A recently published analysis of slant columns of NO2 observed at twilight at 45 ¿S has identified trends of about 5%/decade between 1980 and 1998. This is twice the trend in tropospheric N2O, which is the source of stratospheric NO2. By means of a column photochemical model, we explore the sensitivity of NO2 to the observed trends in stratospheric temperature, O3 and H2O. The resulting calculated trends in NO2 are smaller than observed, and we cannot force agreement by varying the ozone or temperature trends. The calculated sensitivity of NO2 to stratospheric aerosol is large, and a 20% per decade decrease in aerosol surface area creates agreement. We conclude that a small residual in the statistical fit of aerosol to the NO2 measurements may remain, and is a likely cause of the trends found in the NO2 measurements. ¿ 2000 American Geophysical Union