Abstract View
Determination of the Refractive Index of Carbon Black Engulfed by Ammonium Sulfate from Effective Medium Approximations
JAMES RADNEY, Christopher Zangmeister, National Institute of Standards and Technology
Abstract Number: 182
Working Group: Carbonaceous Aerosol
Abstract
Calculation of aerosol optical properties in radiative transfer models requires accurate knowledge of the size distribution, number density, mixing state, hygroscopicity and complex refractive index (RI) of the input particles. These input particle types can take many forms, but are often split into sulfate, black carbon, organic carbon and mineral dust. While the optical properties of external mixtures of these components can be easily calculated, internal mixtures pose challenges and an effective refractive index (RIeff) of the mixed particle must be determined; often using an effective medium approximation (EMA). The chosen EMA can vary depending upon whether the particles are homogeneously or heterogeneously mixed. Here, we investigate whether, for well-characterized core-shell particles under well-controlled laboratory conditions, the refractive index of the absorbing core (an aged black carbon mimic, CB) could be isolated from the ammonium sulfate shell using optical measurements of absorption and extinction by photoacoustic and cavity ring-down spectroscopy, respectively. Refractive index retrievals of size- and mass-selected particles were performed as a function of CB mass fraction to determine the RIeff and we observe that the real (n) and imaginary (k) components are composition dependent; neff = (1.47 ± 0.01) + (0.21 ± 0.07)wCB and keff = (0.76 ± 0.07)wCB. Various EMAs (volume fractions, average dielectric, Maxwell-Garnett and Bruggeman) are then tested to retrieve the RI of the CB core (RICB). We find that the retrieved RICB is EMA dependent, but the calculated optical properties always agree with the measured data to better than 5 %. In addition, the calculated values are less sensitive to RICB of and the choice of EMA than they are to the assumed refractive index of the ammonium sulfate shell.