A one‐dimensional numerical model to simulate formation and balance of sulfate during radiation fog events

Abstract

A one‐dimensional numerical model of the atmospheric boundary layer is coupled with a bulk phase aqueous chemistry model to simulate meteorological conditions, sulfate production, and pH values during radiation fog events. The boundary layer model includes prognostic equations for atmospheric temperature, specific humidity, liquid water content, wind speed, and the concentrations of the gas phase and liquid phase chemical reactants. Within the soil, temperature and liquid water content are predicted. A hybrid equilibrium and kinetic aqueous chemistry model is used to predict reactant concentrations in the fog water. The model is used to investigate the influence of soluble aerosol material and of trace gases on SO2 oxidation, on fog water pH, and on deposition fluxes due to sedimentation. As the meteorological conditions have an influence on fog height, liquid water content, and duration, not only the initial aerosol composition and the trace gas concentrations but also thermodynamic and dynamic conditions of the atmosphere and soil properties have an impact on liquid phase sulfate production during the fog event. With the exception of the dissipation stage, the turbulent transport of oxidants from aloft into the fog layer has only a small influence on the production of sulfate within a fog layer. An important mechanism for downward supply of fresh material is the rise of the fog top, followed by sedimentation of droplets. Sedimentation also results in the removal of a considerable amount of aerosol material from the fog layer.