Ambient air concentrations of nitric acid vapor and aerosol nitrate were measured above the remote North Pacific between May 1 and June 5, 1988, during the Mauna Loa Observatory Photochemistry (MLOPEX). Nighttime (free tropospheric) mean values were 30 pptv nitrate aerosol and 103 pptv nitric acid. Daytime values, representative of island-modified marine boundary layer air, averaged 60 pptv aerosol and 130 pptv nitric acid. The measurements were generally consistent with previous measurements in the remote North Pacific including free tropospheric measurements of nitric acid during NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE 2). MLOPEX observations are smaller than model HNO3/NOx ratios. However, the magnitude of this discrepancy is obscured by uncertainties, mainly in the model OH concentration and nitric acid photolysis rates. Nonetheless, large observed variations in the HNO3/NOx ratio likely indicate significant loss of nitric acid, at least part of the time, by some process in addition to vapor phase photolysis and destruction by OH. Several processes are considered, but no firm conclusion is reached. ¿ American Geophysical Union 1992