Nucleation simulations in the wake of a jet aircraft in stratospheric flight

An efficient method for calculating the nucleation rates of H 2SO 4H 2O aerosol droplets is developed using classical nucleation theory for a binary vapor system. The nucleation model is used to predict binary homogeneous and heterogeneous nucleation rates in the exhaust stream of a supersonic aircraft flying in the stratosphere. Calculations indicate that, assuming at least 0.01% of sulfur is emitted as H 2SO 4, the H 2SO 4H 2O particle formation rate is large enough to enhance the background sulfate aerosol concentration substantially. Heterogeneous nucleation, although somewhat slower, may convert a substantial fraction of the soot particles into acid aerosols (notwith-standing rapid water vapor condensation on the soot). Model results are compared with a balloon measurement taken in the aircraft exhaust plume in which the sulfuric acid aerosol concentration was greatly enhanced above the background. The observed aerosols consist of pure sulfuric acid-water solutions without a nucleating “core”, which is consistent with the prompt homogeneous binary sulfuric acid nucleation in the jet plume. Simulations do not support an alternative delayed mechanism that requires photooxidation of SO 2 into H 2SO 4 followed by homogeneous nucleation in the humid air (like the formation of new sulfate aerosols in volcanic eruption clouds).