The interaction between gas-phase nitric acid and the graphite surface is taken as a simple model of interactions occurring at the surface of atmospheric soot particles. In particular, we study the heterogeneous processes that lead to the dissociation of the nitric acid and the production of nitrous acid. The atomistic details of the reaction mechanisms are reproduced by use of the new metadynamics method. The binding interactions of the HNO3 molecule and its fragments with the graphite surface are calculated, and the role of the surface in catalyzing the reaction is taken into account. From the reactive trajectory generated by the metadynamics, it is seen that the path goes through several different intermediate states. We analyze in detail the electronic structures and spin density distributions of the relevant products and report on the mechanisms and the main features of the transition regions relative to all the activated processes observed.