The DOAS instrument consists of grating spectrometer covering the visible and near ultraviolet spectral region. Zenith-scattered sunlight is collected by simple one-lens telescopes and fed via optical fiber bundles into the spectrometers, where atmospheric absorption spectra are obtained. The instrument runs automatically. Total column densities of the stratospheric trace species ozone, NO2, BrO, and OClO are retrieved from the spectra using the DOAS algorithm. These are species that play a major role in ozone chemistry, either by themselves in ozone destruction (BrO) or as indicators of chlorine activation/deactivation (OClO). The chemistry and dynamics of ozone destruction is investigated, e.g. with respect to the location of the polar vortex during the winter. The instrument is also useful for detection of polar stratospheric clouds using the zenith-sky colour index method.