Simultaneous measurements are reported of the nitrate radical (NO₃), nitrogen dioxide (NO₂), ozone (O₃), and dimethylsulfide (DMS) in the nighttime marine boundary layer over Biscayne Bay in South Florida. These field observations are analyzed and used to initialize a boundary layer box model which examines the relative importance of the various sinks for NO_x in the marine boundary layer. The results show that the observed lifetime of NO₃ (≤6 min.) is probably controlled both by the loss of nitrogen pentoxide (N₂O₅) to reaction with water vapor and aerosols and by the reaction between NO₃ and DMS. The model is then extended to investigate the loss of nitrogen oxides from an air parcel that remains in the boundary layer with a constant sea-to-air DMS flux for several days. The principal conclusions are (1) that DMS is a much more important sink for NO₃ at lower NO₂ levels and (2) that the reaction between NO₃ and DMS is an important sink for DMS in the marine boundary layer and could exceed that of the daytime removal by OH.