The impact of uncertainty in black carbon's refractive index on simulated optical depth and radiative forcing

Abstract. The radiative forcing of black carbon (BC) is subject to many complex, interconnected sources of uncertainty. Here we isolate the role of the refractive index, which determines the extent to which BC absorbs and scatters radiation. With other parameterizations held constant, varying BC's refractive index from m_550nm = 1.75–0.44i to m_550nm = 1.95–0.79i increases simulated absorbing aerosol optical depth (AAOD) by 42 % and the effective radiative forcing from BC-radiation interactions (BC ERFari) by 47 %. The AAOD increase is comparable to that from recent updates to aerosol emission inventories, and in BC source regions, a third as large as the difference in AAOD retrieved from MISR and POLDER-GRASP satellites. The BC ERFari increase is comparable to the scale of the uncertainty in recent literature assessments. Although model sensitivity to the choice of BC refractive index is modulated by other parameterization choices, our results highlight the importance of considering refractive index diversity in model intercomparison projects.