An analysis of ozone measurements at Cerro Tololo (30 degrees S, 70 degrees W, 2200 m.a.s.l.) in Chile

Increases in tropospheric ozone (O-3) abundance are likely to take place in the near future in the populous and rapidly developing countries in the tr opics and subtropics, An accurate evaluation of the future impact of increa sing industrial activities in tropical and subtropical areas requires knowl edge of the background levels of ozone. New ozone monitoring stations have been installed at several sites by the World Meteorological Organization (W MO) since the mid-90s. We analyze ozone data collected during two years sin ce April 1996 at Cerro Tololo (30 degrees S, 70 degrees W, 2200 m.a.s.l.) s ome 50 km east from the city of La Serena. In this paper, we describe some of the atmospheric chemistry and meteorology that characterizes the Tololo site. The data show a seasonal variation with maximum mixing ratios in late winter and spring and minimum mixing ratios in late summer and early fall. These variations are most likely associated with the large-scale subsidenc e of the Hadley circulation and the location of the subtropical jet stream (STJ). Also? there is a diurnal variation that is probably partly associate d with a mountain wind flow which is strongest in late spring and summer mo nths. No significant mixing with marine boundary layer air perturbed by ant hropogenic activities is apparent from the data We find the Cerro Tololo si te to be generally representative for background conditions of free-troposp heric air in the subtropics of the Southern Hemisphere. This work is done w ithin the framework of a larger effort recently started by several Chilean institutions in cooperation with research centers abroad.