We present remote-sensing measurements of the abundance of nitric acid (HNO3) in the lower and middle stratosphere (between 16 and 40 km) covering the period 1989--1997. The measurements were made with the Smithsonian Astrophysical Observatory Far-Infrared Spectrometer (FIRS-2) under a wide range of aerosol surface area density. We compare our measurements with the results of a photochemical steady state model to test our understanding of the chemistry of HNO3 under a variety of conditions. We find that HNO3 is significantly overestimated by the model at altitudes above 22 km, with the difference increasing with increasing altitude and decreasing aerosol surface area density. The agreement between modeled and measured HNO3 can be improved by either decreasing the rate of OH+NO2 by 35% or by using newly measured rate constants for the reactions OH+NO2 and OH+HNO3, but significant differences remain. We discuss these observations in the context of possible uncertainties in the calculated photolysis rate of HNO3 at wavelengths near 200 nm, uncertainties in the observations, errors caused by the use of constrained steady state models, and possible missing sink reactions for HNO3. ¿ 1999 American Geophysical Union