Optical properties of black carbon in cookstove emissions coated with secondary organic aerosols: Measurements and modeling

Cookstoves are a major source of black carbon (BC) particles and associated organic compounds, which influence the atmospheric radiative balance. We present results from experiments that characterize BC emissions from a rocket stove coated with secondary organic aerosol. Optical properties, namely, BC mass absorption cross-section (MAC(BC)) and mass scattering cross-section (MSC), as a function of the organic-to-black carbon ratio (OA:BC) of fresh and aged cookstove emissions were compared with Mie and Rayleigh-Debye-Gans (RDG) calculations. Mie theory reproduced the measured MAC(BC) across the entire OA: BC range. However, Mie theory failed to capture the MSC at low OA: BC, where the data agreed better with RDG, consistent with a fractal morphology of fresh BC aggregates. As the OA: BC increased, the MSC approached Mie predictions indicating that BC-containing particles approach a core-shell structure as BC cores become heavily coated. To gain insight into the implications of our findings, we calculated the spectral simple forcing efficiency (dSFE) using measured and modeled optical properties as inputs. Good agreement between dSFE estimates calculated from measurements and Mie-modeled dSFE across the entire OA: BC range suggests that Mie theory can be used to simulate the optical properties of aged cookstove emissions.