Effective Radiative Forcing of the Internally Mixed Sulfate and Black Carbon Aerosol in the GFDL AM4 Model: The Role Played by Other Aerosol Species

We compute the effective radiative forcing (ERF) of the internally mixed sulfate-black carbon (SBC) aerosol species in the Geophysical Fluid Dynamics Laboratory's (GFDL) Atmospheric Model version 4 (AM4) model using five different formulations. The formulations differ in how they account for the presence of other aerosol species. The global mean ERF of SBC in the GFDL AM4 model ranges from −0.51 ± 0.1 to −1.06 ± 0.1 W m⁻². The three most realistic configurations of the five, in which the emissions of other aerosol species vary between 1850 and 2010 states, depict a tighter distribution of ERF (−0.51, −0.55, and −0.57 ± 0.1). The two outlier configurations completely exclude one or more other aerosol species, which is slightly unrealistic but included for completeness. The former three configurations, however, result in substantially different ERFs over the regional hot spots of aerosols, e.g., over the land-mass of East China; the choice of the emission conditions for organic carbon (i.e., present-day or preindustrial) affects the ERF of SBC by ∼37%. The component of ERF related to aerosol-cloud interactions (ACI) gets principally affected by the presence of other aerosol species. The higher the emissions of other aerosol species, the lesser is the ERF of SBC associated with ACI. This finding suggests that for ERF estimates, the choice of the emission level/concentrations of the other aerosol species significantly affects the estimates of SBC, especially over the aerosol hot spots.