Atmospheric Chemistry of Methane

4 Theoretical Maximum Yield of Ozone from CO Oxidation 

The theoretical maximum yield of 03 per CO + OH reaction would occur if NO, concentrations were sufficiently high that every H02 radical reacts with NO rather than with itself and termination of the chain by the OH + N02 reaction were neglected. The resulting mechanism would be 

While it is informative to see that one O3 molecule could theoretically result from each CO + OH reaction, this condition can never be achieved. If NO, levels are sufficiently high to keep H02 from reacting with itself, they are also sufficiently high so that some N02 must react with OH to form HNO3, thereby terminating the chain reaction. 

The principal oxidation reaction of methane, CH4, is with the hydroxyl radical  

As in the case of the hydrogen atom, the methyl radical, CH3, reacts virtually instantaneously with 02 to yield the methyl peroxy radical, CH302 

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The central role of hydroxyl radicals in atmospheric chemistry is well illustrated by examining the atmospheric cycles of methane and carbon monoxide. A quantitative assessment of both of these species was carried out in the 1920s in Belgium by Marcell Migeotte, who detected their absorption lines in the spectrum of infrared solar radiation reaching Earth s surface. 

Once the importance of DMS to the global sulfur cycle was established, numerous measurements of DMS concentrations in the marine atmosphere have been conducted. The average DMS mixing ratio in the marine boundary layer (MBL) is in the range of 80-1 lOppt but can reach values as high as 1 ppb over entrophic (e.g., coastal, upwelling) waters. DMS mixing ratios fall rapidly with altitude to a few parts per trillion in the free troposphere. After transfer across the air-sea interface into the atmosphere, DMS reacts predominantly with the hydroxyl radical and also with the nitrate (N03) radical. Oxidation of DMS is the exclusive source of methane sulfonic acid (MSA) in the atmosphere, and the dominant source of S02 in the marine atmosphere. We will return to the atmospheric chemistry of DMS in Chapter 6. 

In 1992, a complete mechanism for the oxidation of Ci and C2 hydrocarbons was formulated by the LACTOZ Steering Committee. The intended application is in global models for studies of methane oxidation and the tropospheric ozone budget. These studies have provided useful input to the Intergovernmental Panel on Climate Change (IPCC) assessment of the atmospheric chemistry of greenhouse gases. 

Aselmann 1, Crutzen PJ. 1989. Global distribution of natural fresh-water wetlands and rice paddies their net primary productivity, seasonality and possible methane emissions. Journal of Atmospheric Chemistry 8 307-358. 

Seiler W, Holzapfel-Pschom A, Comad R, Scharffe D. 1984. Methane emission from rice paddies. Journal of Atmospheric Chemistry 1 241-268. 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

The most intriguing of Saturn s moons is Titan, larger than the planet Mercury. It is the only moon known to have an atmosphere. Nitrogen and methane gasses shroud Titan with dense clouds which our cameras cannot penetrate. The chemistry of this atmosphere is unlike that of any other. If we could descend to the surface of Titan, we might see ice mountains softly eroded by a persistent rain of complex chemicals, and a deep chemical ocean, a strange parody of the oceans of earth. Titan s atmosphere, like the ancient atmosphere of earth, contains prelife chemicals, but is too cold for life to evolve. 

Thus the lifetime of a constituent with a first order removal process is equal to the inverse of the first order rate constant for its removal. Taking an example from atmospheric chemistry, the major removal mechanism for many trace gases is reaction with hydroxyl radical, OH. Considering two substances with very different rate constants for this reaction, methane and nitrogen dioxide... 

Crutzen P.J., Gas-phase nitrogen and methane chemistry in the atmosphere. In Physics and Chemistry of the Upper Atmosphere, edited by B. McCormac, D. Reidel, Dordrecht, Netherlands (1973). 

Your point is certainly well taken there are many aspects of tropospheric chemistry that are uncertain, and that involving methyl peroxide is without doubt a prime example. From my own estimation of how the methane system works in the atmosphere, I believe that a significant fraction of the methyl peroxide is removed by heterogeneous reactions before it has a chance to react. This terminates the chain making methane oxidation a net sink for OH regardless of the details of the chemistry of methyl peroxide and its daughter molecules. Nevertheless we certainly need to be aware of the many uncertainties in this chemistry. 

Methane is an important atmospheric trace gas which affects the chemistry of the troposphere [10] and of the stratosphere [11]. It is... 

The chemistry of the ozone layer in the tropics has not been extensively studied. We do not know the level of the ozone concentration with exactitude. We have demonstrated that termites do emit large quantities of methane in the tropics. The emitted methane is transformed into CO which can affect the ozone layer. Therefore, studies of ozone layer profile in the tropics would be an essential component towards our understanding of the atmospheric chemistry. 

Methane is the most abundant hydrocarbon in the atmosphere. It plays important roles in atmospheric chemistry and the radiative balance of the Earth. Stratospheric oxidation of CH4 provides a means of introducing water vapor above the tropopause. Methane reacts with atomic chlorine in the stratosphere, forming HCl, a reservoir species for chlorine. Some 90% of the CH4 entering the atmosphere is oxidized through reactions initiated by the OH radical. These reactions are discussed in more detail by Wofsy (1976) and Cicerone and Oremland (1988), and are important in controlling the oxidation state of the atmosphere. Methane absorbs infrared radiation in the troposphere, as do CO2 and H2O, and is an important greenhouse gas (Lacis et al., 1981 Ramanathan et al., 1985). 

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