Black carbon refers to tiny carbon particles that form during incomplete combustion of carbon-based fuels. Black carbon particles absorb sunlight, so they are considered to contribute to global warming. However, the contribution of black carbon to the heating of the Earth's atmosphere is currently uncertain. Models that can accurately assess the warming effect of black carbon on our atmosphere are needed so that we can understand the contribution of these tiny carbon particles to climate change. The mixing state of black carbon particles and their particle size strongly influence their ability to absorb sunlight, but current models have large uncertainties associated with both particle size and mixing state.