Ozone destruction

Chlorine atoms are also very efficient ozone destruction catalysts, as noted originally by Stolarski and Cicerone (4) ... 

Since the recognition of the role of chlorine in catalytic ozone destruction, increasing effort has been devoted to finding replacements. In most cases reported so far, the replacements are partially halogenated molecules that retain one or more hydrogen atoms (HCFCs and HFC s). The presence of H-atoms gives HO a handle (via H-atom abstractions such as R4) for their tropospheric... 

This simple oxygen-only mechanism consistently overestimates the O3 concentration in the stratosphere as compared to measured values. This implies that there must be a mechanism for ozone destruction that the Chapman model does not account for. A series of catalytic ozone-destroying reactions causes the discrepancy. Shown below is an ozone-destroying mechanism with NO/NO2 serving as a catalyst ... 

It is interesting to compare the rate constants of the oxygen-only ozone destruction reaction with those of the catalytic ozone destruction cycle. The rate constants for reactions 4-6 at 30 km are given below in units of cm molecules s . 

Consider the NO/N02-catalyzed ozone destruction cycle, reactions 5 and 6 in Section 5.4.3. One could perform a calculation to determine which reaction is the rate-limiting step (i.e., the slowest step that determines the rate of the overall reaction) in this cycle. In this case, a theoretical doubling of ks reduces the ozone concentration by about 2%. On the other hand, doubling kf, reduces the ozone concentration by nearly 50%. (a) Which reaction is the rate-limiting step in N0/N02-catalyzed ozone destruction (b) The concentrations of NO and NO2 are [NO] = 2.9 X 10 /cm and [NO2] = 6.1 x 10 /cm. How do these data support or refute your answer to (a) ... 

The chemical reactions in the oxygen-only mechanism. Sections 5.4.3 and 10.4 substantially underestimate the ozone destruction rate ... 

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... 

Ozone in turn absorbs a different band of life-threatening ultraviolet light. The rate of ozone destruction in the pristine atmosphere is slow and is due to a reaction such as... 

Kerwin et al. [41] determined methyl bromide soil fumigant by cyrotrapping and electron capture gas chromatography. Down to 0.23pM of methyl bromide could be detected by this procedure. Kerwin et al. [41] found levels of methyl bromide in the stratosphere and claimed that this contributed to ozone destruction. 

Ozone destruction at the wall to form oxygen molecules would explain the lower limit. Lewis and von Elbe explain the upper limit by the third-order reaction... 

The reaction sequence (8.156)-(8.157) is a catalytic chain for ozone destruction and contributes to the net destruction. However, even given the uncertainty possible in the rates of these reactions and the uncertainty of the air motions, this system could not explain the imbalance in the ozone throughout the stratosphere. 

It is possible to similarly estimate the effect of the various cycles upon ozone destruction. The results can be summarized as follows between 15 and 20 km, the N03 catalytic cycle dominates between 20 and 40 km, the N02 cycle dominates between 40 and 45km, the N02, HO, and O mechanisms are about equal and above 45 km, the HO reactions are the controlling reactions. 

These two reactions add up to the overall ozone destruction reaction... 

Figure 8-17 The chain reaction by which small concentrations of organic chlorine compounds produce ozone destruction in the stratosphere.

Chlorine and CIO cause ozone destruction in the stratosphere by catalyzing the reaction... 

With the addition of Reactions 2-173 and 2-174, the production and consumption of ozone include both chain and parallel reactions. The method of solution is nonetheless similar to the case without anthropogenic ozone destruction. To solve for the concentration of [O3], it is necessary to solve for [XO], [O], and [O3] from three equations d[XO]/df=0, d[O]/df=0, and d[O3]/df=0. 

There are numerous natural contributors of chlorine to the stratosphere, for example, volcanic eruptions. The main concern regarding ozone destruction in recent years is associated with human activities that have increased chlorine and other synthetic chemical input into the stratosphere. At the top of the list of such chemicals are chlo-rofluorocarbons, or CFCs. CFCs are compounds that contain carbon, chlorine, and fluorine they were first developed in 1928. Common CFCs are called Freons, a trade name coined by the DuPont chemical company. CFC compounds are nonreactive, nontoxic, inflammable gases. Because of their... 

Chemical Family a group of elements that share similar chemical properties and share the same column in the periodic table, for example, halogens, alkali earth Chirality condition that describes the handedness of a molecule or whether a molecule exists in forms that can be superimposed on each other Chlorofluorocarbons also called CFCs, compounds consisting of chorine, fluorine, and carbon that are responsible for stratospheric ozone destruction Coagulation precipitation or separation from a dispersed state Coefficient of Thermal Expansion measure of the rate at which a substance will expand when heated... 

In 1974, Cicerone and Stolarski suggested that if there were sources of atomic chlorine in the stratosphere, the following catalytic ozone destruction cycle... 

After the first reports of this phenomenon, major field campaigns were launched, which clearly established a relationship between ozone destruction and chlorine chemistry. For example, Fig. 1.8 shows simultaneous aircraft measurements of ozone and the free radical CIO as the plane flew toward the South Pole. As it entered the polar vortex, a relatively well-contained air mass over Antarctica, 03 dropped dramati-... 

As discussed earlier, the N02 then photolyzes to 0(3P), which adds to 02 to form 03. Under these conditions, O, will be formed. The concentration of NO at which this crossover from ozone destruction to ozone formation occurs is central to the chemistry of both remote and polluted regions. 

Barrie, L. A., J. W. Bottenheim, R. C. Schnell, P. J. Crutzen, and R. A. Rasmussen, Ozone Destruction and Photochemical Reactions at Polar Sunrise in the Lower Arctic Atmosphere, Nature, 334, 138-141 (1988). 

Finlayson-Pitts, B. J., F. E. Livingston, and H. N. Berko, Ozone Destruction and Bromine Photochemistry at Ground Level in the Arctic Spring, Nature, 343, 622-625 (1990). 

Sander, R., and P. Crutzen, Model Study Indicating Halogen Activation and Ozone Destruction in Polluted Air Masses Transported to the Sea, J. Geophys. Res., 101, 9121-9138 (1996). 

Tuckermann, M., R. Ackermann, C. Golz, H. Lorenzen-Schmidt, T. Senne, J. Stutz, B. Trost, W. Unold, and U. Platt, DOAS-Ob-servation of Halogen Radical-Catalysed Arctic Boundary Layer Ozone Destruction during the ARCTOC-Campaigns 1995 and 1996 in Ny-Alesund, Spitsbergen, Tellus, 49B, 533-555 (1997). 

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