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James B. Pollack, Owen B. Toon, Andrey Summers, Warren Van Camp, and Betty Baldwin


Aircraft and Space Shuttles flying through the stratosphere over the next several decades will add sulfuric acid and aluminum oxide particles, respectively, to this region of the atmosphere. To evaluate the effect of these additional aerosols on the global heat balance, we have performed solar and terrestrial radiative transfer calculations. The solar calculations employed an accurate numerical method for solving the multiple-scattering problem for unpolarized light to determine the dependence of the global (spherical) albedo on the optical depth perturbation Δτ. Correct allowance was made for absorption by gases. Using these results, and those obtained from calculations of the terrestrial thermal flux at the top of the atmosphere, we determined the resulting change in the mean surface temperature, ΔT, as a function of Δτ. In both calculations, we used the measured optical constants of the aerosol species.

To apply these results to the problem of interest, we used engine exhaust properties of the various types of vehicles to estimate their optical depth perturbation and examined the record of past climate changes to set a threshold value, 0.1 K, on the mean surface temperature change, below which no significant impact is to be expected. Using the above information, we find that no significant climate change should result from the aerosols produced by Space Shuttles, SST's, and other high flying aircraft, operating at traffic levels projected for the next several decades. However, the effect of SST's is sufficiently close to our threshold limit to warrant a reevaluation as their characteristics are updated.

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James B. Pollock, Edwin F. Erickson, David Goorvitch, Betty J. Baldwin, Donald W. Strecker, Fred C. Witteborn, and Gordon C. Augason


We summarize the evidence showing that the first optical depth of the Venus cloud layer is composed of a water solution of sulfuric acid, including our earlier aircraft observations of Venus’ reflectivity in the 1–4 μm region obtained at a phase angle of 120° (Pollack et al.). Analyses of these aircraft results indicated that of all the proposed cloud candidates only a sulfuric acid solution with a concentration of 75% or more H2SO4, by weight was consistent with the observed 3 µm cloud feature. We present new aircraft observations of Venus obtained in the 1–4 µm region at a phase angle of 40° and in the 3–6 µm region at a phase angle of 136°. Comparing the two sets of observations in the 1–4 µm region, we find a striking phase effect: the reflectivity is much lower in the 3 µm region and there is a much more marked decline between 1.3 and 2.5 µm for the data obtained at the smaller phase angle. The observations made at the 40° phase angle are consistent with the theoretical behavior of a sulfuric acid cloud and imply that the sulfuric acid is present to at least many tens of optical depth below the cloud tops. Arguments concerning the concentration of the solution are reviewed and we conclude that the best current estimate is about 85% H2SO4 by weight.

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