Ground-based measurements of stratospheric slant column NO2 amounts made at Halley Bay, Antarctica in 1987 are compared with ozone and temperature profiles from balloon-borne sondes. Sunrise-to-sunset (am/pm) ratios of NO2 have been calculated in autumn and spring by using the sonde data in conjunction with a simple photochemical model for the conversion of NO2 to N2O5. These calculations can be reconciled with the spectrometric measurements of column NO2, provided that the bulk of the NO2 layer is assumed to lie at a height of about 25 km. The small amounts of NO2 that are present in the stratospheric column during the first 6 weeks of spring are therefore confined to altitudes above the ozone depletion region. Slow recovery of the NO2 column in spring compared with the rate of its decline in autumn indicates slow photolysis of depleted levels of N2O5 inside the polar vortex. ¿American Geophysical Union 1991