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 NOx 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. ¿ American Geophysical Union 1996