Seasonal tropospheric distributions of ozone, carbon monoxide and aerosols and their relationship with sources over southern Africa are compared for two airborne sampling campaigns during southern hemisphere spring 1992 (SAFARI-92) and autumn 1994 (SAFARI-94). Average trace gas and aerosol concentrations from both campaigns are compared for equal spatial areas between 15ºE, 15ºS and 35ºE, 30ºS. This study presents a first estimate of air chemistry climatology over southern Africa for two seasons and uses trajectory analysis to identify possible sources of atmosphere trace species. Elevated chemical concentrations in the troposphere are attributed to two principal sources -- industrial emissions and biomass burning. Differences in southern African tropospheric chemistry are the result of seasonal variability in the frequency of fire emissions in the north and the permanence of industrial emissions in the southeast. Seasonal variation in trace gas and aerosol concentrations over this region is largely influenced by fire and regulated by dominant air flow patterns and strong stratification into layers of polluted and unpolluted air. The season-independent signal is shown to be that of industrial emissions that prevail throughout the year and determine a significant portion of the chemical conditions of the troposphere over the subcontinent.