Stratosphere, upper, ozone

We have addressed several aspects of STE of ozone and the impact on tropospheric ozone levels. Using ozone observations in the upper troposphere and lower stratosphere from MOZAIC, we have examined the rdation between ozone and PV in the lower stratosphere. A distinct seasonality in the ratio between ozone and PV is evident, with a maximum in spring and minimum in fall associated with the seasonality of downward transport in the meridional circulation and of the ozone concentrations in the lower stratosphere. The ozone-PV ratio is applied in our tropospheric chemistry-climate model to improve the boundary conditions for ozone above the tropopause, to improve the representativity of simulated ozone distributions near synoptic disturbances and realistically simulate cross-tropopause ozone transports. It is expected that the results will further improve when the model is applied in a finer horizontal and vertical resolution. 

Reinsel G.C., Tiao G.C., DeLuisi J.J., Mateer C.L., Miller A.J. and Frederick J.E., Analysis of upper stratospheric umkehr ozone profile data for trends and effects of atmospheric aerosols. J. Geophys. Res. (1984). 

This section is mostly concerned with the presence of phosgene in the atmosphere, its formation and removal in both the troposphere (lower atmosphere) and stratosphere (upper atmosphere) and its possible effects on that part of the stratosphere known as the ozone layer. 

Most ozone is formed near the equator, where solar radiation is greatest, and transported toward the poles by normal circulation patterns in the stratosphere. Consequendy, the concentration is minimum at the equator and maximum for most of the year at the north pole and about 60°S latitude. The equihbrium ozone concentration also varies with altitude the maximum occurs at about 25 km at the equator and 15—20 km at or near the poles. It also varies seasonally, daily, as well as interaimuaHy. Absorption of solar radiation (200—300 nm) by ozone and heat Hberated in ozone formation and destmction together create a warm layer in the upper atmosphere at 40—50 km, which helps to maintain thermal equihbrium on earth. 

Despite their instability (or perhaps because of it) the oxides of chlorine have been much studied and some (such as CI2O and particularly CIO2) find extensive industrial use. They have also assumed considerable importance in studies of the upper atmosphere because of the vulnerability of ozone in the stratosphere to destruction by the photolysis products of chlorofluorocarbons (p. 848). The compounds to be discussed are ... 

In the upper atmosphere (the stratosphere), the situation is quite different. There the partial pressure of ozone goes through a maximum of about 10-5 atm at an altitude of 30 km. From 95% to 99% of sunlight in the ultraviolet region between 200 and 300 nm is absorbed by ozone in this region, commonly referred to as the "ozone layer." The mechanism by which this occurs can be represented by the following pair of equations ... 

The chlorine-containing product species (HCl, CIONO2, HOCl) are "inert reservoirs" because they are not directly involved in ozone depletion however, they eventually break down by absorbing solar radiation or by reaction with other free radicals, returning chlorine to its catalytically active form. Ozone is formed fastest in the upper stratosphere at tropical latitudes (by reactions 1 and 2), and in those regions a few percent of the chlorine is in its active "free radical" form the rest is in the "inert reservoir" form (see Figure 3). 

Considering natural stratospheric ozone pro-duction/destruction as a balanced cycle, the NO.v reaction sequence is responsible for approximately half of the loss in the upper stratosphere, but much less in the lower stratosphere (Wennberg et al, 1994). Since this is a natural steady-state process, this is not the same as a long term O3 loss. The principal source of NO to the stratosphere is the slow upward diffusion of tropospheric N2O, and its subsequent reaction with O atoms, or photolysis (McElroy et ai, 1976). 

We begin our exploration of delocalized bonds with ozone, O3. As described in Chapter 7, ozone in the upper stratosphere protects plants and animals from hazardous ultraviolet radiation. Ozone has 18 valence electrons and a Lewis stmcture that appears in Figure 10-36a. Experimental measurements show that ozone is a bent molecule with a bond angle of 118°. 

The net reaction for this two-step mechanism is the conversion of an O3 molecule and an oxygen atom into two O2 molecules. In this mechanism, chlorine atoms catalyze ozone decomposition. They participate in the mechanism, but they do not appear in the overall stoichiometry. Although chlorine atoms are consumed in the first step, they are regenerated in the second. The cyclical nature of this process means that each chlorine atom can catalyze the destruction of many O3 molecules. It has been estimated that each chlorine atom produced by a CFC molecule in the upper stratosphere destroys about 100,000 molecules of ozone before it is removed by other reactions such as recombination CF2 Cl -b Cl CF2 CI2... 

Ozone is a very reactive compound present in the upper atmosphere (stratosphere) and the lower atmosphere (troposphere). Whilst ozone is vital in the stratosphere, its presence at ground levels is a danger to human health and contributes to the formation of other pollutants. 

Halogen oxide radicals such as CIO and BrO are important reactive intermediates in the catalytic cycles of ozone destruction in the middle and upper stratosphere. The first absorption band CIO(/l211 <— X2 I) starts from 318 nm and has a series of vibronic bands that converge to a broad continuum at wavelengths shorter than 264nm (Fig. 8).98-101 In this continuum region four dissociation pathways are thermodynamically possible,33... 

Another example of gas-phase catalysis is the destruction of ozone (03) in the stratosphere, catalyzed by Cl atoms. Ultraviolet light in the upper atmosphere causes the dissociation of molecular oxygen, which maintains a significant concentration of ozone ... 

Photochemically-generated radicals are encountered as reactive intermediates in many important systems, being a major driving force in the photochemistry of ozone in the upper atmosphere (stratosphere) and the polluted lower atmosphere (troposphere). The photochemistry of organic carbonyl compounds is dominated by radical chemistry (Chapter 9). Photoinitiators are used to form radicals used as intermediates in the chain growth and cross-linking of polymers involved in the production of electronic circuitry and in dental treatment. 

Ozone is main component in many oxidation processes assembled imder the term ozonation processes. In these processes ozone is applied either alone (O3 process) or with the addition of oxidant, e.g. H2O2 (O3/H2O2 process), UV radiation (explained in above subchapter), catalyst, activated carbon, ultrasoimd etc. Ozone is inorganic molecule constituted by three atoms of oxygen. It is present in nature in upper atmosphere in the form of stratospheric layer aroimd the earth, and it is formed by the photolysis of diatomic oxygen and further recombination of atomic and diatomic oxygen, shown by equations (25) and (26) [35] ... 

CFCs are nearly ideal substances for attacking ozone molecules and damaging the ozone layer. On the one hand, they tend to be very stable, even in the stratosphere. Many CFCs have half-lives of 100 years or more that means that once they have escaped into the upper atmosphere, they are likely to remain there for very long periods. On the other hand, some small number of CFC molecules do dissociate to form chlorine free radicals, with the ability to destroy ozone molecules. Although the number of CFC molecules that do dissociate is relatively small, the actual number is not important since chlorine free radicals that are generated in the process are used over and over again. That is, they are catalysts in the destruction of ozone and are not, themselves, used up in their reactions with ozone molecules. 

Although there has been some controversy over whether there is indeed a true ozone deficit problem (e.g., Crutzen et al., 1995), a combination of measured concentrations of OH, HOz, and CIO with photochemical modeling seems to indicate that it may, indeed, exist (Osterman et al., 1997 Crtuzen, 1997), although the source of the discrepancy remains unclear. Measurements of CIO in the upper stratosphere have found concentrations that are much smaller (by a factor of 2) than predicted by the models (e.g., Dessler et al., 1996 Michelsen et al., 1996). Because of the chlorine chemistry discussed later, model overestimates of CIO will also result in larger predicted losses of 03 and hence smaller concentrations. 

There are several important points with respect to the effects of any future HSCT emissions. First, ozone concentrations at a particular location and time depend not only on the local chemistry but on transport processes as well. In the lower stratosphere, transport processes occur on time scales comparable to the rates of ozone formation and loss so that taking into account such transport is particularly important. However, in the middle and upper stratosphere, production and removal of 03 are much faster than transport so that a steady state exists between these two processes. 

Because the concentration of oxygen atoms increases with altitude, the reaction cycle represented by (26) and (27) is important primarily in the middle and upper stratosphere (e.g., Garcia and Solomon, 1994 WMO, 1995). For the lower stratosphere, however, it is only responsible for about 5% of the portion of the total ozone loss that is due to halogens at 15 km and 25% at 21 km (see Fig. 12.8 Wennberg et al., 1994). Most of the 03 loss associated with C10x and BrO at the relatively low altitudes in Fig. 12.8 is due to the following cycle (Solomon et al., 1986 Crutzen and Arnold, 1986) ... 

Increased production of oxides of nitrogen through solar proton events associated with the 11-year cycle in solar activity would be expected to be most important in the upper stratosphere, above the region where the majority of the ozone depletion was observed in addition, lower, rather than higher, concentrations of gas-phase oxides of nitrogen appear to be associated with the ozone depletion (e.g., see Noxon, 1978 McKenzie and Johnston, 1984 Thomas et al., 1988 Keys and Gardiner, 1991 and Solomon and Keys, 1992). Hence both of these explanations are not consistent with atmospheric observations. 

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. 

Brasseur, G. P., X. Tie, P. J. Rasch, and F. Lefevre, A Three-Dimensional Simulation of the Antarctic Ozone Hole Impact of Anthropogenic Chlorine on the Lower Stratosphere and Upper Troposphere, J. Geophys. Res., 102, 8909-8930 (1997). 

Eluszkiewicz, J., and M. Allen, A Global Analysis of the Ozone Deficit in the Upper Stratosphere and Lower Mesosphere, J. Geophys. Res., 98, 1069-1082 (1993). 

Stone, R. S., J. R. Key, and E. G. Duton, Properties and Decay of Stratospheric Aerosols in the Arctic Following the 1991 Eruptions of Mount Pinatubo, Geophys. Res. Lett., 20, 2359-2362 (1993). Strand, A, and 0. Hov, The Impact of Man-Made and Natural NOA. Emissions on Upper Tropospheric Ozone A Two-Dimensional Model Study, Atmos. Enriron., 30, 1291-1303 (1996). 

In addition, there is an obseived correlation between total column ozone and the El Nino Southern Oscillation (ENSO) in the tropical troposphere, with decreases in total ozone in middle and sometimes polar latitudes following the ENSO by several months the period associated with the ENSO is 43 months (Zerefos et al., 1992). While the association between the ENSO and ozone is not well understood, it has been proposed that the warming of the troposphere in the tropics over the Pacific Ocean causes increases in the upper troposphere air temperatures and tropopause height and an upwelling in the lower stratosphere. If sufficiently large, this could have more widespread impact than just in the tropics (e.g., see Zerefos et al., 1992 and Kalicharran et al., 1993). 

Chandra, S C. H. Jackman, and E. L. Fleming, Recent Trends in Ozone in the Upper Stratosphere Implications for Chlorine Chemistry, Geophys. Res. Lett., 22, 843-846 (1995). 

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