Following the massive eruption of Mt. Pinatubo during June of 1991, twilight slant column abundances of NO2 were observed to decrease by 30--40% at Lauder, New Zealand (45 ¿S). Johnston et al. [1992> proposed that at most, 10--15% of the observed decrease could be due to alterations of the effective optical path of light received by the detector associated with increased scattering, the balance likely being the result of heterogeneous conversion of odd-nitrogen chemical species to nitric acid on the surfaces of the volcanic aerosols. In this study, it is shown that nearly all of the observed decrease in twilight NO2 slant column abundance is likely to be due to heterogeneous chemistry. Airmass factors calculated with a spherical Monte Carlo radiative transfer model are shown to be relatively insensitive to the presence of the Pinatubo volcanic aerosols above about 20 km, although some significant differences are obtained below this altitude. Changes in photolysis rates due to increased scattering are also shown to have only a small effect on the total twilight NO2 column abundances. ¿ American Geophysical Union 1992