Stratospheric Chemistry Understanding The Ozone Layer

Ozone was discovered in 1839 by the German scientist Christian Frederich Schonbein at the University of Basel in Switzerland. Because of its pungent odor, its name was taken from the Greek word ozein, meaning "odor." Schonbein's research, subsequent to his discovery, focused on verifying his hypothesis that ozone was a natural trace constituent of the atmosphere. As a result of interest in the late nineteenth century, there are a surprisingly large number of ambient measurements during that time.

The primary study of ozone focused on the chemistry of the stratosphere when it was hypothesized and then verified that most of Earth's ozone was located at an altitude of 20 to 50 km (also called the ozonosphere) high above Earth's surface. The British physicist Sir Sidney Chapman put forth the premise that sufficiently intense ultraviolet radiation [at wavelengths (1); X < 242 nm) breaks apart molecular oxygen into two oxygen atoms. This reaction is commonly written:

where hv is the standard notation for a photon.

Stratospheric Chemistry

Figure 1 Simplified schematic diagram showing the interaction of the chemical families (large boxes) in the stratosphere. The round-cornered boxes define the sources and sinks for the chemical families. Reservoir (longer-lived) trace species are enclosed by ovals. Nitric acid (HN03) is a reservoir species for both reactive hydrogen (HX) and reactive (NX) families; chlorine nitrate (C10N02) is a reservoir species for the reactive chlorine (C1X) and NX families. The chemistry has been simplified by omitting bromine and iodine chemistry from the figure.

Figure 1 Simplified schematic diagram showing the interaction of the chemical families (large boxes) in the stratosphere. The round-cornered boxes define the sources and sinks for the chemical families. Reservoir (longer-lived) trace species are enclosed by ovals. Nitric acid (HN03) is a reservoir species for both reactive hydrogen (HX) and reactive (NX) families; chlorine nitrate (C10N02) is a reservoir species for the reactive chlorine (C1X) and NX families. The chemistry has been simplified by omitting bromine and iodine chemistry from the figure.

As the air becomes denser at lower altitudes in the stratosphere, most of this high-energy radiation is absorbed, and the oxygen molecules can no longer be broken apart. At these altitudes, the oxygen atoms will efficiently combine with the oxygen molecules and the formation of ozone occurs through the reaction:

where M is a nonreactive third body that absorbs any excess collisional energy that may be present. Thus, there is a preferred region in the atmosphere where sufficient ultraviolet energy is concurrently present with the proper amount of molecular density to create ozone, and the altitude region at which these processes are most prevalent is commonly referred to as the ozone layer.

Ozone can also be photolyzed in the atmosphere by weaker ultraviolet radiation (A < 320 nm) to give back molecular and atomic oxygen:

and also by visible radiation (A < 600 nm) to yield atomic oxygen in its ground state, 0(3P), rather than the more energetic 0(lD) state; furthermore ozone can react with atomic oxygen (in either its ground or excited state) to give two molecules of oxygen:

To complete the possible reactions in a "pure oxygen" atmosphere, two atoms of oxygen can combine in a three-body reaction to give molecular oxygen back to the system:

The set of five reactions involving only the various states of oxygen in the stratosphere are commonly referred to as "Chapman chemistry" and did a remarkable job of describing qualitatively why the ozone layer existed where it did. The speeds at which the five reactions took place in the atmosphere were measured independently in the laboratory and are called reaction rate constants (denoted k4 for reaction 4, k5 for reaction 5, etc.). Reaction rate constants are often temperature and pressure dependent. The rates of photolysis are noted by the letter j (e.g., /3 for photolytic reaction 3, etc.) and are primarily dependent on the cross section of the individual molecule as a function of wavelength (those that have weaker bonds and can be broken apart more easily have larger cross sections) and the number of incident photons at those wavelengths (commonly called the photon flux).

As the field of chemistry progressed, other reactions were measured in the laboratory that were also believed to occur in the atmosphere with sufficient speed that they were eventually hypothesized to take an active role in the destruction and formation of ozone. These reactions dealt with derivatives of various forms of hydrogen in the

1 STRATOSPHERIC CHEMISTRY: UNDERSTANDING THE OZONE LAYER 7

stratosphere. The chemistry of the stratosphere was modified accordingly to account for this new "wet photochemistry," which involved reactions being measured in the laboratory, was in the 1950s and 1960s. The rationale behind this new chemistry was that atomic oxygen, O('O), could react with water vapor to form the hydroxyl radical, OH:

Another important source of reactive hydrogen in the stratosphere is degradation of methane CH4, by 0(' D). Regardless of the initial source of the OH radical, it could then react with ozone to form another radical, H02, the hydroperoxy radical, which can lead to a catalytic cycle that becomes an efficient mechanism by which ozone can be removed from the atmosphere:

The above reactions helped to explain some of the observed differences between the measurements that were routinely made in the 1950s and 1960s and the calculated distribution of ozone determined from an oxygen-only atmosphere.

The next major modification to atmospheric chemistry came about from the inclusion of nitrogen chemistry into the reaction scheme of the stratosphere. Nitrous oxide, N20, was known to be a natural trace gas in the troposphere which did not have any identifiable removable mechanisms in lower atmosphere. Consequently, it could drift to the stratosphere where it was eventually attacked by the O('O) atom to form nitric oxide, NO:

With the presence of NO in the stratosphere, another catalytic cycle of ozone destruction could occur through the following reaction sequence:

Net cycle 03 + O 202

The importance of reactive nitrogen chemistry in the stratosphere was independently brought to light circa 1970 by Paul Crutzen, a recent Ph.D. in meteorology at the time from the University of Stockholm, and Harold Johnston, a chemistry professor at the University of California.

These catalytic ozone destruction cycles involving nitrogen and hydrogen species were the impetus behind the Climatic Impact Assessment Program (CIAP) of the

1970s, which became the rationale for determining the potential damage to the ozone layer that might result from flying a fleet of supersonic transport (SST) planes in the lower stratosphere. These planes would emit NO and H20 directly into the stratosphere, and a confederation of U.S. federal agencies was charged with the task of determining how the ozone layer would be harmed by such a fleet. Although economic considerations eventually lay behind the decision for the United States not to pursue the development of a commercial fleet of SSTs, the environmental debate that developed during the early 1970s also contributed to the decision not to pursue the building of this new type of airplane.

But the environmental concern became even more of a reason to spend an increasing amount of money on stratospheric chemistry when Ralph Cicerone and Richard Stolarski, both at the University of Michigan in the early 1970s, introduced the possibility that chlorine chemistry might also provide another important means by which stratospheric ozone might be destroyed:

Shortly after the chlorine cycle was identified as a potential mechanism for stratospheric ozone destruction, Mario Molina and F. Sherwood Rowland, both chemists at the University of California at Irvine, proposed that a group of anthropogenic chlorine-containing compounds could provide the source of significant amounts of chlorine in the stratosphere (Molina and Rowland, 1974). These compounds, known as chlorofluoro-carbons (CFC13 and CF2C12) were used primarily in air-conditioning systems and as propellants for aerosol spray cans that proliferated the use of these compounds in the 1960s. These substances had no known removal mechanism in the troposphere, and Molina and Rowland hypothesized that their only eventual sink would be drifting to the upper stratosphere where they would be destroyed by high-energy ultraviolet radiation resulting in the release of their reactive chlorine atoms into the chemistry of the stratosphere. Figure 1 shows the chemical reactions within each reactive family [e.g., the reactive nitrogen family (NX) the reactive hydrogen family (HX), etc.] and also how each of these individual chemical cycles would influence stratospheric ozone chemistry. The circled trace gas in each box in Figure 1 is the longest-lived species for that particular reactive group. Chlorine nitrate (C10N02) and nitric acid (HN03) are long-lived trace gases that serve as reservoirs of more than one reactive family.

As predicted, the buildup in chlorine led to a "thinning" of the ozone layer. Not predicted by the atmospheric chemists, however, was that the depletion of ozone intensified in the Antarctic stratosphere because of the unique meteorological conditions there. Stratospheric dynamics are such that an enhanced circulation develops during austral winter, which severely inhibits meridional heat exchange (unlike the Northern Hemisphere, where the position of major mountain ranges closer to the pole results in a more favorable situation for heat from middle and low latitudes to be

CI + 03 -> CIO + 02 followed by CIO + O -> CI + 02 Net cycle: 03 + O -> 202

1 STRATOSPHERIC CHEMISTRY: UNDERSTANDING THE OZONE LAYER

transported poleward). Thus, temperatures in the Antarctic lower stratosphere reach temperatures that are cold enough to allow for the formation of polar stratospheric clouds (PSCs) that provide ice surfaces that greatly perturb stratospheric chemistry by turning the long-lived (and relatively nonreactivc) chlorine-containing compounds (chlorine nitrate, ClONO:, and hydrochloric acid. HCi) into chlorine atoms, thereby greatly enhancing the destructive power of the reactive chlorine. Some of the main reactions that are influenced by PSCs are also shown in Figure 1 within the CIX box in the upper left of the figure. The net result has been the formation of the ozone hole whereby more than two-thirds of the normal amount of stratospheric ozone can be destroyed w ithin a period few weeks as the austral winter ends (see Chapter 21, "Stratospheric Ozone Observations"). This phenomenon was first identified from ozonesonde measurements made by Joe Farinan of the British Antarctic Survey in the early 1980s (Farman et al. 1985), By the early 1990s, more than 80% of the chlorine in the atmosphere was determined to be of anthropogenic origin (see Figure 2).

Figure 2 Diagram showing chlorine-containing compounds that release reactive chlorine to the stratosphere and the percentage that each contributes to stratospheric-reactive chlorine family {1994 estimates). Compounds that are completely produced by humans are shaded. Approximately 82% of the chlorine present in the stratosphere is of anthropogenic origin.

Entirely HumanMade

Natural Sources Contribute

Figure 2 Diagram showing chlorine-containing compounds that release reactive chlorine to the stratosphere and the percentage that each contributes to stratospheric-reactive chlorine family {1994 estimates). Compounds that are completely produced by humans are shaded. Approximately 82% of the chlorine present in the stratosphere is of anthropogenic origin.

The environmental problem of stratospheric ozone depletion was successfully addressed by an international treaty in 1987 referred to as the Montreal Protocol, whereby a plan was set forth to phase out and eventually eliminate the manufacture and use of primary ozone-depleting chlorinated compounds (see Albritton et al., 1999). Figure 3 shows how the amount of man-made chlorine has decreased as a result of the effort to minimize the destruction of the ozone layer. As the amount of chlorine goes down in the stratosphere, model predictions suggest that the ozone hole should return to its pre-1980s level by the second or third decade of the new millennium. As a result of their important work on understanding the ozone layer and the chemical processes that drive the formation and destruction of ozone in both the stratosphere and the troposphere, Paul Crutzen, Mario Molina, and F. Sherwood Rowland were awarded the Nobel Prize for chemistry in 1995, the first time that atmospheric chemists received this coveted award. Whereas Rowland and Molina were both trained as chemists, Crutzen is the first meteorologist to receive the Nobel Prize.

Despite the complexity of Figure l, it has been simplified by excluding the chemistry of two other halogen compounds: bromine and iodine. Reactive family chemistry of these halogens is similar to that shown for the reactive chlorine family.

Stratospheric Chemistry

Sampling Date

Figure 3 Monthly hemispheric and global tropospheric chlorine content measured between 1992 and 1996. Because of the international effort to curb human-produced chlorinated gases outlined in the Montreal Protocol of 1987 and subsequent amendments to that original agreement, the amount of chlorine in the troposphere showed an annual global decrease for the first time in the early 1990s. It is expected that stratospheric chlorine content will decline in the early 2000s, at which point the amount of ozone in the stratosphere should end its long-term declining trend.

Sampling Date

Figure 3 Monthly hemispheric and global tropospheric chlorine content measured between 1992 and 1996. Because of the international effort to curb human-produced chlorinated gases outlined in the Montreal Protocol of 1987 and subsequent amendments to that original agreement, the amount of chlorine in the troposphere showed an annual global decrease for the first time in the early 1990s. It is expected that stratospheric chlorine content will decline in the early 2000s, at which point the amount of ozone in the stratosphere should end its long-term declining trend.

Anthropogenic bromine compounds (called halons) are used as fumigants in agriculture and as a fire retardant on clothing. The most abundant iodine compound is methyl iodide, CH3I, and has been observed in the atmosphere as a biomass burning product.

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