Understanding Black Carbon (BC) Atmospheric Aging: Radiative Properties and Timescale
Speaker: Dr. Cenglin He, UCLA, USA
Time: 15:00, 7 Sept, 2016
Black carbon (BC), commonly known as soot, has been identified as the second most important anthropogenic global warming agent in the current atmosphere by virtue of its strong absorption of solar radiation and its role as cloud condensation nuclei in cloud formation. BC climatic effects are significantly influenced by BC aging process in the atmosphere, which transforms BC from an external to internal mixing state and increases its hygroscopicity and light absorption. This presentation seeks to understand variations of BC radiative properties and aging timescales during atmospheric aging from a modeling perspective. In the first part, we develop and examine a microphysics-based BC aging scheme that accounts for condensation, coagulation, and heterogeneous chemical oxidation processes in a global 3-D chemical transport model (GEOS-Chem) by interpreting the BC measurements from the HIAPER Pole-to-Pole Observations (HIPPO, 2009–2011) using the model. The microphysics-based aging scheme significantly improves model simulations of BC across the Pacific and shows important implications for global BC budget. In the second part, we develop a theoretical BC aging model to account for three typical evolution stages, namely, freshly emitted aggregates, BC coated by soluble material, and BC particles undergoing further hygroscopic growth. We employ the geometric-optics surface-wave (GOS) approach to compute BC single-scattering properties at each aging stage, which are subsequently compared with laboratory measurements. The optical cross sections of coated BC particles vary by more than a factor of 2 due to different coating structures, which suggests that an accurate estimate of BC radiative effects requires the incorporation of a dynamic BC aging process that accounts for realistic coating structures in climate models.