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 Astrophy sical 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 chem istry of HNO3 under a variety of conditions. We find that HNO3 is significa ntly overestimated by the model at altitudes above 22 km, with the differen ce 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 r ate constants for the reactions OH + NO2 and OH + HNO3, but significant dif ferences remain. We discuss these observations in the context of possible u ncertainties in the calculated photolysis rate of HNO3 at wavelengths near 200 nm, uncertainties in the observations, errors caused by the use of cons trained steady state models, and possible missing sink reactions for HNO3.