Incomplete combustion is when a substance does not fully burn, causing it to release harmful byproducts like smoke, soot, or carbon monoxide. This happens when there is not enough oxygen present during the burning process. Full definition
Black carbon (BC) is a primary aerosol emitted directly at the source from incomplete combustion processes such as fossil fuel and biomass burning and therefore much atmospheric BC is of anthropogenic origin. (sindark.com)
Black carbon is an aerosol emitted by incomplete combustion from vehicles and factories. (scientificamerican.com)
PAH are toxic tar substances formed during incomplete combustion in, for example, car engines and cigarettes. (sciencedaily.com)