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 Ver sus latitude using a modified version of the Oslo two-dimensional glob al tropospheric photochemical model. A photochemically calculated hydr oxyl radical distribution, which has been validated with methylchlorof orm data, and seasonally varying surface measurements of the title spe cies 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 c alculated annual chemical removal rates of these compounds show distin ct latitudinal distributions. Because their steady state global lifeti mes are less than the model interhemispheric exchange time (about 1 ye ar), the calculated north to south ratios of the deduced surface emiss ion strengths of C2H6, C3H8, C2H2, and C2Cl4 probably reflect the loca tions of their sources. Within the limits of previously estimated indu strial emissions of C2Cl4 (3-4 kT) for the southern hemisphere, our ca lculations indicate that about 47 kT of additional southern hemispheri c source of C2Cl4 is required for 1989-1990 to attain steady state mas s balance in this region. There are two possibilities for this needed source: either other industrial sources are missing, or there are unid entified natural sources of C2Cl4. So far, oceans have been suggested as a natural source. Normalization of monthly varying ratios of hemisp herically averaged calculated surface mixing ratios of C2H6, C3H8, and C2H2 and their respective observed mixing ratios with respect to thos e for C2Cl4 indicates that the sources of these hydrocarbons are seaso nal in nature. It is also shown that convective transport effectively redistributes these short-lived species but their calculated surface s ource strengths are relatively independent of this transport process.