Carbon black showed the smallest NP-to-NP variation in initial EEO2 values, averaging ~1.2 x 10 -3 Da/O2 collision, with total variation from the least to most reactive NP being a factor of ~2.5. This variation is small enough that an increase in EEO2 with mass is noticeable, with the values for individual NPs lying within ±40% of the linear regression line plotted in the figure. The small scatter implies that the carbon black NPs must be reasonably homogeneous in the sense that the NP-to-NP variations in shape and reactive site densities are small. This is not the case for any of the other carbon NP types.