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 c arbon (EC) as completely externally mixed or well-mixed internally. No glob al study has treated EC as a core in an internal mixture. It is hypothesize d 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 treatmen t may be more representative than the external-mixture treatment. Global si mulations 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 mo re than offset negative forcing due to all other anthropogenic aerosol comp onents combined. Further studies are needed to understand the mixing state of EC and determine the accuracy of the core treatment.