Visible absorption spectroscopy was employed for stratospheric measurements at McMurdo Station, Antarctica, during the summer and fall seasons in 1990, 1991, 1992, and 1993. Observed column amounts of NO2 were as much as 50% smaller in 1992 and 1993 than in 1990 and 1991. The measured decreases in NO2 are believed to be due to the hydrolysis of N2O5 on the enhanced volcanic aerosols injected by the massive eruption of Mount Pinatubo in mid-1991. Such decreases in NO2 are broadly consistent with observations at other latitudes and with model calculations. However, substantially reduced NO2 was also observed during the polar day period in summer under conditions of continuous local solar illumination. The formation of the key N2O5 intermediate and reduction of NO2 at this time of year probably occurs via two mechanisms: (1) excursions of air parcels to lower latitudes where nighttime chemistry can produce N2O5 and (2) conversion of a small fraction of the NO3 formed during the day to N2O5. Model calculations show that the latter mechanism can be important in spite of the fast daytime photolysis of NO3.