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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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Patrick Hamill
,
R. D. Cadle
, and
C. S. Kiang

Abstract

The homogeneous- and heterogeneous-heteromolecular nucleation of H2SO4-H2O solution droplets in the stratosphere is investigated and order-of-magnitude nucleation rates are evaluated. The heterogeneous processes considered are nucleation (i) onto soluble particles, (ii) onto flat insoluble surfaces, (iii) onto spherical insoluble particles and (iv) onto ions. The relative importance of the various nucleation mechanisms is determined for conditions assumed to correspond to 18 km altitude. Under the assumed conditions the heterogeneous nucleation rate onto insoluble particles is shown to be about 1069 times larger than the homogeneous nucleation rate and 1057 times larger than nucleation onto ions.

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

Abstract

We have performed sensitivity tests on a one-dimensional physical-chemical model of the unperturbed stratospheric aerosols and have compared model calculations with observations. The sensitivity tests and comparisons with observations suggest that coagulation controls the particle number mixing ratio, although the number of condensation nuclei at the tropopause and the diffusion coefficient at high altitudes are also important. The sulfate mass and large particle number (r > 0.15 µm) mixing ratios are controlled by growth, sedimentation, evaporation at high altitudes and washout below the tropopause. The sulfur gas source strength and the aerosol residence time are much more important than the supply of condensation nuclei in establishing mass and large particle concentrations. The particle size is also controlled mainly by gas supply and residence time. OCS diffusion (not SO2diffusion) dominates the production of stratospheric H2SO4 particles during unperturbed times, although direct injection of SO2 into the stratosphere could be significant if it normally occurs regularly by some transport mechanism. We suggest a number of in-situ observations of the aerosols and laboratory measurements of aerosol parameters that can provide further information about the physics and chemistry of the stratosphere and the aerosols found there.

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C. S. Kiang
,
D. Stauffer
,
G. H. Walker
,
O. P. Puri
,
J. D. Wise Jr.
, and
E. M. Patterson

Abstract

The homogeneous nucleation theory of liquid droplets in supersaturated vapors is reviewed. Taking into consideration the microscopic surface tension and extrapolating from the triple point to the critical point (T = Te ) of the liquid-gas phase transition, we reexamine homogeneous nucleation theory. A calculation of the growth rate for microscopic clusters due to the incorporation of much smaller clusters (instead of single molecules) is given and an appropriate variable, scaled supersaturation, is presented to study the mechanism of homogeneous nucleation. For a given nucleation rate, the scaled supersaturation is expected to be nearly independent of temperature below Te .; this is confirmed by experimental data. A generalized form for the droplet model is proposed. Previous theories (“classical,” Lothe-Pound, Reiss, Katz, Cohen) are shown to be special cases of this generalized form and all are shown to be invalid near the critical point. A quantitative theory is made by extrapolating Fisher's droplet model from the critical region to the triple point. For the free energy of embryo formation we include the contributions due to internal vibrations and self-avoiding walks. The microscopic surface tension for the droplet is estimated from the measured coexistence curve; it is found to agree with the bulk macroscopic surface tension near the triple and critical points and to be smaller in the intermediate region. With these considerations we have calculated the scaled supersaturation for water vapor as a function of temperature for given measured nucleation rates. The proposed theory is in good agreement with experiment.

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