Reconciling modeling with observations of radiative absorption of black carbon aerosols

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AbstractThe physical treatment of internal mixing and aging of black carbon (BC) aerosols that allow for enhanced solar absorption of the BC is an important parameterization in climate models. Many climate models predict a factor of 2–3 lower aerosol absorption optical depth (AAOD) than the atmospheric columnar absorption observed from ground‐based networks such as AERONET, likely because these models do not parameterize properly the BC absorption enhancement (EMAC). Models that are configured with an internal mixing have predicted large variations of EMAC, which are poorly constrained from ambient measurements. We determined the BC EMAC from aerosol coatings with a two‐step solvent experiment to remove both organic and inorganic coatings in ambient fine particulate matter (PM2.5). Observations in a rural North China site showed that the EMAC varied from 1.4 to 3. The EMAC increases simultaneously with SO42−/EC ratios, suggesting the photochemical production of sulfate coatings enhanced BC absorption. A global climate model, parameterized to account for these observational constraints, verifies that sulfates are primary drivers of the BC absorption enhancement in severely polluted area in China. This magnification of the radiative forcing of coated BC is stronger by a factor of ~2 than predicted by the standard parameterization (external mixing) in the climate model and is in better agreement with AERONET observations of AAOD. This result would be useful for testing the representation of solar absorption by BC‐containing particles in the newer generation of climate models.