AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Effect of Aggregation and Mixing on Optical Properties of Black Carbon
BARBARA SCARNATO, NASA Ames
Abstract Number: 516 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption and scattering of solar radiation, and then radiative forcing. These mixing state effects are not yet fully understood.
Multiple studies have demonstrated a strong variability in the observed mass absorption efficiency of BC, when it becomes internally mixed with non-absorbing organic compounds. Recent modeling studies show that, BC absorption enhancement depends strongly on the BC aggregate compactness and on the resulting mixing with other aerosol compounds.
The impact of morphology and mixing state on aerosol optical properties is a relevant topic, as well, for interpretation of remote sensing measurements. In radiative transfer calculations, that are also used to interpret space or ground-based observations of Earth, it is common to approximate aerosol shape to homogeneous spherical or spheroidal particles, ignoring the effect of realistic morphology and realistic mixing with other aerosol compounds, which can lead to significant errors in retrieved parameters, such as the aerosol type, optical thickness, particle size distributions and composition, and so forth.
This paper describes a sensitivity study on the effects of aggregation and internal mixing on optical properties BC, including absorption, scattering, linear polarization, scattering phase function, and others. A discrete dipole approximation model has been applied to examine these effects for BC particles bare and mixed with ammonium sulfate, sodium chloride, dust and others. Simulations are in accordance with observed morphology and mixing state published in literature.
The discrete dipole approximation model presented captures complexities in particle morphology that other models do not, and thus may be useful in predicting aerosol optical properties in climate modeling and interpreting data remotely measured with satellites.