A model of sulfate aerosol dynamics in atmospheric plumes

Abstract

The evolution of atmospheric aerosols is currently a subject of concern because of its relationship to environmental issues. Recently, a rigorous approach for modeling the dynamics of a sectional aerosol distribution has been developed by Gelbard, Tambour and Seinfeld (1980) J. Colloid Interface Sci., 76, 541–556. This paper makes use of this approach and extends it through the presentation of a mathematical model that describes advective transport, turbulent diffusion, gas-phase chemistry and aerosol dynamics in atmospheric plumes. This mathematical model incorporates the Reactive Plume Model, which is a Lagrangian model used to describe plume dynamics consisting of six contiguous cells that expand as the plume is dispersed by atmospheric turbulence. It assumes a Gaussian distribution of the plume arid takes into account interactions with the ambient air. The mathematical model also incorporates the Carbon-Bond Mechanism to model the gas-phase chemistry, which involves 73 reactions among 36 chemical specks. The aerosol population distribution for this model is represented by means of seven sections that have a particle diameter range of 0.01–2.15 μm. Equations describing aerosol dynamics are derived from the classical General Dynamic Equation (CDE) for a sectional aerosol distribution. The dynamic processes described in the model include inter- and intra-sectional coagulation, condensation of acid sulfate or ammonium sulfate and heterogeneous oxidation of sulfur dioxide (SO 2) in each section, as well as new-particle formation in the lowest section. Condensation is diffusion-limited and new-particle formation is estimated from monomer balance theory. Heterogeneous oxidation of SO 2 in the aerosol phase is parameterized by means of a pseudo-first-order chemical reaction. The aerosol dynamics module is evaluated with smog chamber data obtained for an aerosol-H 2SO 4-H 2O system. Aerosol plume model predictions are compared with airborne measurements obtained from the plume of the Navajo power plant on four different days during the VISTTA program, at downwind distances ranging from 25 to 88km. Important characteristics of aerosol dynamics are well described by the model. Secondary aerosol formation is most apparent in the 0.01–0.1 μm size range. Maximum predicted SO 2 homogeneous oxidation rates are 0.5 and 0.6 % h −1 for the summer and winter cases, respectively. Sensitivity studies performed with the aerosol plume model showed that background hydrocarbon concentrations, relative humidity and photolysis rates affect secondary aerosol formation most, whereas atmospheric stability has little effect.