Comparison of parameterized nitric acid rainout rates using a coupled stochastic-photochemical tropospheric model

Abstract

A major tropospheric loss of soluble species such as nitric acid (HNO₃) results from scavenging by water droplets. Several theoretical formulations have been advanced which relate an effective time independent loss rate for soluble species to statistical properties of precipitation such as the wet fraction and length of a precipitation cycle. There is not enough data available on trace species concentrations to test the computational results of alternative loss rate models against observations. Therefore in this paper, various “effective” loss rates that have been proposed are compared with the results of detailed time dependent model calculations carried out over a seasonal time scale. The model which we have developed for this purpose is a stochastic precipitation model coupled to a tropospheric photochemical model. The results of numerous time dependent seasonal model runs are used to derive numerical values for the nitric acid residence time for several assumed sets of precipitation statistics. These values are then compared with the results obtained by utilizing theoretical “effective” loss rates in time independent models.