The goal of this proposal is to develop a state-of-the-art model representation of nighttime chemistry for the Los Angeles basin. To achieve this, the investigators will build on existing model capabilities of the community model WRF/Chem and on results from prior research by the PI. They will validate the model with field observations from CalNex 2010. Central to this task is the modeling of the transformations of nocturnal nitrogen oxides, NO3 and N2O5. This includes gas-phase reactions of NO3 with ozone and VOC, and the heterogeneous hydrolysis of N2O5.