Global atmospheric distributions and source strengths of light hydrocarbons and tetrachloroethene

The atmospheric distributions of CH4. C2H6, C3H8, C2H2, and C2Cl4 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 CH4, C2H6, C3H8, C2H2, and C2Cl4 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 C2H6, C3H8, C2H2, and C2Cl4 probably reflect the locations of their sources. Within the limits of previously estimated industrial emissions of C2Cl4 (3-4 kT) for the southern hemisphere, our calculations indicate that about 47 kT of additional southern hemispheric source of C2Cl4 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 C2Cl4. So far, oceans have been suggested as a natural source. Normalization of monthly varying ratios of hemispherically averaged calculated surface mixing ratios of C2H6, C3H8, and C2H2 and their respective observed mixing ratios with respect to those for C2Cl4 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.