A chemical mechanism for the gas phase atmospheric oxidation of nitrogen oxides (NOx) and sulfur dioxide (SO2) in the presence of reactive hydrocarbons (RHC) has been developed for use in long-range transport modeling. The mechanism includes numerous reactions important for nighttime and multiday simulations which have been previously ignored in photochemical mechanisms designed for urban scale applications. This mechanism is capable of simulating the decay of NOx/SO2/RHC precursors and the formation of oxidation products such as nitric acid, sulfuric acid, ozone, hydrogen peroxide, hydro-peroxides, and organic peroxides which have recognized roles in the acidification of precipitation. Testing of detailed and condensed versions of the chemical mechanism is reported.