Column measurements of NO2 and HNO3 were made over Fairbanks, Alaska, between August 1992 and April 1994. The seasonal minimum of the NO2 column occurs in late December, while the minimum of the HNO3 column is in early August. The high HNO3 column amounts observed in February as opposed to the lower winter value predicted by gas phase two-dimensional models suggest that the storage reservoir species N2O5 does not accumulate during the winter. Instead, it appears to be converted to HNO3 on sulfate aerosols. Three-dimensional model calculations which include the heterogeneous reaction of N2O5 with aerosols simulate the anti-correlation relationship between NO2 and HNO3 well for the summer and predict the magnitude of the NO2 maximum but overestimate the HNO3 minimum. ¿ 1998 American Geophysical Union