In the spring of 1999 airborne measurements of NO, O3, peroxyacetyl nitrate (PAN), CO, CH4, H2O, volatile organic compounds, aerosols (particle count, light scattering, and light absorption), NO2 photolysis frequency, and standard meteorological variables were made off the coast of Washington State as part of the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) experiment. These measurements were used to constrain a photochemical box model to calculate the ozone photochemical tendency, T(O3), in this region. T(O3) in marine flow from the remote Pacific was found to be weakly ozone destroying from the surface up to 8 km. Values of T(O3) increased from -0.83 ppbv d-1 in the 0--2 km layer to -0.11 ppbv d-1 in the 6--8 km layer. These results are compared to T(O3) from other photochemistry experiments in the springtime Pacific. We also used the model to investigate the impacts of PAN decomposition on the mixing ratio of NOx (defined here as NO+NO2+NO3+2N2O5+HNO2+HNO4) and on T(O3). PAN decomposition was found to contribute from 11 to 30% toward NOx production and to enhance T(O3) by 0.13 to 0.41 ppbv d-1. The impacts of PAN decomposition were further investigated in a case study where measurements were made in a strongly subsiding air mass. In this air mass, PAN induced perturbations to NOx and T(O3) reached 20.1 pptv and 1.45 ppbv d-1, respectively, more than three times that found in marine background average. Finally, we estimate how T(O3) in the northeast Pacific atmosphere may change as a result of increasing anthropogenic NOx emissions from Asia. The calculations suggest that while O3 mixing ratios in the northeast Pacific are likely to increase, T(O3) will remain close to its current value as a result of offsetting factors. ¿ 2001 American Geophysical Union