Microphysical studies of mesospheric sulfate aerosol as PMC nuclei in WACCM3

We present the first three-dimensional calculations of the mesospheric sulfate layer. Since this new class of particles was proposed, it has been suggested as a source of nuclei for polar mesospheric clouds (PMCs). Homogeneous nucleation of water vapor is too slow to account for observed PMC particles, necessitating the existence of nuclei, the character of which is as yet unresolved. The leading candidates are dust particles generated by recondensation in meteor trails, ion nucleation resulting from proton hydrates, and nucleation on sulfate particles generated in situ near the mesopause. However, neither the smoke particles, nor the sulfate aerosols, nor the proton hydrates have been measured directly and unambiguously. Recent modeling studies have raised questions about whether sufficient concentrations of dust particles exist in PMC nucleation regions. We have incorporated sulfur chemistry and aerosol microphysics into the Whole Atmosphere Community Climate Model 3 (WACCM3), a comprehensive model that spans the range of altitudes from the Earth's surface to the thermosphere. WACCM3 reproduces well the unique structure of the mesopause region that is critical to this study, comparing well to observations of water vapor and temperature. We have merged WACCM3 with the Community Aerosol and Radiation Model for Atmospheres (CARMA), a bin microphysics model that has been used extensively for simulations of a wide range of aerosol and cloud types, including PMCs and sulfates. Above about 35 km, increasing temperatures evaporate the stratospheric sulfate layer, producing H2SO4 vapor. Although visible light does photolize H2SO4 by vibrational overtone excitation, we calcuate that sufficient H2SO4 survives this weak photolysis to produce sulfate in the cold summer upper mesosphere, where PMCs form. We present calculations of this mesospheric sulfate layer, and discuss its suitability for PMC nucleation. We also discuss the effects of volcanic eruptions on the number of sulfate particles and nucleation sites.