Quantifying the impact of particle composition on absorption by black carbon

Abstract:

Black carbon is the most strongly absorbing component of atmospheric aerosol, but absorption per black carbon mass is not well constrained. Black carbon that is coated with non-absorbing material absorbs more strongly than the same amount of black carbon in an uncoated particle, but there is a large discrepancy between absorption enhancement predicted by models and absorption enhancement observed in the atmosphere. Modeling studies and laboratory measurements find absorption enhancement (E_abs) greater than two for thickly coated particles, but ambient observations find only weak absorption enhancement (E_abs = 1–1.2). Through a detailed modeling study of black carbon populations, we show that unrealistic representations of particle composition lead to large errors in modeled absorption. For example, if particles in BC populations were assumed to have uniform composition, similar to the representation applied in modal models, absorption enhancement was overestimated by as much as a factor of two relative to more realistic treatments of particle composition. By applying realistic distributions of particle composition from a particle-resolved aerosol model, we found weak absorption enhancement at low relative humidity (E_abs = 1–1.3), consistent with ambient observations that are performed at low relative humidity. On the other hand, we found strong absorption enhancement (E_abs > 1.8) in many locations if particles were modeled using the global variation in relative humidity.