Stratospheric absorption due to NO2 has been identified 1-3 in infrared spectra of the solar radiation recorded from balloon borne gondolas floating at mid-latitudes in the altitude range of 30 km when the solar zenith angle is larger than 91°. In this particular geometric situation most of the absorption by the atmosphere takes place at altitudes smaller than float altitude over very long optical paths. This offers two great advantages: the sensitivity of the absorption method is maximum and the quantitative interpretation is easier because absorption takes place mostly at constant total pressure, constituent concentration and temperature where the solar radiation passes closest to the Earth's surface.