Each YSI was measured three times and then averaged. There are three main sources of uncertainty: (1) random uncertainty, which is estimated to be ±2% based on the standard deviation of the internal standard measurements (see SI G); (2) systematic uncertainty, which is estimated to be ±1%, mainly due to possible error in the ratios of the mass densities between the test compounds, and (3) the uncertainty in comparing the current measurements with the previous measurements in , which is estimated to be ±2% based on the differences observed for the internal standard. Overall, we estimate that the uncertainty in the measured YSIs is ±5% or ±5.0, whichever is larger. The figure plots the data as YSI/LHV, where LHV is lower heating value, since the oxygen atoms in the lactones significantly reduce their heat of combustion. The units of LHV are molar (i.e., MJ/mol) since YSI measures the amount of soot produced per mole of fuel consumed. The conventional fuels include several test gasolines (the TG fuels) , a Jet A aviation fuel , and a diesel fuel . The TG fuels were produced by conventional fuel refineries and mostly meet the specifications for gasoline, but they were formulated to have enhanced concentrations of various hydrocarbon classes and they bracket the composition space of commercial gasolines. The LHVs of the conventional fuels are measured values, while the LHVs of the lactones were estimated with an empirical relationship . SI H provides further details and the raw data. See SI H for further details and the raw data.