Graphite, and particularly defective graphite, is chosen to model soot particles. Quantum mechanical calculations are first carried out on molecular polycyclic-aromatic-hydrocarbon-type systems and then extended to a periodic representation of one graphite layer. The features of the interaction of H, HO, NO, NO2, and NO3 with these model systems are examined, with the aim of defining a suitable representation of the atmospheric or combustion gas-solid interactions by which functionalization reactions can take place. The more interesting interactions with small reactive molecules regard the edge of the graphene sheet and the in-plane carbon vacancies. While these interactions can be well described by sufficiently extended molecular models, periodic models are necessary to describe accurately the equilibrium geometries because they introduce the necessary geometric constraints. The ability of a graphene sheet to easily accommodate unpaired electrons in sigma or pi orbitals is the basis for its interesting interactions with the small molecules present in tropospheric chemistry or combustion processes.