Observations of the stratospheric NO2 column amount over the Colorado mountains during January, February, and March, 1992 are compared to concurrent lidar measurements. During this period, large aerosol amounts due to the volcanic material injected into the stratosphere by Mount Pinatubo were observed over Colorado. The observed NO2 column displays a strong anti-correlation with increasing amounts of stratospheric aerosol near 25--30 km for relatively low aerosol loadings. At higher aerosol loadings, the NO2 response appears to saturate, displaying little further reduction. The reasons for the altitude dependence of this relationship are probed with a one-dimensional stratospheric model, and it is shown that such a saturation response is to be expected for conditions where the heterogeneous reaction between N2O5 and H2O dominates the NOx chemistry, as in Prather (1992). The results therefore provide evidence that heterogeneous reactions influence the chemical composition of the mid-latitude stratosphere, and in the general manner predicted by theory. ¿ American Geophysical Union 1993