Raymond et al (2014] analyzed black carbon (BC] emissions and found a proportional relationship with the aromatic content of alternative jet fuels. The RF of aviation BC emissions is approximately 34% of the aviation CO2 RF (Stettler etal., The results indicate a need for further research and development of a holistic framework for quantification, as an extension to the current GHG metric, before absolute comparisons of the climate impact of alternative jet fuels can be fully trusted.