Department of Earth System Science

We simulate accumulation of Al₂O₃ particles in the atmosphere produced by solid-fueled rocket motors by using the Goddard Institute for Space Studies/University of California at Irvine three-dimensional (3-D) chemistry-transport model (CTM). Our study differs from Jackman et al. (1998) by applying a 3-D CTM, considering 13 size bins for the emitted particles from 0.025 to 10 μm and taking into account their washout, gravitational sedimentation, and coagulation with background sulfate aerosol. We assume an initial trimodal size distribution of Al₂O₃ particles (Beiting, 1997) with 2.8% by mass of the alumina emitted as particles with radius of less than 1 μm. Our test case adopts a stratospheric source of 1120 tons/yr equivalent to nine space Shuttle and four Titan IV launches annually. The calculated steady state surface area density (SAD) and mass density for the scenarios with sedimentation of alumina particles have maximum values in the lower stratosphere in the Northern Hemisphere of up to 7×10⁻⁴ μm²/cm³ and 0.09 ng/m³, respectively, or about 1000 times smaller than those of the background sulfate aerosol. Our results are sensitive to the emitted mass fractionation of alumina (EMFA) showing the values for the SAD or mass density higher or lower by an order of magnitude owing to a poorly known EMFA. Chemical implications of alumina particle accumulation for the ozone balance are estimated by using the Atmospheric and Environmental Research 2-D model assuming chlorine activation on Al₂O₃ surfaces via the C1ONO₂ + HCl → Cl₂ + HNO₃ reaction with a probability of 0.02 (Molina et al., 1997). Owing to the very small Al₂O₃ SAD, any additional ozone depletion due to Al₂O₃ emissions is also small (0.0028% on a global annually averaged basis for the scenario with sedimentation, or about 4 times smaller than the ozone response to chlorine emissions only). The ozone depletion potential of the alumina emissions is about 0.03–0.08 for the scenarios using the EMFA of Beiting (1997) and larger by an order of magnitude for the EMFA of Brady and Martin (1995).