Methane, tropospheric lifetime

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... 

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). 

Since TO is a greenhouse gas, emissions of it can indirectly affect the formation of atmospheric greenhouse effect by influencing the TO concentration field. Moreover, MGC/TO precursors change the hydroxyl concentration field and, hence, the oxidation power of the troposphere. In its turn, the distribution of hydroxyl concentration in the troposphere controls the lifetime and, thus, the level of concentration of methane at the global scale. 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

Methane is found throughout the troposphere in concentrations now exceeding 1.6 parts per million by volume (1 ppmv = 10" ), and is the most abundant source of C-H bonds in the atmosphere. Its primary atmospheric removal process is also reaction with HO radicals, as in (6). The atmospheric lifetimes for CHa and CILCCh can be connected through the relative rates of reactions (5) and (6), and the value observed in the laboratory... 

Quantitative understanding of the sources, sinks and atmospheric lifetime for CHa is an important future goal for several reasons. The direct increase in tropospheric CHa concentrations adds another important infrared absorbing contributor to the greenhouse effect. The calculated contribution from a CHa increase of 0.18 ppmv in a decade is a tropospheric temperature increase of 0.04 C [N.A.S., 1983], about 1/3 as large as that calculated for the observed 12 ppmv increase for CO2 over the decade from 1970-1980. As described earlier, increasing concentrations of CHa in the stratosphere have an influence on ozone-depletion by ClOx through diversion of Cl into HCl, and should in addition after oxidation increase the upper stratospheric concentrations of H2O. Methane is also a participant in tropospheric chemical reaction sequences which lead under some conditions to the formation of ozone. 

One may conclude that approximately 600 Tg of methane are produced each year. Since the total atmospheric burden of methane is about 4900 Tg (corresponding to a mean tropospheric mixing ratio of about 1.75 ppmv), a global atmospheric lifetime of 8 years can be derived. 

The oxidizing properties of the troposphere have a strong influence on the lifetime of chemical compounds in the atmosphere, and hence on the probability for a molecule to reach the middle atmosphere. Most hydrocarbons, for example, including hydrogenated halocarbons, are efficiently destroyed by the OH radical in the troposphere before they can penetrate into the stratosphere. Compounds that are not oxidized in the troposphere (e.g., chlorofluorocarbons) or weakly oxidized (e.g., methane) reach the stratosphere more easily. 

Methyl chloride reacts with OH radicals by hydrogen abstraction in the same way as methane, except that the rate coefficient is about five times greater. From the known rate coefficient and the usual assumption of an average OH number density of 5 x 105 molecules/cm3 in the troposphere, one infers an atmospheric lifetime for CH3C1 of about 1.8 yr. The uniform distribution of methyl chloride in the troposphere would be incompatible with such a relatively short lifetime, if the substance were mainly human-... 

The oxidizing power of the atmosphere has likely decreased significantly, especially in the Northern Hemisphere, as a result of human activities. As a result, the lifetime of methane may have increased by 10-15% since the preindustrial era. At the same time, the abundance of tropospheric ozone has increased perhaps by as much as a factor of 2-3 in the Northern Hemisphere. Enhanced biomass burning fluxes of NO CO, and hydrocarbons from tropical ecosystems are likely to be important. Future changes in tropospheric ozone are predicted to be largest in the tropics (India, China). These projected increases in tropical emissions are likely to have a... 

Methane oxidation leads to a net loss of OH in the atmosphere, thereby lengthening the lifetime of CH4 itself (we will discuss this later in this chapter). It is estimated that this longer lifetime increases the radiative forcing of CH4 by 25-35% over that in the absence of this feedback effect. Methane oxidation also leads to tropospheric O3 this indirectly increases the greenhouse effect by another 30-40% through the effect of the added O3 itself. Finally, increases in CH4 also indirectly lead to further climate forcing by increasing stratospheric H20 (about 7% of CH4 is oxidized in the upper troposphere). 

To illustrate the nonlinear chemical feedbacks in the atmospheric system, let us consider methane. One kilogram of CH4 released from the surface becomes well mixed in the troposphere. A portion of this CH4 is transported into the stratosphere. As we have just described, the added kilogram of CH4 is removed with an adjustment time of about 12 years (and not with its global lifetime of 8.4 years due to OH reaction and stratospheric loss). That amount of the CFLt perturbation that makes it into the stratosphere directly affects stratospheric chemistry that controls stratospheric 03 abundance. More CH4 will... 

The ability of these compounds to absorb infrared radiation varies widely from compound to compound, as does their life in the atmosphere before they undergo photochemical reactions or are absorbed in the oceans or on land. Methane has a concentration of only l.Vppmv in the troposphere, which is much less than that of carbon dioxide. On the other hand, each molecule of methane has a global warming potential (GWP) value that is 21 times that of carbon dioxide over the course of 100 years. (Note the GWP value has been developed to compare the ability of each greenhouse gas to trap infrared radiation over 100 years relative to another gas by convention, carbon dioxide has a GWP of 1.) Although methane has a relatively short lifetime (a few years)... 

The troposphere has an estimated 155 Tg of hydrogen gas (H2), with approximately a two-year lifetime (Chapter 2.8.2.10). Many sources of hydrogen gas and a few major sinks account for this relatively short lifetime. The main pathway in the production of hydrogen atoms in the air is the methane (CH4) conversion by the OH radical and subsequent photolysis of formaldehyde (HCHO) see reactions (5.42) to (5.48). This process accounts for about 26 Tg H yr (Novelli et al. 1999). 

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