A three-dimensional simulation of the evolution of HNO3 has been run for the winter of 1979. Winds and temperatures are taken from a stratospheric data assimilation analysis, and the chemistry is based on Limb Infrared Monitor of the Stratosphere (LIMS) observations. The model is compared to LIMS observations to investigate the problem of “missing” nitric acid chemistry in the winter hemisphere. Both the model and observations support the contention that a nitric acid source is needed outside of the polar vortex and north of the subtropics.
Observations show that nitric acid and potential vorticity are uncorrelated in middle latitudes outside the polar vortex. This suggests that HNO3 is not dynamically controlled in middle latitudes. The model shows that given the time scales of conventional chemistry, dynamical control is expected. Therefore, an error exists in the conventional chemistry or additional processes are needed to bring the model and data into agreement. Since the polar vortex is dynamically isolated from the middle latitudes, and since the highest HNO3 values are observed in October and November, a source associated solely with polar stratospheric clouds cannot explain the deficiencies in the chemistry. The role of heterogeneous processes on background aerosols is reviewed in light of these results.