Precipitation of energetic charged particles into the polar winter stratosphere leads to an enhancement of nitric acid. Three different ion-molecule reaction mechanisms have been suggested as the cause of the HNO3 enhancement: 1) OH production by ion-molecule reactions and subsequent reaction with NO2 to produce HNO3, 2) positive ion-negative ion recombination, and 3) positive hydrated ion reaction with N2O5. It is shown that the OH reaction can be disregarded. While H+⋅(H2O)n+N2O5 may be important below 40 km, it accounts for less than half the HNO3 production at 45 km and is negligible in comparison with ion-ion recombination at higher altitudes. The inclusion of ion-ion recombination greatly reduces the ion pair production requirements for HNO3 production.