A kinetic aerosol model for the formation and growth of secondary sulfuric acid particles

Abstract A numerical kinetic aerosol model is utilized to study conditions under which secondary sulfuric acid particle formation followed by condensation and coagulation growth may substantially increase the concentration of submicron size aerosols which can contribute significantly to visibility degradation, adverse health effects, injuries to agricultural products and inadvertent weather modification. This model incorporates the mechanisms of photochemical reactions, heteromolecular nucleation, heteromolecular condensation and thermal coagulation. Microphysical and chemical input parameters include: the rate constant for the formation of gaseous sulfuric acid molecules, the surface tension and equilibrium vapor pressure for sulfuric acid solution droplets and the accommodation coefficient for nucleation and condensation growth of secondary sulfuric acid aerosols. Sensitivity of the model calculations to uncertainties in the rate constant data, surface tension and vapor pressure data and the accommodation coefficient is analyzed. Possible errors arising from the numerical approximations used in the calculations are investigated. The difficulties of model validation by comparison with laboratory and field experiments are also discussed. The main purpose of this model calculation is to demonstrate qualitatively the sensitivity of various chemical and microphysical input parameters and the nonlinear interaction between the mechanisms in the study of secondary aerosol formation and growth. Secondary aerosol formation and growth in the presence of preexisting particles is studied under different atmospheric conditions by varying relative humidity, temperature, initial particle size distribution and rate of production of gaseous sulfuric acid molecules. Our model calculations show that the rate limiting factor for the formation and growth of secondary acid aerosols into the submicron size range is the amount of sulfuric acid gaseous molecules available in the atmosphere. It is also shown that the increased number of submicron size particles which results from secondary aerosol formation can contribute an increase in the light extinction coefficient and, therefore, a decease in visibility.

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