[1] In situ measurements of the mass, mixing state, and optical size of individual black-carbon (BC) particles in the fine mode (90 -600 nm) have been made in fresh emissions from urban and biomass burning sources with an airborne single-particle soot photometer. Contrasts between the two sources are significant and consistent. Urban BC tends to smaller sizes, fewer coated particles, thinner coatings, and less absorption per unit mass than biomass-burning BC. This suggests that urban BC may have a longer lifetime in the atmosphere and a different impact on BC radiative forcing in the first indirect effect than biomass-burning BC. These measurements bound the likely variability in the microphysical state of BC emissions from typical continental processes, and provide direct measurements of the size distribution and coating state of fine-mode BC for use in constraining climate and aerosol models. These results highlight the need for the integration of sourcespecific information into such models. Citation: Schwarz, J. P., et al. (2008), Measurement of the mixing state, mass, and optical size of individual black carbon particles in urban and biomass burning emissions, Geophys. Res. Lett., 35, L13810,