Black carbon (BC) is estimated to have the second largest anthropogenic radiative forcing in earth-systems models (ESMs), but there is significant uncertainty in its impact due to complex mixing with organics. Laboratory-generated particles show that co-mixed non-absorbing material enhances absorption by BC by a factor of 2-3.5 as predicted by optical models. However, weak or no enhancements are often reported for field studies. The cause of lower-than-expected absorption is not well understood and implies a lower radiative impact of BC compared to how many ESMs currently treat aerosols. By analyzing BC aerosol particle-by-particle we reconcile observed and expected absorption for ambient smoke plumes varying in geographic origin, fuel types, burn conditions, atmospheric age and transport. Although particle-by-particle tracking is computationally prohibitive for sophisticated ESMs we show that realistic BC absorption is reliably estimated by bulk properties of the plume providing a suitable parameterization to constrain black carbon radiative forcing.
