Ground-based observations of nitrogen dioxide and ozone total columns have been performed since June 1990 at the International Scientific Station at the Jungfraujoch (45°N, 8°E). Measurements are made twice a day, at sunrise and sunset, by looking at the sunlight scattered at zenith in the visible range. NO2 and O3 vertical abundances are deduced by the differential absorption method. The error sources of the method are discussed in detail. For most observation conditions, the precision of measurement is estimated at about 11% for NO2 and 6% for O3. However larger errors might be encountered occasionally due to tropospheric pollution or enchanced multiple scattering in thick clouds. It is shown that the signature of such events can be detected in the evolution of the retrieved vertical column during the twilight period. A systematic method to detect and reject data significantly biased is described and applied to the NO2 time-series of measurements.