A physically-based treatment of elemental carbon optics: Implications for global direct forcing of aerosols

To date, global models of direct radiative forcing have treated elemental carbon (EC) as completely externally mixed or well-mixed internally. No global study has treated EC as a core in an internal mixture. It is hypothesized that the well-mixed treatment is unphysical and reality lies between the externally-mixed and core treatments. It is also suggested, but not proven, that most EC particles are coated to some degree; hence, the core treatment may be more representative than the external-mixture treatment. Global simulations with the core treatment resulted in EC forcing 50% higher and 40% lower than forcings obtained with the externally-mixed and well-internally-mixed treatments, respectively. In the core case, EC's positive forcing more than offset negative forcing due to all other anthropogenic aerosol components combined. Further studies are needed to understand the mixing state of EC and determine the accuracy of the core treatment.