Conceptually Characterizing the Radiative Effects of Black Carbon Internal Mixing

Abstract:

Black carbon (BC), as a strongly absorbing aerosol, is distinct from most other climate forcers, as it not only has positive top of atmosphere (TOA) radiative forcing, but also redistributes the absorbed radiation vertically through surface dimming and enhancement of atmospheric absorption. Internal mixing (IM) between BC and other aerosol species, e.g. sulfate and organic carbon (OC), primarily from fossil fuel and biomass burning respectively, further enhances its absorbing ability. Most studies of BC focus on particle-scale changes or TOA radiative forcing enhancement. Our work identifies three layer-scale radiative fluxes (at TOA, atmospheric absorption, and at the surface) due to IM and connects them to particle-scale effects through a new conceptual radiative transfer model (RTM). We also employ a Mie calculation for particle-scale effects and a comprehensive RTM for evaluation of the conceptual model. We find that, although scattering decreases and absorption increases by the same amount at the particle scale due to IM, a weakening in scattering is one order of magnitude less at the layer scale, and thus can be neglected to simplify the conceptual RTM. Our result after simplification indicates that IM enhances atmospheric absorption by increasing TOA forcing and decreasing surface forcing the same amount. This is supported by similar findings both globally and over major BC source regions using the comprehensive RTM. Our conceptual RTM well captures layer-scale radiative effects of IM by reducing the complexity of computing and understanding IM-radiation interactions. Using the conceptual RTM, we estimate a global average increase of 0.42 W/m² when internal mixing of BC with sulfate and OC is included relative to a case where internal mixing with OC is absent. We conclude that including OC in IM with BC is important, especially when analyzing the climate effects of biomass burning and sulfate mitigation.