Huebert et al. observed anomalous vertical profiles of nitric acid vapor and nitrate aerosol near the Earth's surface. The average deposition velocity of nitrate aerosol was often larger than that of HNO3. Moreover, about 10% of the profiles implied an apparent surface emission of HNO3. The complexity of the numerical model required to simulate these anomalies depends on the relative size of the time scale for attainment of chemical equilibrium &tgr;c for the system of HNO3, NH3, and NH4NO3 and of the time scale for turbulent mixing &tgr;t. Because &tgr;c may not be well known and because chemistry may alter the effective eddy diffusivity K, we develop and use three numerical models, all of which can qualitatively explain the observations. For chemically conserved variables the gradient transfer assumption for vertical turbulent transfer is noncontroversial, but for chemically nonconserved variables the gradient transfer is potenially invalid.

One numerical model simulates the evolution of total nitrate and total ammonia, justifiably assumes gradient transport for these chemically conserved species, but requires instantaneous chemical equilibrium. This model produces acceptable results only if the aerosol does not exist so close to the surface that &tgr;dt<&tgr;c. The other two models are valid when aerosol exists close to the Earths's surface. They simulate the evolution of the three chemically nonconserved individual species. Both models allow relaxation toward chemical equilibrium of the three nonconserved species, but only one model explicitly predicts the effect of the chemical relaxation on the vertical turbulent fluxes. We find that this explicit consideration of the effect of chemical relaxation on R may be unnecessary. The most general numerical model may also be applied to other problems and was used to identify various limits for K. For example, for rapid decay &tgr;c replaces &tgr;t in K. Also K derived from observations of a decaying tracer, example, radon or thoron, may be significantly less than K for a conserved tracer. ¿ American Geophysical Union 1988