Constraining Aging Processes of Black Carbon in a Climate Model

The direct radiative forcing of atmospheric black carbon aerosols (BC) remains highly uncertain in the current climate assessment, due to the complexity of their optical properties, lifetime and distribution. In particular, much of the uncertainty arises from the poorly understood BC aging process, i.e. the coating of BC by sulfate and organics that changes the morphology, hygroscopicity and optical properties of BC. A novel environmental chamber approach was developed recently to assess BC aging timescales under different urban polluted conditions, providing estimates of the BC aging timescales to be 4.6 h and 18 h in Beijing and Houston, respectively. The optical absorption of BC is amplified by a factor of 2.4 due to this process. Those in situ characterizations of the BC aging process provide unique observational constraints for both regional and global climate models. In this study, we evaluate the simulated BC aging efficiency and associated changes in the optical properties in a newly developed aerosol module (MAM4) in the NCAR CAM5.3 model. We further assess the sensitivity of key uncertain parameters in the treatment of BC aging process, and use the chamber measurements to constrain the modeled BC aging timescales. The sensitivity of global BC radiative forcing to different BC aging setups in CAM5.3 will be presented. Additional constraints from space borne measurements of absorbing aerosols will be discussed as well.