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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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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. C. Whitten
,
W. J. Borucki
,
I. G. Poppoff
, and
R. P. Turco

Abstract

Free chlorine atoms produced from HCl deposited by solid-fueled rocket engines react to destroy odd oxygen in the stratosphere. Using a one-dimensional photochemical model, we have computed the reduction of ozone likely to be caused by long-term operation of such vehicles powered by large solid-fueled rocket engines. It is likely that ozone reduction will not be greater than a few tenths of a percent, although the value is dependent upon ambient NO x concentrations.

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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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Sajal K. Kar
,
Richard P. Turco
,
Carlos R. Mechoso
, and
Akio Arakawa

Abstract

A splitting method is presented for eliminating the need to directly solve for a two-dimensional Helmholtz-type difference equation in a semi-implicit scheme for a global gridpoint shallow-water model. In the proposed method, the model equations are split so that the gravity-oscillation terms are integrated implicitly in two locally one-dimensional steps. It is required that such splitting must preserve the irrotationality properties of the gradients of pressure in finite-difference form so that no spurious sources of vorticity and divergence are introduced into the flow. The semi-implicit scheme, thus derived, provides a locally one-dimensional method of solving the two-dimensional Helmholtz-type difference equation. This method requires the solutions of two linear tridiagonal systems of equations, which can be solved more easily and efficiently than the original two-dimensional Helmholtz-type equation. Using idealized large-scale flows on the sphere, it is shown that the scheme provides stable and accurate model solutions at considerable computational economy.

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