HR: 16:30h
AN: A14B-03    [Abstracts]
TI: Sensitivity of sulfate direct climate forcing to the hysteresis of particle phase transitions
AU: Martin, S T
EM: smartin@seas.harvard.edu
AF: Harvard University, 29 Oxford Steet, Cambridge, MA 02138, United States
AU: * Wang, J
EM: jwang7@unl.edu
AF: Harvard University, 29 Oxford Steet, Cambridge, MA 02138, United States
AU: * Wang, J
EM: jwang7@unl.edu
AF: University of Nebraska - Lincoln, 303 Bessey Hall, Lincoln, NE 68588, United States
AU: Jacob, D J
EM: djacob@fas.harvard.edu
AF: Harvard University, 29 Oxford Steet, Cambridge, MA 02138, United States
AB: We investigate the effect of the hysteresis of solid-aqueous phase transitions of sulfate-ammonium particles on sulfate direct climate forcing (SDCF) by using both a column model and a global chemical transport model. Aqueous particles have a larger mass extinction efficiency but a smaller backscattered fraction than their solid counterparts. The column model shows that hysteresis can result in an uncertainty in the SDCF of 20%. The global chemical transport model explicitly accounts for the relative humidity processing of particles and the associated hysteresis. The model also treats the extent of sulfate neutralization by ammonia. The base case simulation finds that solid particles contribute 41% of the global sulfate burden of the anthropogenic component, 26% of the clear-sky optical thickness, 31% of the clear-sky SDCF, and 37% of the full-sky SDCF, a trend that reflects the correlation of solid particles with clear skies. A perturbation to the model, omitting hysteresis by assuming that all particles are aqueous, results in an overestimate of the SDCF by +8% compared to base case. A converse assumption that crystallization occurs at the deliquescence relative humidity underestimates the SDCF by -8%. A case that assumes that aqueous particles occur whenever the ambient relative humidity exceeds the crystallization relative humidity biases the SDCF by +5%. A case that includes hysteresis but omits the difference in the fraction of radiation backscattered to space by aqueous compared to solid particles changes the SDCF by +15%. Seasonal and regional differences can be much larger. We recommend that the ratio of the sulfate aerosol optical thickness calculated with vs. without consideration of particle hygroscopicity be reported as a standard parameter to facilitate meaningful SDCF intercomparisons among different models.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0360 Radiation: transmission and scattering
DE: 0365 Troposphere: composition and chemistry
DE: 1009 Geochemical modeling (3610, 8410)
DE: 3359 Radiative processes
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting