POSSIBLE VARIATIONS IN ATMOSPHERIC METHANE

A model coupling photochemistry and vertical transport in the troposphere has been used to investigate the magnitude of possible future perturbations to atmospheric CH4. We have studied the response of atmospheric CH4 to al(cid:127) increase in the concentration of CO or to a variation in stratospheric Oa. Both of the above mechanisms, which could be caused by man's activities, arise because a change in the concentration of tropospheric OH will lead to a change in the CH(cid:127) abundance. Our calculations imply that a perturbation of (cid:127)30-40% in atmospheric CH(cid:127) could occur if stratospheric Oa were perturbed by 10% or if man-made CO continued to increase. The possible consequences of a CH(cid:127) perturbation may entail a perturbation in stratospheric photochemistry or in the thermal balance of the atmosphere.


INTRODUCTION
Atmospheric CH4 is produced at the earth's surface primarily by biological fermentation in anaerobic environments such as swamps, tropical rain forests, and paddy fields [Koyama, 1963]. Tropospheric CH4 is characterized by a well-mixed profile with a vertically constant mixing ratio of 1.
Calculations indicate that owing to destruction by reaction (1) in the troposphere, less than 10% of the CH• produced at the earth's surface enters the stratosphere. Note also that the oxidation of methane initiates a long series of reactions, known as the methane oxidation chain, which is believed to be a large source of tropospheric H•. and CO and which may also play a major role in controlling the abundance of tropospheric ozone [Levy, 1972;Wofsy et al., 1972;Crutzen, 1973Crutzen, , 1974a Charneides and Walker, 1973Walker, , 1976]. There is, however, a possibility that the methane oxidation chain does not lead to CO exclusively; instead, the direct evolution of CO•. may result, depending upon several formyl radical reaction paths (H. Niki, private communication, 1976). The tropospheric CH• abundance is determined by a balance between biological production at the ground, transport upward, and destruction by reaction (1). In the steady state the CH• continuity equation may be written as where F is the vertical methane flux, n is the number density, and kx is the rate coefficient for reaction (1), and we assume no net horizontal transport. Integrating (2) from z = 0 to z = zt,, the tropopause, we obtain f gtp As Chameides and Walker [1975] have pointed out, (4) implies that a perturbation in tropospheric OH will result in a change in the abundance of atmospheric CHq. Since CH4 takes part in several key stratospheric reactions [Crutzen, 1974b] and also contributes significantly to the atmospheric 'greenhouse effect' [Wang et al., 1976], a perturbation in atmospheric CH• could result in perturbations to stratospheric Oa and the thermal balance of the atmosphere. In this work, two possible causes of a perturbation to atmospheric CH• are investigated: an increase in the anthropogenic production rate of CO and a variation in stratospheric ozone. Both processes cause a change in tropospheric OH, which then results in a variation in CHq. To determine the dependence of CH• upon these parameters, numerical calculations were made by using a steady state one-dimensional tropospheric model which couples photochemistry and transport, as described by Chameides and Stedman [1977] (their 'standard model'), with input data appropriate for 30øN latitude at equinox. In the calculations an iterative scheme with a 1% convergence test on all species was used to determine the mixing ratios of the long-lived gases, such as CH4, as a function of stratOspheric Oa or anthropogenic CO, given a fixed flux condition at the earth's surface.
Note that in these steady state calculations we calculated the equilibrium CH• abundance. Since CH• has a residence time of about 10 years, it would probably take a decade or so for CH• to adjust to its equilibrium value after a perturbation. We have also assumed that CH4 is presently in equilibrium and that the CH4 production rate is constant at 1 X 10 • cm -• s -•. Note also that in our model, for every CH4 molecule oxidized, a CO molecule is produced.

X 10 • cm -•' s -• (1530 Mt yr -•) and a CO production rate via CH4 oxidation of I X 10 • cm -2 s -• (730 Mt yr -•)
Since it is estimated that other natural CO sources produce about 3 X 10 •ø cm -2 s - However, as Seller [1974] points out, since the uptake of CO by soils depends upon the CO density above the soil, the soil temperature, and the type of soil, the global uptake rate extrapolated from laboratory data is highly uncertain.
While further research is needed to determine more accurately the role of anthropogenic activities in the CO budget, models based on our present understanding of tropospheric photochemistry indicate that anthropogenic CO has already begun to perturb the atmosphere, and it• effect will likely increase if anthropogenic CO production continues to grow. ]. Given these assumptions, the photochemical model predicts that by the year 2000 the steady state C H4 abundance will have increased by almost 40%. Of course, improved pollution control devices and/or a switch from fossil fuel burning to alternative power sources could slow the anthropogenic production of CO and thereby lessen a perturbation to atmospheric CH4. In fact, pollution control in the United States has probably already slowed the global anthropogenic production of CO [Jaffe, 1973]. It is also possible that as the CO abundance increases, the magnitude of the soil sink for CO will also increase, thereby moderating the effect of anthropogenic activities. Finally, it is possible that future increases in anthropogenic NO• emissions may also perturb OH levels and thus affect the CH4 abundance. However, if CO production continues to grow, a significant increase in atmospheric CH4 may result in the coming d•cades. Further measurements, especially of CO, CH4, and OH, are needed to understand further the relationship between anthropogenic CO and atmospheric CH4.  *Present-day conditions are given by N(Oa) = 9 X 10 •8 cm -•, f(O3)(l I km) = 1.3 X 10 -7, and Xc., = 1.4 ppm. followed by
Since the O(•D) density can be altered either by changing the flux of near-UV radiation or by changing the local Oa density, a perturbation to stratospheric Oa can affect the OH and therefore also the CH4 tropospheric abundance in two ways: by changing the flux of near-U¾ radiation penetrating the tr0popause and by changing the Oa density in the lower stratosphere and thus the injection rate of stratospheric Oa into the troposphere. (Note that in our model, approximately 60% of the ozone found in the troposphere ori•ginates in the stratos. phere, the remainder being produced locally by photochemical processes [Chameides and Stedman, 1977].) For instance, for equinoctial conditions at 30øN near the ground we calculated that a change in N(Oa), t,he ozone column above 10 km, from 9 x 10 •8 to 8 x 10 •8 cm -2 resulted in a change in the production of O(•D) from 4.2 X 106 to 4.8 X 106 cm -3 s -•.
Similarly, by changing f(O3)(11 kin), the O3 mixing ratio at 11 kin, from 1.30 X 10 -7 to 1.65 X 10 -7 and keeping N(O3) constant we obtained an increase in the O(•D) production from 4.2 X 106 to 4.6 X 106 cm -3 s -•. Table 1 illustrates the calculated dependence of Xc., upon both N(O3) and f(O3)(l 1 kin). These results, which could be incorporated into stratospheric model calculations as a lower boundary condition upon CH4, indicate that a significant CH• perturbation could result from a change in stratospheric 03. For example, our calculations predict an approximate 30% decrease in Xc., if a 10% decrease in N(O3) is aCCompanied by a 25% increase in f(O3)(11 kin). Note that stratospheric model calculations predict that a decrease in N(O3) is generally accompanied by an increase in the ozone density near the tropopause due to photolytiC healing. However, Liu et al. [1976] found the opposite effect when they used a combination of rate coefficients which maximized the odd hydrogen density in the lower stratosphere.
Since stratospheric CH•, which originates in the troposphere, takes part in several key photochemical reactions, the possibility exists for feedback effects whereby an initial O3 perturbation is reduced or enhanced by a subsequent variation in atmospheric CHq. CH•, as a source of stratospheric odd hydrogen via reactions (1) and (7), may act to increase or decrease stratospheric O3 [Crutzen, 1971;McElroy et al., 1974;Rao-Vupputuri, 1974;Liu et al., 1976]  Whether CH• acts to decrease or increase stratospheric O3 and thus whether the system described above will stabilize or destabilize stratospheric O3 depend upon the importance of reactions (8) and (9) relative to reactions (10) and (11). Further study, both in the laboratory and with numerical models, is needed to help clarify this point. CONCLUSION Our calculations indicate that a 30-40% perturbation in atmospheric CH• may occur in the coming decades. The effects of a CH• perturbation are not clear, and further research is needed to understand fully the consequences of such a perturbation. Model calculations of the effects of CH4 upon stratospheric O3 are conflicting. The findings of Liu et al. [1976] and Rao-Vupputuri [1974] imply that an increase in CH• will result in a decrease in stratospheric 03, while Crutzen [1971] and McElroy et al. [1974] apparently find the opposite effect,. A careful reexamination of the importance of stratospheric CH4 as an odd hydrogen source and of the photochemical role of odd hydrogen in the lower stratosphere•is necessary to clarify this discrepancy.
Another possible consequence of a CH• variation is a perturbation to the thermal equilibrium of the atmosphere. Wang et al. [1976] have found that the infrared absorptio n bands of CH4 in the 7-to 14-urn spectral region contribute to the atmospheric greenhouse effect. Their calculations using a onedimensional radiative-convective model imply that a 0.3øK increase in surface temperature will result from doubling the CH• concentration. Thus a 30-40% perturbation in atmospheric CH• could have a significant climatic impact. Further calculations are necessary to establish more accurately the magnitude and nature of this climatic effect.
It is interesting to note that other long-lived species, such as H2, CH3CI, CHFCI2, and CHF2CI, which are attacked by OH in the troposphere, should undergo variations similar to those discussed for CHq. These variations are particularly significant in the case of the Cl-containing molecules, since they may affect stratospheric O3 [cf. Cicerone et al., 1975].
In view of the remaining gaps in our understanding of the tropospheric photochemical system, large uncertainties are associated with our results, and our predictions should therefore be taken as rough estimates of possible future trends. In order to better understand tropospheric photochemistry and therefore the magnitude of future perturbations to atmospheric CH•, further observational and theoretical research is necessary. A global program for measurement of OH, odd nitrogen, and CO levels is of primary importance.