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Patrick Hamill
,
O. B. Toon
, and
C. S. Kiang

Abstract

Physical processes which affect stratospheric aerosol particles include nucleation, condensation, evaporation, coagulation and sedimentation. We carry out quantitative studies of these mechanisms to determine if they can account for some of the observed properties of the aerosol. We show that the altitude range in which nucleation of H2SO4-H2O solution droplets can take place corresponds to that region of the stratosphere where the aerosol is generally found. Since heterogeneous nucleation is the dominant nucleation mechanism, the stratospheric solution droplets are mainly formed on particles which have been mixed up from the troposphere or injected into the stratosphere by volcanoes or meteorites. Particle growth by heteromolecular condensation can account for the observed increase in mixing ratio of large particles in the stratosphere. Coagulation is important in reducing the number of particles smaller than 0.05 µm radius. Growth by condensation, applied to the mixed nature of the particles, shows that available information is consistent with ammonium sulfate being formed by liquid phase chemical reactions in the aerosol particles. The upper altitude limit of the aerosol layer is probably due to the evaporation of sulfuric acid aerosol particles, while the lower limit is due to mixing across the tropopause.

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R. P. Turco
,
P. Hamill
,
O. B. Toon
,
R. C. Whitten
, and
C. S. Kiang

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.

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O. B. Toon
,
R. P. Turco
,
D. Westphal
,
R. Malone
, and
M. Liu

Abstract

The numerical algorithms which we use to simulate the advection, diffusion, sedimentation, coagulation and condensational growth of atmospheric aerosols are described. The model can be used in one, two, or three spatial dimensions. We develop the continuity equation in a generalized horizontal and vertical coordinate system which allows the model to be quickly adapted to a wide variety of dynamical models of global or regional scale. Algorithms are developed to treat the various physical processes and the results of simulations are presented which show the strengths and weaknesses of these algorithms. Although our emphasis is on the modeling of aerosols, the work is also applicable to simulations of the transport of gases.

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R. P. Turco
,
O. B. Toon
,
J. B. Pollack
,
R. C. Whitten
,
I. G. Poppoff
, and
P. Hamill

Abstract

We have estimated the potential effects on stratospheric aerosols of supersonic transport emissions of sulfur dioxide gas and submicron soot granules, and space shuttle rocket emissions of aluminum oxide particulates. Recently, exhaust particles from large aircraft and rocket engines have been characterized experimentally, and we have adopted new data where appropriate. We use an interactive particle-gas model of the stratospheric aerosol layer to calculate changes due to exhaust emissions. We also employ an accurate radiation transport model to compute the effect of aerosol changes on the earth's average surface temperature. Our major conclusions are as follows. The release of large numbers of small particles (soot or aluminum oxide) into the stratosphere should not lead to corresponding significant increase in the concentration of large, optically active aerosols. On the contrary, the increase in large particles is severely limited by the total mass of sulfate available to make large particles in situ, and by the rapid loss of small seed particles via coagulation. We find that a fleet of several hundred advanced supersonic aircraft operating daily at 20 km, or the launch of one space shuttle rocket per week, could produce roughly a 20% increase in the large-particle concentration of the stratosphere. We find, in addition, that aerosol increases of this magnitude would reduce the global surface temperature by less than 0.01 K, a negligible climate change.

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