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.
