Possible causes of stratospheric NO2 trends observed at Lauder, New Zealand

A recently published analysis of slant columns of NO2 observed at twilight at 45 degreesS has identified trends of about 5%/decade between 1980 and 19 98. This is twice the trend in tropospheric N2O, which is the source of str atospheric NO2. By means of a column photochemical model, we explore the se nsitivity of NO2 to the observed trends in stratospheric temperature, O-3 a nd H2O. The resulting calculated trends in NO2 are smaller than observed, a nd we cannot force agreement by varying the ozone or temperature trends. Th e calculated sensitivity of NO2 to stratospheric aerosol is large, and a 20 % per decade decrease in aerosol surface area creates agreement. We conclud e that a small residual in the statistical fit of aerosol to the NO2 measur ements may remain, and is a likely cause of the trends found in the NO2 mea surements.