Ozone hole formation

Antarctic ozone hole formation. Outflow to lower latitudes then provides a source of air that has been processed by the polar vortex and PSCs (e.g., see Proffitt et al., 1990, 1993 Randel and Wu, 1995). 

McElroy et al (1986) and Tung et al (1986) emphasized the role of bromine chemistry in ozone hole formation (in particular, its coupling to chlorine through the reaction between CIO and BrO) this cycle is now known to contribute about 20% to the annual formation of the Antarctic ozone hole (e.g., Anderson et al, 1989). Both McElroy et al (1986) and Tung et al (1986) also emphasized the need for reduced NO2 in order for CIO to remain active (noting the links to the Noxon cliff ), and McElroy et al (1986) also emphasized the Japanese ozonesonde observations, particularly the observation of ozone loss at low altitudes, where bromine can be very effective for ozone destruction. 

Present the mechanism of ozone hole formation at the Antarctic pole. Is a similar phenomenon recorded at the North Pole ... 

The solubility of HNO3 is low enough that reactions (7) to (10) will produce predominantly gas phase HNO3. These heterogeneous reactions convert odd nitrogen to the more stable species nitric acid, but will not remove nitrogen completely from circulation as is observed during Antarctic ozone hole formation when denitrification by polar stratospheric clouds occurs. 

Chlorine CI2, and bromine monochloride BrCl are formed in the reactions of CIONO2, Br0N02, HCl, HBr, HOCl, HOBr in the heterogeneous reaction in the polar stratospheric clouds (see Sect. 6.5), and their photolyses play an important role in the chain reactions of the ozone hole formation. In the troposphere, CI2 is known to be produced in the heterogeneous reactions on sea salts, but observational data is still limited. Bromine Bra is known to be produced by the heterogeneous chain reactions in the tropospheric ozone destruction in the arctic region. Meanwhile, iodine I2 is released from sea weeds in coastal regions. 

Fig. 8.8 Comparison of the ozone vertical profiles in 1986 before ozone hole formation (August 25) and after its development (October 26) (Adapted from Hofmann et al. 1987)...

The formation of photochemically driven layers in the atmosphere, such as the ozone hole a natural consequence of atmospheric structure... 

Figure 12.17 shows the ozone profiles over the U.S. Amundsen-Scott Station at the South Pole in 1993 on August 23 prior to formation of the ozone hole and on October 12 after the ozone hole had developed. The total column ozone decreased from 276 DU on August 23 to only 91 DU on October 12, and, in addition, there was essentially no ozone in the region from 14 to 19 km (Hofmann et al., 1994a). During the same period at the McMurdo Station in Antarctica, the total column ozone decreased from 275 to 130 DU (B. J. Johnson et al., 1995). While similar profiles have been observed since the discovery of the ozone hole, these data show some of the most extensive ozone destruction ever observed, although 1994 and 1995 showed almost as much 03... 

There are also important differences in the gas-phase chemistry of the Antarctic ozone hole compared to the chemistry at midlatitudes. One is the formation and photolysis of the CIO dimer. In the Antarctic spring, recycling of CIO back to chlorine atoms via reaction (27) with oxygen atoms does not play a major role because of the relatively small oxygen atom concentrations at the low UV levels at that time. Molina and Molina (1987) proposed that the formation of a dimer of CIO could, however, lead to regeneration of atomic chlorine through the following reactions ... 

In short, the overall features of the chemistry involved with the massive destruction of ozone and formation of the ozone hole are now reasonably well understood and include as a key component heterogeneous reactions on the surfaces of polar stratospheric clouds and aerosols. However, there remain a number of questions relating to the details of the chemistry, including the microphysics of dehydration and denitrification, the kinetics and photochemistry of some of the C10x and BrOx species, and the nature of PSCs under various conditions. PSCs and aerosols, and their role in halogen and NOx chemistry, are discussed in more detail in the following section. 

Crutzen, P. J., and F. Arnold, Nitric Acid Cloud Formation in the Cold Antarctic Stratosphere A Major Cause for the Springtime Ozone Hole, Nature, 324, 651-655 (1986). 

Hofmann, D. J and T. Deshler, Stratospheric Cloud Observations during Formation of the Antarctic Ozone Hole in 1989, J. Geophys. Res., 96, 2897-2912 (1991). 

As discussed in Chapter 12, trends in stratospheric ozone in the Antarctic spring during formation of the ozone hole are clear. However, as treated in detail in... 

R. S. Stolarski, The Antarctic Ozone Hole, Scientific American, January 1988. The 1995 Nobel Prize in Chemistry was shared by Paul Crutzen, Mario Molina, and F. Sherwood Rowland for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. Their Nobel lectures can be found in P. J. Crutzen, My Life with 03, NO, and Other YZO Compounds, Angew. Chem. lnt. Ed. Engl. 1996,35, 1759 M. J. Molina, Polar Ozone Depletion, ibid., 1779 F. S. Rowland, Stratospheric Ozone Depletion by Chlorofluorocarbons, ibid., 1787. 

Crutzen, P.J., and Arnold, F. (1986) Nitric acid cloud formation in the cold Antarctic stratosphere a major cause for the springtime ozone hole. Nature 324,651-655. 

Over recent years, scientists have become aware of a reduction in the amount of ozone in our atmosphere and of the formation of ozone holes in the stratosphere (Figure 11.5). The reduction of ozone in our atmosphere has led to an increased risk of skin cancer as more harmful ultraviolet radiation has reached the surface of the Earth. This is a different type of problem from the greenhouse effect and associated global warming, which is caused mainly by an increase in the amount of carbon dioxide in the atmosphere. For a further discussion of the greenhouse effect and global warming see p. 212. 

Chlorofluorocarbons, such as CF3C1, catalyze this reaction and are responsible for the formation of the ozone hole. The decomposition is a chain reaction involving chlorine atoms as the chain-carrying species. Suggest a mechanism for this reaction. 

Since ROS are formed from the absorption of UVR by DOM and its subsequent photochemical decay, any changes in the atmosphere such as tropospheric warming or stratospheric ozone depletion should affect steady state concentrations of ROS in the water column. Initial studies with H2O2 suggest that the formation of an ozone hole will increase production rates by 20-50%. Changes in atmospheric ozone levels are also expected to affect production rates of other... 

When high concentrations ( 1 ppbv) of CIO are present in the lower stratosphere (e.g., in air masses wherein the reservoir s HC1 and CIONO2 have been processed by polar stratospheric clouds), additional cycles must be considered (see Figure 5.68) one of them involves the formation of the CI2O2 dimer which dominates the destruction of polar ozone (e.g., Antarctic ozone hole) ... 

Halogen Chemistry on PSCs. The cold temperatures that occur in polar winter can lead to formation of clouds within the stratosphere, and there are visual sightings of such Arctic clouds dating back hundreds of years. In the unpopulated Antarctic, the earliest explorers noted unusually colorful high clouds in winter. The term polar stratospheric clouds (or PSCs) was coined by McCormick et al. (1982), who first presented satellite observations of high-altitude clouds in the Antarctic and Arctic stratospheres, but the clouds were considered little more than a scientific curiosity until the ozone hole was discovered. 

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