The nitrate radical, NO3, and dinitrogen pentoxide, N2O5, are key reactive nocturnal nitrogen oxides in the troposphere. The daytime impact of NO3 and N2O5, however, is restricted by photochemical recycling of NO3 to NO2 and O3. In this paper, we report daytime measurements of N2O5 on board the NOAA research vessel Ronald H. Brown in the Gulf of Maine during the New England Air Quality Study--Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) campaign in the summer of 2004. Daytime N2O5 mixing ratios of up to 4 pptv were observed, consistent with predictions from a steady state analysis. Predicted and observed NO3 mixing ratios were below the instrumental detection limit of ~1 pptv; the average calculated concentration was 0.09 pptv. Important impacts of daytime NO3 and N2O5 in the marine boundary layer included increased rates of VOC oxidation (in particular dimethyl sulfide) and enhanced NOx to HNO3 conversion, both of which scaled with the available NOx. Smaller effects of daytime NO3 and N2O5 included chemical destruction of O3 and a shift of the NO2:NO ratio. Because the rates of heterogeneous conversion of N2O5 and NO3 to HNO3 scale with the surface area available for uptake, the importance of daytime fog is discussed.