Methane lifetime

GLOBAL IMPACT OF AIRCRAFT EMISSION ON OZONE CONCENTRATIONS AND METHANE LIFETIME RESULTS FROM THE 1999IPCC AIRCRAFT ASSESSMENT... 

Table 3. Base background scenarios and subsonic aircraft NOx scenarios used in the global model studies. These scenarios are used to study ozone increases, non linearity in ozone productions from aircraft emissions and the impact on methane lifetime and methane concentrations for future aircraft emissions.

Table 6. Calculated change in methane lifetime from aircraft emissions up to 300 hPa ( 10 km).

Karlsdottir, S., I.S.A. Isaksen, Changing methane lifetime Cause for reduced growth,... 

Methane is the atmospheric hydrocarbon least reactive with HO, losing a hydrogen after an atmospheric lifetime of about a decade ... 

HO oxidation of CO is much faster than the reaction with methane, resulting in a mean CO lifetime of about two months, but considerably slower than reaction with the majority of the nonmethane hydrocarbons. Table I gives representative removal rates for a number of atmospheric organic compounds their atmospheric lifetimes are the reciprocals of these removal rates (see Equation E4, below). The reaction sequence R31, R13, R14, R15 constitutes one of many tropospheric chain reactions that use CO or hydrocarbons as fuel in the production of tropospheric ozone. These four reactions (if not diverted through other pathways) produce the net reaction... 

Fig. 3.1.10 Molecular lifetimes xintra and. aii in H-ZSM-5 crystallites obtained using the NMR tracer desorption technique and calculated via Eq. (3.3.15), respectively. Tracing by probe molecules (methane, measurement at 296 K) after an H-ZSM-5 catalyst has been kept for different coking times in a stream of n-hexane (filled symbols) and mesitylene (open symbols) at elevated temperature. The inserts present the evidence provided by a comparison of xintra and r]1,]]], with respect to the distribu-...

Lifetimes were measured at 77°K and adjusted to room temperature using 98/ J7. 8 Not a di-w-methane reaction. 

The rate constant for the singlet di-7r-methane photorearrangement can be determined from the quantum yield for the reaction and the experimentally measured singlet lifetime ... 

Fluorescence Lifetimes. Fluorescence lifetimes were determined by the phase shift method, utilizing a previously-described phase fluorimeter. The emission from an argon laser was frequency doubled to provide a 257 nm band for excitation. Fluorescence lifetimes of anisole and polymer 1 in dichloro-methane solution were 2.2 and 1.4 nsec, respectively. Fluorescence lifetimes of polymer films decreased monotonically with increasing DHB concentration from 1.8 (0) to 0.7 nsec (9.2 x 10 3 MDHB). Since fluorescence lifetimes (in contrast to fluorescence intensities) are unaffected by absorption effects of the stabilizer, these results provide direct evidence in support of the intensity measurements for RET from polymer to stabilizer. 

The observation of extremely facile formation of an acetyl complex and the finding that oxidative addition is the rate-determining step are almost certainly related to the high selectivity observed in the reaction. Thus, the extremely short lifetime of any CH3—Rh species makes it unlikely that it would be reacted off to methane in the presence of hydrogen (and/or metal hydrides). 

In many gaseous state reactions of technological importance, short-lived intermediate molecules which are formed by the decomposition of reacting species play a significant role in the reaction kinetics. Thus reactions involving the methane molecule, CH4, show the presence of a well-defined dissociation product, CH3, the methyl radical, which has a finite lifetime as a separate entity and which plays an important part in a sequence or chain of chemical reactions. 

LeUeveld I, Crutzen PI, Dentener FI. 1998. Changing concentration, lifetime and climate forcing of atmospheric methane. Tellus Series B-Chemical and Physical Meteorology 50 128-150. 

Laser flash photolysis of 46 showed results similar to those obtained for 45. The lifetimes and yields of Z and E photoenols from 46 are comparable to those obtained for 56. Similarly, laser flash photolysis of 47 reveals that the major reactivity pattern of 47 is intramolecular H-atom abstraction to form Z-58 and E-58 even though no products were observed that can be attributed to the formation of photoenol 58. Laser flash photolysis of 47 in methanol showed formation of biradical 57 ( max 330 nm, r = 22ns), which was efficiently quenched with oxygen (Scheme 32). Biradical 57 intersystem crosses to form Z-58 and E-58, which have maximum absorption at 400 nm. Enols Z-58 to E-58 were formed in the approximate ratio of 1 4. Enol Z-58 had a lifetime of 6.5)0,s in methanol, but its lifetime in dichloro-methane was only 110 ns. The measured lifetime of E-58 in methanol was 162)0,s, while it was 44 ms in 2-propanol. Thus, E-58 is considerably shorter-lived than E-56. Furthermore, E-58 is also shorter-lived than the analogous E-59 (Scheme 33), which cannot decay by intramolecular lactonization and has a lifetime of 3.6 ms in methanol. Thus, we proposed that E-58 undergoes solvent-assisted reketonization that is facilitated by the intramolecular H-atom bonding, as shown in Scheme 34. 

It is important to consider the magnitude of the recombination rate in studies of this type. For methane, is 1.7 x 10 sec at 93 K [81]. Thus if a concentration of ions of 0.1 pM was formed in the pulse, the electrons would disappear with a first half-life of 50 psec. For 2,2,4-trimethylpentane, k,. is 3.6 x 10 sec and for a similar concentration of electrons, the recombination lifetime would be a few nanoseconds. Where the electron mobility is lower, the recombination rate is slower. For methylcyclohexane,... 

A relevant example is the use of lifetimes to characterize the reactivity of organics. Compressed natural gas (CNG), for example, is a widely used fuel whose major component is methane, CH4. The only known significant chemical loss process for CH4 is reaction with OH ... 

The first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

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