Absorption Enhancement of Atmospheric Black Carbon Driven by Coating Morphology

CYPRIEN JOURDAIN, Yu Wang, David Neubauer, Jonathan Symonds, Ulrike Lohmann, Adam M Boies, University of Cambridge

     Abstract Number: 690
     Working Group: Aerosols, Clouds and Climate

Abstract
Black carbon (BC) particles are short-lived climate forcers that strongly alter the regional direct radiative forcing (RF) due to the rapid mixing with semi volatile compounds present in the atmosphere. While recent models have highlighted the need to account for soot morphology to reduce the large uncertainties associated with BC RF estimation, studies of the coating evolution - a key element responsible for absorption enhancement - have been limited, especially for non-uniformly coated (NUC) soot agglomerates. In this work, we evaluate the effects of both the intrinsic BC structure and the non-uniform coating of fractal BC agglomerates on the physico-optical properties of coated BC by interfacing the CA²M generation tool with a discrete dipole approximation model (DDSCAT). Realistic parametrizations in term of mass absorption cross-section (MAC) have been found and used in the ECHAM-HAM global model. Significant improvement in the parametrization using the NUC model that shows a small absorption enhancement for low coating fraction, in agreement with several ambient measurements reported in the literature. Above a critical mass ratio threshold (~14), the BC particles become encapsulated in droplets that eventually have a spherical morphology, maximizing the lensing effect. A comparison of MAC with current coated BC models reveals a closer agreement of NUC with the core-shell (CS) Mie theory than with the uniformly-coated BC models found in literature. Comparing these models highlights the need to account for coating morphology, which impacts the BC absorption, significantly more that the BC internal structure itself. Global simulation results show a positive RF induced by BC between the pre-industrial era and the present days, especially in regions of China, India, and some regions of the Arabic Peninsula, Western Africa and South America.