Abstract View
Quantifying the Effects of Mixing State on Aerosol Optical Properties
YU YAO, Jeffrey H. Curtis, Nicole Riemer, University of Illinois at Urbana-Champaign
Abstract Number: 310
Working Group: Aerosols, Clouds and Climate
Abstract
Calculations of the aerosol direct effect on climate rely on simulated aerosol fields. The model representation of aerosol mixing state potentially introduces large uncertainties into these calculations, since the simulated aerosol optical properties are sensitive to mixing state. The aim of this study is to systematically quantify the impact of aerosol mixing state on aerosol optical properties.
To this end, we created a large number of model scenarios using the particle-resolved model PartMC-MOSAIC. PartMC is a Lagrangian aerosol parcel model that tracks the evolution of composition and sizes of individual aerosol particles due to emission, coagulation, condensation and dilution without any a priori assumptions about aerosol mixing state. The model is coupled with the chemistry module MOSAIC, which deals with gas and aerosol phase chemistry. Aerosol optical properties are calculated using Mie calculations assuming spherical particles.
To cover a wide range of possible mixing states, we varied 41 input parameters that govern aerosol aging, including aerosol initial size distribution parameters, gas and aerosol emission rates, relative humidity, and temperature. The combinations of these variables for the individual scenarios are created by Latin Hypercube sampling where we prescribed a parameter range that represents the range encountered in different regions of the globe. The simulated aerosol optical properties in this scenario group capture the full diversity of the aerosols and can be used as the reference case. In order to quantify the errors due to the internal mixture assumptions used by many current aerosol models, we created another sensitivity scenario group with the same number and mass size distributions as the base scenarios but with averaged aerosol composition in prescribed size bins. Finally, we quantified the errors introduced by this composition averaging by linking the differences of optical properties with the mixing state metric chi. Preliminary results show the single scattering albedo (SSA) in the internally-mixed size bin cases are all lower than the particle-resolved reference cases, especially for the aerosols of intermediate mixing state (chi between 60% to 80%). The overestimation of the ensemble absorption in these internally-mixed particles is responsible for the SSA errors.