Methane, tropospheric sources

Recent estimates indicate that the level of carbon dioxide in the atmosphere has increased by a third since the beginning of the industrial age, and that it contributes significantly to global warming. Other major contributors include methane, tropospheric ozone, and nitrous oxide. Methane is the principal component of natural gas, but it is also produced by other sources such as rice paddies and farm animals. Tropospheric ozone is generated naturally and by the sunlight-... 

This leads to a reaction chain in which O3 is destroyed and not formed as under usual conditions in the troposphere. This type of chemistry is important in very clean environments such as in the unpolluted marine boundary layer in the southern hemisphere. Ozone also strongly regulates the oxidant capacity of the troposphere, because photolysis of O3 is the most dominant tropospheric source of OH radicals. These radicals are most important for the chemical degradation of the gaseous reduced compounds which include important greenhouse gases such as, e.g., methane. 

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

Air pollution in cities can be considered to have three components sources, transport and transformations in the troposphere, and receptors. The sources are processes, devices, or activities that emits airborne substances. When the substances are released, they are transported through the atmosphere, and are transformed into different substances. Air pollutants that are emitted directly to the atmosphere are called primary pollutants. Pollutants that are formed in the atmosphere as a result of transformations are called secondary pollutants. The reactants that undergo the transformation are referred to as precursors. An example of a secondary pollutant is troposphere ozone, O3, and its precursors are nitrogen oxides (NO = NO + NO2) and non-methane hydrocarbons, NMHC. The receptors are the person, animal, plant, material, or urban ecosystems affected by the emissions (Wolff, 1999). 

Taylor JA, Brasseur GP, Zimmermann PR, Cicerone RJ. 1991. A study of the sources and sinks of methane and methyl chloroform using a global three-dimensional Lagrangian tropospheric tracer transport model. Journal of Geophysical Research 96D 3013-3044. 

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. 

Chanton, J.P., Crill, P.M., Baertlett, K.B., and Martens, C.S. (1989b) Amazon Capims (floating grassmats) a source of 13C-enriched methane to the troposphere. Geophys. Res. Lett. 16, 799-802. 

The fraction of 0( D) atoms that form OH is dependent on pressure and the concentration of H2O typically in the marine boundary layer (MBL) about 10% of the 0( D) generate OH. Reactions (2.7 and 2.8) are the primary source of OH in the troposphere, but there are a number of other reactions and photolysis routes capable of forming OH directly or indirectly. As these compounds are often products of OH radical initiated oxidation they are often termed secondary sources of OH and include the photolysis of HONO, HCHO, H2O2 and acetone and the reaction of 0( D) with methane (see Figure 9). Table 2 illustrates the average contribution of various formation routes with altitude in a standard atmosphere. 

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... 

Swinnerton, Linnebom and Cheek (239) and Seiler and Junge (226) reported that the ocean, which they found to be supersaturated with CO, might be an important source. It is also possible that the oxidation of nonmethane hydrocarbons, particularly those whose production occurs over land, may be another huge source. Since, as we will see later, methane is thought to have a uniform distribution throughout the troposphere, some... 

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. 

By comparison, the average CO mixing ratio in Earth s troposphere is —0.12 ppmv and it is produced from a variety of anthropogenic and biogenic sources such as fossil fuel combustion, biomass burning, and oxidation of methane and other hydrocarbons. Most of the CO in Earth s troposphere is destroyed by reaction with OH radicals, which are also important for the catalytic... 

Methane is oxidized primarily in the troposphere by reactions involving the hydroxyl radical (OH). Methane is the most abundant hydrocarbon species in the atmosphere, and its oxidation affects atmospheric levels of other important reactive species, including formaldehyde (CH2O), carbon monoxide (CO), and ozone (O3) (Wuebbles and Hayhoe, 2002). The chemistry of these reactions is well known, and the rate of atmospheric CH4 oxidation can be calculated from the temperature and concentrations of the reactants, primarily CH4 and OH (Prinn et al., 1987). Tropospheric OH concentrations are difficult to measure directly, but they are reasonably well constrained by observations of other reactive trace gases (Thompson, 1992 Martinerie et al., 1995 Prinn et al., 1995 Prinn et al., 2001). Thus, rates of tropospheric CH4 oxidation can be estimated from knowledge of atmospheric CH4 concentrations. And because tropospheric oxidation is the primary process by which CH4 is removed from the atmosphere, the estimated rate of CH4 oxidation provides a basis for approximating the total rate of supply of CH4 to the atmosphere from aU sources at steady state (see Section 8.09.2.2) (Cicerone and Oremland, 1988). 

Crutzen PJ, Gidel LT. 1983. A two-dimensional photochemical model of the atmosphere. 2 The tropospheric budgets of the anthropogenic chlorocarbons, carbon monoxide, methane, chloromethane and the effect of various nitrogen oxides sources on the tropospheric ozone. Journal of Geophysical Research... 

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... 

Oxidation of methane is one of the sources of atmospheric CO. Another internal source of importance is the oxidation of terpenes and isoprenes emitted by forests (Crutzen, 1983). The carbon monoxide concentration in the atmosphere ranges from 0.05 to 0.20 ppmv in the remote troposphere (with considerable differences between the northern and southern hemispheres), which means that about 0.2 Pg of carbon is present as CO in the atmosphere. 

According to Levy (1971) the average concentration of OH radicals is 2.5 x 106 cm 3 in the troposphere. This calculated value is essentially confirmed by recent atmospheric measurements (e.g. Pemer et al., 1976). On the basis of the above figures a global tropospheric CH4 destruction rate of 900 x 106 t yr can be estimated, which is comparable to the total source strength given above. If this figure is correct the majority of CH4 is destroyed in the troposphere. Thus, Ehhalt and Schmidt (1978) speculate that about 85-90 % of the methane is removed from the air in the troposphere the remainder is destroyed in the stratosphere by the same process. 

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