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.

Kinetic data imprecisions in photochemical rate calculations: Means, medians, and temperature dependence

“…Our treatment of reaction rates as random variables is necessitated by imprecision in their measurement, not by intrinsic properties of the rates themselves. This contrasts with the treatment of variables such as precipitation occurrence, which are intrinsically random [Stewart et al, 1989]. We view reaction rates as precise quantities.…”

Tropospheric Chemistry: 4 Years of U.S. Research, 1987–1990

“…The rise of atmospheric CH 4 could be due in part to a decrease of OH concentrations, caused by increasing emissions of anthropogenic compounds (in particular CO) which compete with CH 4 for available OH [Thompson et al, 1989, 1990]. Khalil and Rasmussen [1988] estimated that OH concentrations may have decreased by about 20% since preindustrial times.…”

“…Khalil and Rasmussen [1988] estimated that OH concentrations may have decreased by about 20% since preindustrial times. Assessment of anthropogenic influences on the OH budget is however complicated by a number of factors, including NO x emissions and depletion of stratospheric O 3 which both tend to mitigate the OH depletion caused by increases in CO [Liu and Trainer, 1988; Thompson et al, 1989]. Guthrie [1989] has argued that the CH 4 ‐CO‐OH system in the atmosphere is in a fragile chemical equilibrium maintained only by a narrow balance between CH 4 and CO emissions on the one hand, and OH production on the other hand; he proposed that the remarkable constancy of CH 4 concentrations from glacial to pre‐industrial times implies a natural covariance of CH 4 and CO emissions with OH production.…”

“…Tropospheric O 3 is of major concern as a public health hazard, a phytotoxicant, a greenhouse gas [Lacis et al, 1990], and a regulator of the oxidizing power of the troposphere [Thompson et al, 1989]. It is supplied by downward transport from the stratosphere, and is also produced within the troposphere by photochemical oxidation of hydrocarbons and CO in the presence of NO x .…”

“…Water‐soluble species may be scavenged by precipitation in the updraft, and the resulting sink depends on the precipitating area (again typically subgrid scale) and on the microphysical properties of cloud and rain. Recent theoretical studies have examined the scavenging efficiencies of water‐soluble gases in relation to gas‐droplet mass transfer [Schwartz, 1988b; Topalian and Montague, 1989; Lopez et al, 1989; Wexler and Seinfeld, 1990], cloud microphysics [Xing and Chameides, 1990], and rainfall frequency [Stewart, 1988; Stewart et al, 1989, 1990]. Advances in cloud chemistry modeling over the past few years have included condensation of chemical mechanisms [Pandis and Seinfeld, 1989a] and inclusion of cloud physics [Chang et al, 1987; Venkatram et al, 1988; Hales, 1989; Pandis and Seinfeld, 1989b].…”