Coupling of highly explicit gas and aqueous chemistry mechanisms for use in 3-D

Abstract

This study discusses the coupling of a near-explicit gas-phase chemical mechanism with an extensive aqueous-phase mechanism in an accurate chemical solver designed for use in 3-D models. The gas and aqueous mechanisms and the solver used are the Master Chemical Mechanism (MCM 3.1), the Chemical Aqueous Phase Radical Mechanism (CAPRAM 3.0i), and the SMVGEAR II ordinary differential solver, respectively. The MCM has over 13,500 reactions and 4600 species, whereas CAPRAM treats aqueous chemistry among 390 species and 829 reactions (including 51 gas-to-aqueous phase reactions). SMVGEAR II is a sparse-matrix Gear solver that reduces the computation time significantly while maintaining any specified accuracy. MCM has been used previously with SMVGEAR II in 3-D, and computer timings here indicate that coupling MCM with CAPRAM in SMVGEAR II is also practical. Gas- and aqueous-phase species are coupled through time-dependent dissolutional growth and dissociation equations. This method is validated with a smaller mechanism against results from a previous model intercomparison. When the smaller mechanism is compared with the full MCM-CAPRAM mechanism, some concentrations are still similar but others differ due to the greater detail in chemistry. We also expand the mechanism to include gas–aqueous transfer of two acids, glycolic acid and glyoxylic acid, and modify the glyoxal Henry's law constant from recent measurements. The average glyoxal partitioning in the cloud changed from 67% aqueous-phase to 87% aqueous-phase with the modifications. The addition of gas–aqueous transfer reactions increased the average gas-phase percentage of glycolic acid to 19% and of glyoxylic acid to 16%. This full gas-phase and aqueous-phase chemistry module is a potentially useful tool for studying air pollution in a cloud or a fog.