Sensitivity Analysis of a Mixed-Phase Chemical Mechanism using Automatic Differentiation

A mixed-phase chemistry box model is applied to study heterogeneous chemistry and its effect on tropospheric gas-phase chemistry, particularly on photochemical production of O3 and photochemical indicators for O3-NOx-hydrocarbon sensitivity, under a variety of atmospheric conditions ranging from remote marine to heavily-polluted atmospheres. A subsequent sensitivity analysis of the mixed-phase chemical mechanism is conducted using the novel automatic differentiation ADIFOR tool, which calculates the local sensitivities of species concentrations in gas, aqueous and aerosol phases with respect to a variety of model parameters. The main chemical reaction pathways in all phases, interfacial mass transfer processes, and ambient physical parameters that affect tropospheric O3 formation, O3-precursor relations and photochemical indicators under all modeled conditions are identified and analyzed. The results show that the presence of clouds and aerosols not only reduces many gas-phase species concentrations and the total oxidizing capacity but alters O3-precursor relations. Cloud chemistry is the dominant heterogeneous process under the remote and marine atmospheres. Aerosols are important scavengers for gaseous species in polluted atmospheres when the total aerosol surface area is larger than 1000 μm2 cm-3. Decreases in concentrations and formation rates of O3 can be up to 27% and 100%, respectively, depending on the initial atmospheric conditions and preexisting surfaces. The significant decreases in photochemical O3 formation are primarily caused by the aqueous-phase reactions of O2with dissolved HO2 and O3 under most cloudy conditions and heterogeneous uptake of O3, HCHO, HO2, H2O2 and NO2 in the presence of aerosols. Heterogeneous chemical processes also affect the photochemical indicators and their sensitivities to many model parameters in a variety of ways.