Effect of Mount Pinatubo H2SO4/H2O aerosol on ice nucleation in the upper troposphere using a global chemistry and transport model -: art. no. 4141

[1] A 2-year simulation of Mount Pinatubo volcanic aerosol is performed using a global chemistry and transport model. The model is driven by meteorological fields from the NASA Goddard Data Assimilation Office (DAO) general circulation model (GCM) with a horizontal resolution of 2degrees latitude by 2.5degrees longitude and 46 vertical levels. The model reproduces the equatorial aerosol reservoir bounded between 20degreesS and 30degreesN several months after the eruption and the detrainment of sulfate aerosol from the equatorial reservoir occurring in connection with anticyclonic circulation systems. Our model results are generally consistent with the dispersal characteristics of sulfate aerosol derived from the Stratospheric Aerosol and Gas Experiment II (SAGE II) aerosol extinction observations. On the basis of global distribution of Pinatubo sulfate, the homogeneous ice nucleation rate of H2SO4/H2O aerosol (number of aerosol particles freezing per cm(3) of air mass and per second) in the lower stratospheric and upper tropospheric regions is calculated and compared with those of natural and anthropogenic sulfate from sources injected near Earth's surface. Large ice nucleation rates from Pinatubo H2SO4/H2O aerosol are found in the region near the equatorial tropopause and generally in the regions along the tropopause in the bottom of the main Pinatubo aerosol layer during the first year following the eruption. Ice nucleation rates of Mount Pinatubo H2SO4/H2O aerosols are much larger than those of sulfate aerosols from the surface sources in many regions of the upper troposphere during the first year and could still be comparable to rates from these background sources during the second year after the eruption. Our results suggest that the Pinatubo sulfate aerosol could have influenced cirrus formation and evolution globally through homogeneous ice nucleation, although the exact impact also requires a more complete calculation that includes cirrus cloud dynamics and microphysics. Available satellite observations support the above assertion.