Direct and semi-direct radiative effects of biomass burning aerosols are subject to large uncertainties in global models, in terms of both magnitude and sign. This is related to the level of complexity in the model representation of aerosol light absorption. While the global simulations of aerosol optical depths are increasingly better constrained by surface and satellite observations, absorption properties of biomass burning aerosols are examined mostly with the observations over or near the sources. Lack of observational constraints over very large spatial scales is in particular problematic for simulating the evolution of organic compounds from biomass burning. In contrast to black carbon (BC), the refractive index of absorbing organics does not remain constant throughout its lifetime, as oxidation and/or photolysis can render initially brownish organic carbon (BrC) to become more transparent (bleaching), leading to a decrease in the light absorption of organic aerosols. In addition, chemical formation and aging of organics have also been suggested to change the absorption and wet deposition of BC-containing particles in the long-distance transport.
In this presentation, I will overview the observational datasets of BC and BrC absorption from recent field experiments including DOE’s BBOP, ENA, LASIC, NASA’s ORACLES and ATom missions. These datasets are representative of biomass burning aerosols over a range of geographical locations, distance from sources, aging times, altitudes, and seasons. Large diversity is displayed in the observed BrC concentration, and contribution to total aerosol absorption relative to BC. I will show in the global climate model simulations that including a parameterization of the BrC bleaching lowers the estimated direct radiative effect due to BrC by nearly a half. The results suggest a strong sensitivity of biomass burning aerosol forcing to the model-simulated evolution of absorbing aerosol properties. Both observational and modeling efforts are needed to resolve the discrepancies in the model-data comparison of BrC and constrain the relative contributions of BrC and BC to total biomass burning aerosol absorption at various aging stages.