A One-Dimensional Model Describing Aerosol Formation and Evolution in the Stratosphere: I. Physical Processes and Mathematical Analogs

Abstract

We have developed a time-dependent one-dimensional model of the stratospheric sulfate aerosol layer. In constructing the model, we have incorporated a wide range of basic physical and chemical processes in order to avoid predetermining or biasing the model predictions. The simulation, which extends from the surface to an altitude of 58 km, includes the troposphere as a source of gases and condensation nuclei and as a sink for aerosol droplets; however, tropospheric aerosol physics and chemistry are not fully analyzed in the present model. The size distribution of aerosol particles is resolved into 25 discrete size categories covering a range of particle radii from 0.01–2.56 µm with particle volume doubling between categories. In the model, sulfur gases reaching the stratosphere are oxidized by a series of photochemical reactions into sulfuric acid vapor. At certain heights this results in a supersaturated H₂SO₄–H₂O gas mixture with the consequent deposition of aqueous sulfuric acid solution on the surfaces of condensation nuclei. The newly formed droplets grow by heteromolecular heterogeneous condensation of acid and water vapors; the droplets also undergo Brownian coagulation, settle under the influence of gravity and diffuse in the vertical direction. Below the tropopause, particles are washed from the air by rainfall. Most of these aspects of aerosol physics are treated in detail, as is the atmospheric chemistry of sulfur compounds. In addition, the model predicts the quantity of solid (or dissolved) core material within the aerosol droplets. Depending on the local physical environment, aerosol droplets may either grow or evaporate; if they evaporate, their cores are released as solid nuclei. A set of continuity equations has been derived which describes the temporal and spatial variations of aerosol droplet and condensation nuclei concentrations in air, as well as the sizes of cores in droplets; techniques to solve these equations accurately and efficiently have also been formulated. We present calculations which illustrate the precision and potential applications of the model.