Black carbon (BC) is the dominant aerosol absorber of solar radiation in the atmosphere and is an important component of anthropogenic climate forcing. BC's role is strongly dependent on its physical state, which can influence the way that BC particles may act as ice and cloud nuclei, as well as the way they interact with solar radiation. In situ measurements made with a single-particle soot photometer flown on a NASA high-altitude research aircraft show the mass and size of individual BC particles in the tropics, as well as their propensity to be found mixed with additional materials. Mie theory was used to connect observed light scattering off BC particles to the optical effects of coatings on the particles. The observations indicate that as BC from ground-based emission sources rises in altitude to the lower stratosphere, coatings on BC particles become both thicker and more prevalent, while BC mass mixing ratios decrease dramatically from their values near the ground. Coatings enhance light absorption by the ambient BC column by at least 30%. These results reveal the microphysical state of BC in the atmosphere while providing important constraints for models evaluating BC's role in climate change. Copyright 2008 by the American Geophysical Union.