This paper considers the-nature of carbonaceous surfaces, the means by which they are activated, the nature of some functional groups that they support, and some reaction mechanisms that may be involved. Because of the strong affinity of carbonaceous surfaces for organic species and because of the ease with which compounds in a high oxidation state can oxidize the carbonaceous surface, it is highly likely that carbonaceous aerosols are interacting chemically with a range of organic species in ways that have, as yet, not been fully characterized but may significantly affect the oxidizing capacity of our atmosphere. If HONO is formed on the surface of carbonaceous aerosols then this could be a significant source of HOx as HONO is readily photolyzed to give OH, and it could explain the large values of HONO often observed in the troposphere. In general, the reduction of NOy on carbonaceous aerosols is an important consideration, and it is addressed here. ¿ 1999 American Geophysical Union