The atmospheric distributions of CH 4 , C 2 H 6 , C 3 H 8 , C 2 H 2 , and C 2 Cl 4 and their annual chemical removal rates in steady state are determined versus latitude using a modified version of the Oslo two‐dimensional global tropospheric photochemical model. A photochemically calculated hydroxyl radical distribution, which has been validated with methylchloroform data, and seasonally varying surface measurements of the title species are used to compute their respective global annual surface source strengths and steady state lifetimes. Computed annual surface source strengths of CH 4 , C 2 H 6 , C 3 H 8 , C 2 H 2 , and C 2 Cl 4 are 490, 10.4, 8.4, 3.1 Tg (1 Tg = 10 12 g), and 432 kT (1 kT = 10 9 g), respectively. The calculated annual chemical removal rates of these compounds show distinct latitudinal distributions. Because their steady state global lifetimes are less than the model interhemispheric exchange time (about 1 year), the calculated north to south ratios of the deduced surface emission strengths of C 2 H 6 , C 3 H 8 , C 2 H 2 , and C 2 Cl 4 probably reflect the locations of their sources. Within the limits of previously estimated industrial emissions of C 2 Cl 4 (3–4 kT) for the southern hemisphere, our calculations indicate that about 47 kT of additional southern hemispheric source of C 2 Cl 4 is required for 1989–1990 to attain steady state mass balance in this region. There are two possibilities for this needed source: either other industrial sources are missing, or there are unidentified natural sources of C 2 Cl 4 . So far, oceans have been suggested as a natural source. Normalization of monthly varying ratios of hemispherically averaged calculated surface mixing ratios of C 2 H 6 , C 3 H 8 , and C 2 H 2 and their respective observed mixing ratios with respect to those for C 2 Cl 4 indicates that the sources of these hydrocarbons are seasonal in nature. It is also shown that convective transport effectively redistributes these short‐lived species but their calculated surface source strengths are relatively independent of this transport process.
Tropospheric concentrations of methane have increased steadily over the past ten years at an average rate of 16.5 ppbv per year, to a value in January 1988 of 1.69 ppmv. Measurements of CH sub 4 concentrations in air bubbles trapped in ice cores have shown concentrations of about 0.7 ppmv 200 years ago, with little further change for thousands of years before that. Interpolation earlier into this century suggests a concentration of about 1.1 to 1.2 ppmv in the 1940's. The only important pathway believed to be important for transfer of air from the troposphere to the stratosphere in through the tropical tropopause which is cold enough to reduce the mixing ratio of H sub 2 O in that air to about 3 ppmv. The only other major pathway for the delivery of H to the stratosphere is through the simultaneous injection of gaseous CH sub 4 in the same rising air. The formation of clouds in the stratosphere is dependent upon very low temperatures, and generally upon the amount of water vapor available. The possibility of a positive feedback exists, especially in well-oxidized methane air, that clouds are easier to form than earlier. This could mean enhancement of PSCs in both Antarctic and Arctic locations. Additional H sub 2 O in the stratosphere can also add to some of the greenhouse calculations.
