Stratosphere halocarbons

As in the case of nitrogen species, the value of theoretical predictions can be evaluated by comparing the measured stratospheric concentrations of halocarbons with calculated profiles. The results of stratospheric halocarbon analyses have been recently reviewed by Volz el al. (1978). Their data show that halocarbon levels decrease rather rapidly in the stratosphere with increasing altitude. The rate of this decrease is comparable to the theoretical value. 

In the last decade, the refrigerant issue is extensively discussed due to the accepted hypothesis that the chlorine and bromine atoms from halocarbons released to the environment were using up ozone in the stratosphere, depleting it specially above the polar regions. Montreal Protocol and later agreements ban use of certain CFCs and halon compounds. It seems that all CFCs and most of the HCFCs will be out of produc tion by the time this text will be pubhshed. 

Many substances react in the gas phase rather than in solution. An important example is the process thought to deplete the ozone layer the reaction between gaseous ozone, O3, and chlorine radicals, high up in the stratosphere. Ultimately, the chlorine derives from volatile halocarbon compounds, such as die refrigerant Freon-12 or the methyl chloroform thinner in correction fluid. 

H. S. Gutowsky et aL, Halocarbons Effects on Stratospheric Ozone, National Academy of Sciences, Washington D. C. 1976. 

This report deals primarily with the origins and effects of ozone and other photochemical oxidants. It is limited, more or less, to the problem of urban pollution and to such closely related topics as natural background in the earth s boundary layer. No consideration is given to the stratospheric ozone layer and the effects produced by supersonic transport (sst) emission or halocarbons. 

The role of biomass in the natural carbon cycle is not well understood, and in the light of predictions of a future atmospheric energy balance crisis caused by carbon dioxide accumulation, in turn the result of an exponential increase in the consumption of carbon fuel, the apparent lack of concern by scientists and policy makers is most troubling. Yet there is no other single issue before us in energy supply which will require action long before the worst effects of excess production will be apparent. The only satisfactory model is the action taken by the R D community with respect to the SST in nitric oxide potential and chloro-halocarbon emissions, when it was realised that the stratospheric ozone layer was vulnerable to interference. Almost all other responses to pollution" have been after definitive effects have become apparent. 

National Research Council, Stratospheric Ozone Depletion by Halocarbons Chemistry and Transport, Panel on Chemistry and Transport, Committee on Impacts of Stratospheric Change, Assembly of Mathematical and Physical Sciences, National Academy of Sciences, Washington, DC, 1979. 

Figure 13.7 shows the effective total tropospheric concentration of chlorine from halocarbons from 1992 to 1996 (Montzka et al., 1996a). The concentration peaked in 1994 at 3.0 ppb, but when methyl chloride (CH-,C1) and other chlorinated organics are taken into account, the peak was likely 3.7 ppb. The total tropospheric chlorine concentration in mid-1995 decreased at a rate of approximately 25 ppt per year, in contrast to increases of 110 ppt per year in 1989 (Montzka et al., 1996a Cunnold et al., 1997). Bromine compounds show the same trend. As a result, the stratospheric levels of chlorine and bromine are expected to peak around the year 2000 (Montzka et al., 1996a World Meteorological Organization, 1995,1999). 

Lovelock, J. E., Atmospheric Halocarbons and Stratospheric Ozone, Nature, 252, 292-294 (1974). 

Montzka, S. A J. H. Butler, R. C. Myers, T. M. Thompson, T. H. Swanson, A. D. Clarke, L. T. Lock, and J. W. Elkins, Decline in the Tropospheric Abundance of Halogen from Halocarbons Implications for Stratospheric Ozone Depletion, Science, 272, 1318-1322 (1996a). 

Ramaswamy, V., M. D. Schwarzkopf, and K. P. Shine, Radiative Forcing of Climate from Halocarbon-Induced Global Stratospheric Ozone Loss, Nature, 355, 810-812 (1992). 

Halogenated Hydrocarbons (Halocarbons) and their Impact on Stratospheric Ozone... 

Stratospheric Ozone depletion is largely due to chlorine and bromine radicals released from halogenated hydrocarbons. This paper describes properties, emission histories and budgets of relevant substances and outlines the pertinent photochemical processes, along with a comprehensive presentation of halocarbon measurements and global distributions. 

Tropical upwelling is the main process through which halocarbons are carried up into the stratosphere. Once there, they are distributed polewards along sloped isentiopic surfaces, and subject to photolysis by UV radiation. As Chou [13] have pointed out, UV absorption cross-sections of halocarbons depend strongly on the number of chlorine atoms attached to a particular carbon atom. Hence, stability and thus atmospheric life times increase with increasing number of fluorine replacing chlorine atoms (see 4. and table 1). 

HF is extremely stable and accumulates in the Middle Atmosphere. Its present abundance is equivalent to the accumulated fluorine amount liberated from fluorine bearing halocaibons up to present, and it increases at the pace of ongoing fluorine species entering the stratosphere. As HF is not a natural atmospheric constituent, the HF budget of the Middle Atmosphere is an important piece of evidence showing the fate and effect of fluorinated halocarbons. 

This presentation focuses on the vertical distribution of halocarbons obtained by analysis of cryogenically collected whole-air samples. The balloon-borne cryogenic samplers developed and flown by the Max-Planck-Institut fur Aeronomie (MPAE) and the Kem-forschungsanlage Jiilich (KFA) are described by Fabian [17] and Schmidt [18]. Between 1977 and 1993, a total of 33 balloon ascents have been carried out by both institutions, 28 at northern midlatitudes (southern France, 44°N), 3 at high latitudes(Kiruna, 69°N) and 2 in the tropics (Hyderabad, 17,5°N). These stratospheric data are limited to balloon altitudes, i.e. up to about 35 km. Tropospheric data were obtained from balloon samples, samples collected aboard aircraft and at ground level. 

Fabian, P. and GOmer D. (1984) The vertical distribution of halocarbons in the stratosphere, Fresenius Z. Analyt. Chem. 319,890-897. 

The halocarbons, which are not destroyed in the troposphere by reactions with hydroxyl, pass into the stratosphere where they are photo-dissociated to liberate chlorine atoms which attack ozone. Only one of them is of natural origin, methyl chloride CH3CI, but there are also several industrial products, especially carbon tetrachloride, CC14, trichlorofluo-romethane, CFC13, and dichlorodifluoromethane. Methyl chloride (Table III) has a natural marine origin (for details, see ref. 12), but it is certainly present also in the smoke produced when polyvinyl and other products containing chlorine are burnt. In addition, it is produced naturally not only in forest fires, but also in tropical agriculture based on the cultivation... 

Carbon tetrachloride, CC14, is generally used in the production of other substances, whereas the halocarbons 13 and 12 are in daily use in sprays and refrigerators. The annual rate of production of CFCI3 was 1000 tonnes in 1947 it rose about to 35,000 tonnes in 1957, 150,000 tonnes in 1967, and to at least 300,000 tonnes in 1979 (for details see ref. 13). If the dispersion of these halocarbons in the atmosphere continues to increase, then the annual rate at which they are produced is so great that it cannot fail to have an effect on the accumulated chlorine in the stratosphere, and on the stratospheric ozone. 

National Academy of Sciences, Halocarbons effects on Stratospheric Ozone, NAS, Washington, D.C., 1976. 

A serious consequence of anthropogenic release of halocarbons to the atmosphere is the depletion of naturally occurring stratospheric ozone. Sane reduction in halocarbon release has been achieved in the United States and a few other countries. Immediate termination of all release worldwide, however, would still leave the world with important stratospheric ozone reductions during the next decade. Reduced upper-air ozone would increase ultraviolet radiation reaching... 

The atmospheric fate of a halocarbon molecule depends upon whether or not it contains a hydrogen atom. Hydrohalomethanes are oxidized by a series of reactions with radicals prominant in the troposphere, predominantly hydroxyl OH. Fully halogenated methanes are unreactive towards these radicals and consequently are transported up through the troposphere into the stratosphere, where their oxidation is initiated by UV photolysis of a carbon-halogen bond. 

There are a number of inorganic molecules such as NO2 and SO2 (See Table 3) which are also lost via reaction with OH. A number of halocarbons also exist that posses insubstantial tropospheric sinks and have importance in the chemistry of the stratosphere (see Section 2.10). 

Under the agreements of the Montreal Protocol on Substances that Deplete the Ozone Layer, the production of chlorofluorocarbons (CFCs), halons and several other halocarbons has been prohibited [1,2]. Consequently, there is an interest in replacing these compounds [3]. As part of the development of such replacing compounds, it is necessary to consider and evaluate the potential environmental effects of their use, especially on stratospheric ozone [2],... 

Long lived compounds in the troposphere are transported into the stratosphere, where they decompose providing a source of inorganic stratospheric bromine [25]. Even short-lived halocarbons such as bromopropane can have significant ozone depletion potentials (ODP) due to rapid transport to the stratosphere by tropical convection [26]. Bromopropane molecules release bromine atoms 2-3 times more effectively than some CFCs would release chlorine atoms in the lower stratosphere [27]. 

N.A.S., Halocarbons effects on stratospheric ozone. National Academy of Sciences, Panel on Atmospheric Chemistry, H.S. Gutowsky, chairman (1976). 

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