It consists of three steps: (1) we sequentially doped 1000 ppm (1000 μmol/mol) of n-heptane (HEP), toluene (TOL), and each test sample (TS) into the fuel of a base methane/air flame; (2) we measured the maximum soot concentration in each flame with line-of-sight spectral radiance (LSSR); and (3) we rescaled the results into a yield sooting index (YSI) defined as: