Effect of Aggregation and Mixing on optical properties of Black Carbon

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 will present a sensitivity study of the effect of the BC aggregate morphology and the mixing state on optical properties, when BC is mixed with ammonium sulfate, sodium chloride, dust and others. Optical properties are computed, using a discrete dipole approximation model (DDSCAT), in accordance with observed BC morphology and mixing state published in literature.