Nitric acid in the middle stratosphere as a function of altitude and aerosol loading

We present remote‐sensing measurements of the abundance of nitric acid (HNO 3 ) 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 HNO 3 under a variety of conditions. We find that HNO 3 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 HNO 3 can be improved by either decreasing the rate of OH + NO 2 by 35% or by using newly measured rate constants for the reactions OH + NO 2 and OH + HNO 3 , but significant differences remain. We discuss these observations in the context of possible uncertainties in the calculated photolysis rate of HNO 3 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 HNO 3 .