they are especially reactive. While many reactions in the troposphere and oceans involve ions, most of the important oxidation reactions in both the stratosphere and troposphere involve reactive radical species, such as OH, HO2, ClO, BrO, NO, and NO2. Although it is also reactive, O3 is not a radical since it has an even number of electrons. An important property of radicals is that they cannot be destroyed except by reaction with another radical. A reaction of a radical and a nonradical must always result in a radical because the two combined have an odd number of electrons.