Methane is the most abundant organic chemical in Earth's atmosphere, and its concentration is increasing with time, as a variety of independent measurements have shown. Photochemical reactions oxidize methane in the atmosphere; through these reactions, methane exerts strong influence over the chemistry of the troposphere and the stratosphere and many species including ozone, hydroxyl radicals, and carbon monoxide. Also, through its infrared absorption spectrum, methane is an important greenhouse gas in the climate system. We describe and enumerate key roles and reactions. Then we focus on two kinds of methane production: microbial and thermogenic. Microbial methanogenesis is described, and key organisms and substrates are identified along with their properties and habitats. Microbial methane oxidation limits the release of methane from certain methanogenic areas. Both aerobic and anaerobic oxidation are described here along with methods to measure rates of methane production and oxidation experimentally. Indicators of the origin of methane, including C and H isotopes, are reviewed. We identify and evaluate several constraints on the budget of atmospheric methane, its sources, sinks and residence time. From these constraints and other data on sources and sinks we construct a list of sources and sinks, identities, and sizes. The quasi‐steady state (defined in the text) annual source (or sink) totals about 310(±60) × 1012 mol (500(±95) × 1012 g), but there are many remaining uncertainties in source and sink sizes and several types of data that could lead to stronger constraints and revised estimates in the future. It is particularly difficult to identify enough sources of radiocarbon‐free methane.

Halley Bay (75.5øS, 26.8øW) that has occurred annually since the mid-1970s in September and October. This Antarctic ozone "hole" which is characterized by 50% or more decreases in column ozone has now been extensively studied using satellite, aircraft, and ground-based instruments. In addition, further analyses of satellite and ground-based Dobson data have shown northern hemisphere (30øN to 64øN) The Halogen Occultation Experiment (HALOE) was conceived to provide critical data for study of the ozone distribution and those processes which affect ozone levels. The experiment uses the principle of satellite solar occultation to sound the stratosphere, mesosphere, and lower thermosphere. Using this technique, absorption of solar energy in selected spectral bands is used to infer vertical profiles of temperature, pressure, and mixing ratios of key gases involved in the ozone chemistry. The HALOE instrument includes both broadband and gas filter channels Together, these observations form a minimum but adequate set which can be used to derive, under appropriate conditions, the unmeasured concentrations of several other gases needed to test understanding of the chemistry (see Figure 1). In this regard, HALOE 03, H20, and CH4 measurements, for example, can be used to derive OH levels. In turn, these parameters can be used to derive atomic chlorine and, from that, C10 through reactions with 03. Chlorine monoxide can be used with NO2 observations to derive chlorine nitrate (C1ONO2). Since C10

This study examines the potential climatic effects of the radiatively active trace gases that have been detected in the atmosphere including chlorofluorocarbons, chlorocarbons, hydrocarbons, fluorinated and brominated species, and other compounds of nitrogen and sulfur, in addition to CO 2 and 0 3. A one-dimensional radiative-convective model is used to estimate trace gas effects on atmospheric and surface temperatures for three cases: (1)

This study proposes that the oxides of chlorine, ClO,, may constitute an important sink for stratospheric ozone. A photochemical scheme is devised which includes two catalytic cycles through which C10, destroys odd oxygen. The individual C1X constituents (HC1, C1, C10, and OC10) perform analogously to the respective constituents (HNOI, NO, NO?, and NO:,) in the NO, catalytic cycles, but the ozone destruction efficiency is higher for CIO,. Our photochemical scheme predicts that C10 is the dominant chlorine constituent in the lower and middle stratosphere and HCI dominates in the upper stratosphere. Sample calculations are performed for several CIX altitude profiles: an assumed 1 p.p.b. volume mixing ratio, a ground level source, and direct injection by volcanic explosions. Finally we discuss certain lin~itations of the present model: uncertainty in stratospheric O H concentrations, the possibility that C 1 0 0 exists, the need to couple C10, cycles with NO, and HO, cycles, and possible heterogeneous reactions.Cette Ctude suggkre que les oxydes de chlore, ClO,, peuvent &tre une raison importante de la diminution de I'ozone stratosphtrique. Un schtma photochimique est tlaborC, lequel inclut deux cycles catalytiques par l'intermtdiaire desquels CIO,. dktruit I'oxygtne impair. Les constituants individuels CIX (HCI, CI, C10 et OCIO) agissent de facon analogue aux constituants respectifs (HNO:,, NO, NO. et NO:,) dans les cycles catalytiques des NO,, mais la destruction de I'ozone est plus efficace pour CIO,. Notre schCma photochimique laisse prtvoir que C10 est le constituant chlore dominant dans la basse et moyenne stratosphkre et que HCI domine dans la haute strntosphkre. Des calculs types sont effectuks pour plusieurs profiles d'altitude des CIX: un rapport de mtlange en volume de 1 p.p.b. est considtrC, une source au niveau du terrain et des injections directes par des explosions volcaniques. Finalenlent nous discutons certaines limitations du modl.le prtsent: incertitude des concentrations en O H dans la stratosphkre, la possibilitC de I'existence de C100, le besoin de r6~1nir les cycles de C10, avec ceux des NO, et HO, et des reactions httirogknes possibles.[Traduit par le journal]Can.

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