Sooting tendencies were measured using a yield-based approach we developed previously . The specific procedure used in this study is described in . It consists of three steps: (1) we sequentially doped 1000 μmol/mol (1000 ppm) of n-heptane (H), toluene (T), and each test fuel (TF) into the fuel stream of a base methane/air flame; (2) we measured the maximum soot concentration in each flame with line-of-sight spectral radiance (L); and