Panel a: Particle size and number concentration can be different even if the total amount (mass) of BC is the same.
Panel b: Particle size and coating amount determine the solar absorption efficiency of BC. Many BC particles are emitted as pure BC particles (BC particles without coating). BC particles are gradually coated by other aerosol species such as sulfate and organic aerosols through aerosol processes in the atmosphere (arrow). These aerosol processes enhance BC absorption efficiency by up to a factor of 2.