Black carbon is a soot that is emitted from power plants that are uncontrolled or diesel engines. It has a relatively short lifetime, in that it doesn't stay in the atmosphere for a long period of time. It is generally not considered a gas, so it doesn't fall under the rubric of greenhouse gases.
It is included in the ACES bill. There is a requirement that the administrator of EPA undertake a study of black carbon and the technologies available to control it, and then it authorizes him to issue regulations to control it.
It's important in the Arctic, in that even in small, small quantities, it changes the albedo of the Arctic, so the Arctic is absorbing a little bit more heat. The ice and snow cover is absorbing a little bit more heat due to that soot, if you will. Eliminating that is something that could be done relatively easily in the short term and could delay the influence of warming on the Arctic.