Abstract

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 tropics and subtropics. An accurate evaluation of the future impact of increasing industrial activities in tropical and subtropical areas requires knowledge of the background levels of ozone. New ozone monitoring stations have been installed at several sites by the World Meteorological Organization (WMO) since the mid-90s. We analyze ozone data collected during two years since April 1996 at Cerro Tololo (30°S, 70°W, 2200 m.a.s.l.) some 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 subsidence of the Hadley circulation and the location of the subtropical jet stream (STJ). Also, there is a diurnal variation that is probably partly associated with a mountain wind flow which is strongest in late spring and summer months. No significant mixing with marine boundary layer air perturbed by anthropogenic activities is apparent from the data. We find the Cerro Tololo site to be generally representative for background conditions of free-tropospheric air in the subtropics of the Southern Hemisphere. This work is done within the framework of a larger effort recently started by several Chilean institutions in cooperation with research centers abroad.