The effects of sulfur emissions from HSCT aircraft: A 2-D model intercomparison

Four independently formulated two-dimensional chemical transport models with sulfate aerosol microphysics are used to evaluate the possible effects of sulfur emissions from high-speed civil transport (HSCT) aircraft operating in the stratosphere in 2015. Emission scenarios studied are those from Baughcum and Henderson [1995], while assumptions regarding the form of emitted sulfur are similar to those of Weisenstein et al. [1996]. All models show much larger increases in aerosol surface area when aircraft sulfur is assumed to be emitted as particles of 10 nm radius rather than as gas phase SO2. If we assume an emission index (EI) for SO2 of 0.4 gm (kg fuel burned)(-1) in 2015, maximum increases in stratospheric sulfate aerosol surface area range from 0.1 mu m(2) cm(-3) to 0.5 mu m(2) cm(-3) with sulfur emitted as SO2 gas and from 1.0 mu m(2) cm(-3) to 2.5 mu m(2) cm(-3) with sulfur emitted as particles. Model differences in calculated surface area are deemed to be due mainly to differences in model transport. Calculated annual average ozone perturbations due to aircraft emissions with EI(NOx)=5, EI(H2O)=1230, and EI(SO2)=0.4 range from -0.1% to -0.6% at 45 degrees N for sulfur emission as SO2 gas and from -0.4% to -1.5% with sulfur emission as 100% particles. The effect of zonal and temporal inhomogeneities in temperature on heterogeneous reactions rates is accounted for in the Atmospheric and Environmental Research model and the Universita degli Studi L'Aquila model and significantly increases the calculated ozone depletion due to HSCT, particularly for the cases with concurrent increases in aerosol surface area. Sensitivities to polar stratospheric clouds, background chlorine amount, additional heterogeneous reactions, and background aerosol loading are also explored.