Atmospheric residence times for soluble species: Differences in numerical and analytical model results

We have compared numerically calculated soluble species residence times with several analytic residence time models. The numerical results are dependent on whether randomly or periodically distributed precipitation events are assumed. A preferred time of occurrence for rain, such as morning or afternoon, influences the computed HNO₃ residence time for precipitation cycle times (wet plus dry period) of about a day, but exerts less influence as the cycle time increases. An analytic residence time model developed by Rodhe and Grandell (1972, Tellus 24, 442-454) is found to agree well with residence times computed for randomly distributed precipitation events while a model developed by Giorgi and Chameides (1985, J. geophys. Res. 90, 7872-7880 is more suitable for periodic rain. The present numerical calculations resolve an apparent ambiguity in HNO₃ residence times computed in earlier numerical studies. Using mean precipitation periods and rainfall amounts characteristic of the northeastern U.S. as a guide, we find that residence times computed under the assumption of randomly distributed wet and dry events agree better with observed aerosol lifetimes than do those computed assuming periodic rain.