Stratosphere methane

Stratospheric methane collected over Japan gave a 8 C-value of —47.5%c at the tropopause and increased to —38.9%c at around 35 km (Sugawara et al. 1998). These authors suggested that reaction with Cl in the stratosphere might be responsible for the C-enrichment. 

Sugawara S, Nakazawa T, Shirakawa Y, Kawamura K, Aoki S, Machida T, Honda H (1998) Vertical profile of the carbon isotope ratio of stratospheric methane over Japan. Geophysic Res Lett 24 2989-2992... 

Atmospheric methane is significant for a number of reasons. It is one of the gases that controls OH radical concentrations in the troposphere. Oxidation of methane is one of the chief sources of water in the stratosphere. Methane is also an active greenhouse... 

Gezari Plate 6. Saturn imeiged in 7.Sftm stratospheric methane with peak brightness 2.7 Jy arcsec (left), 11.6/tm comtinuum peak brightness d.7 Jy arcsec (center), and 12.4/mi ethane band emission peak brightness 20 Jy arcsec" (right). 3-m NASA/IRTF. 

The role of carbon dioxide in the Earth s radiation budget merits this interest in atmospheric CO2. There are, however, other changes of importance. The atmospheric methane concentration is increasing, probably as a result of increasing cattle populations, rice production, and biomass burning (Crutzen, 1983). Increasing methane concentrations are important because of the role it plays in stratospheric and... 

The different greenhouse gases can have complicated interactions. Carbon dioxide may cool the stratosphere which slows the process that destroys ozone. Stratospheric cooling can also create high altitude clouds which interact with chlorofluorocarbons to destroy ozone. Methane may be produced or destroyed in the lower atmosphere at various rates, which depend on the pollutants that are present. Methane can also affect chemicals that control ozone formation. 

Theoretical interpretation of the experimental observations will help in determining the relative roles played by stratospheric injection, plant emission, background methane, and transport to surfaces in the natural portion of the tropospheric ozone cycle. 

This is a very broad conclusion, and additional measurements must be made. Some of this effort (which is current) should address the problem of other pollutants and condensation nuclei that accompany the nonurban oxidant. Interpretation of these measurements will increase the specificity of separating anthropogenic sources from natural background sources. Theoretical assessments of the existing observations will shed light on the relative roles played by stratospheric injection, plant emission, background methane, and diy deposition on surfaces in the natural portion of the tropospheric ozone cycle. 

The chlorofluorocarbon compounds of methane and ethane are collectively known as freons. They are extremely stable, unreactlve, non-toxic, non-corrosive and easily liquefiable gases. Freon 12 (CCI2F2) Is one of the most common freons In Industrial use. It Is manufactured from tetrachloromethane by Swarts reaction. These are usually produced for aerosol propellants, refrigeration and air conditioning purposes. By 1974, total freon production In the world was about 2 billion pounds annually. Most freon, even that used In refrigeration, eventually makes Its way Into the atmosphere where It diffuses unchanged Into the stratosphere. In stratosphere, freon Is able to Initiate radical chain reactions that can upset the natural ozone balance (Unit 14, Class XI). 

Figures 4.44 and 4.45 show absorption spectra of some simple chlorofluoro-methanes and ethanes, respectively (Hubrich and Stuhl, 1980). Tables 4.37 and 4.38 give the recommended absorption cross sections for some of these compounds (DeMore et al., 1997). None of these compounds absorb in the actinic region above 290 nm, but do around 180-200 nm, wavelengths only found in the stratosphere. As discussed in Chapter 12, it is photolysis at these short wavelengths to generate atomic chlorine that is responsible, along with bromine and perhaps in some cases, iodine atoms, for the chain destruction of stratospheric ozone.

Table 14.4 summarizes the estimated total direct radiative forcing calculated for the period from preindustrial times to 1992 for C02, CH4, N20, and O, (IPCC, 1996). The estimate for CH4 includes the effects due to its impacts on tropospheric ozone levels or on stratospheric water vapor, both of which are generated during the oxidation of methane. That shown for 03 is based on the assumption that its concentration increased from 25 to 50 ppb over the Northern Flemi-sphere. The total radiative forcing due to the increase in these four gases from preindustrial times to the present is estimated to be 2.57 W m 2. 

Methane is removed continually from the atmosphere by reaction with OH radicals (Section 8.3). In contrast, chlorofluorocarbons and related volatile compounds are inert under the conditions of the lower atmosphere (troposphere), so atmospheric concentrations of these refrigerants and solvents will tend to increase as long as releases continue. The chief concern over chlorofluorocarbons is that they are a major factor in destruction of the stratospheric ozone layer (Section 8.3). They have been banned under the Montreal Protocol of 1988, but it is important that whatever substitutes (inevitably greenhouse active) are introduced to replace them degrade relatively quickly in the troposphere to minimize any contribution they may be capable of making to greenhouse warming. 

A pioneer in rocket and satellite technology, Singer devised the basic instrument for measuring stratospheric ozone, was the principal investigator on a satellite experiment retrieved by the space shuttle in 1990, and was the first scientist to predict that population growth would increase atmospheric methane—an important greenhouse gas. 

The photochemistry of halogenated methanes has been of great interest recently ever since it was recognized that chloromethancs in the stratosphere may release Cl atoms upon absorption of solar radiation and that Cl atoms so produced may catalytically decompose 03. 

Reactions R1 - RIO are also key reactions in determining OH distribution in the troposphere and lower stratosphere. The key point here is that increases in ozone and nitrogen oxides enhances the OH distribution through reactions R1 and RIO, while enhanced carbon monoxide and methane reduces OH through reactions R4 and R6. Furthermore, reactions with OH (R4 and R6) represent the main loss of CO and methane. 

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