Numerical modeling of dry deposition coupled to 22 photochemical reactions

Three Eulerian models for the dry deposition of photochemically reactive species were formulated and evaluated: a K-theory model with independent transport and deposition of each species, a K-theory model coupled with 22 gas-phase reactions, and a second-order flux-budget model coupled with 22 reactions. Operator splitting was used to separately solve the reaction and dispersion terms in the models including photochemistry. In an evaluation of numerical method performance, the Adams–Moulton method with a pseudo-steady-state approximation for the free radicals was found to be consistent with, but more computationally efficient than Gear’s method for the solution of the reaction terms. The sensitivity of profiles of vertical concentration and flux to the Eulerian model formulation varied according to the species’ Damköhler number. Although a K-theory model is adequate for weakly depositing species with Damköhler numbers <10-3, a second-order flux-budget model is required for species with Damköhler numbers that exceed unity, such as nitrogen oxides, free radicals, and those sensitive to net flux production by chemical reactions. Selection of an appropriate model formulation for the entire system depended on the most reactive species. Simple K-theory models may not accurately predict dry deposition fluxes in the urban surface layer.