Measurements from the Halogen Occultation Experiment (HALOE) on board the UARS satellite and assimilated winds, temperatures, and diabatic heating rates from the NASA Goddard data assimilation office (DAO) are used with the NASA Langley Research Center (LaRC) Lagrangian photochemical model to compute 3-D air parcel trajectories with photochemistry for all Northern Hemisphere HALOE observations during the period March--September 1997. Results from ensemble means of the photochemical trajectory calculations provide a global perspective for the interpretation of constituent measurements made from the ER-2 and balloon platforms during the POLARIS aircraft campaign. Lagrangian photochemical predictions are shown to compare favorably with ER-2, balloon, Total Ozone Mapping Spectrometer (TOMS), and subsequent coincident HALOE observations. Model predictions show large-scale photochemical ozone loss in high latitudes at ER-2 flight altitudes of over 10% per month in June and July, in good agreement with steady state photochemical calculations constrained with ER-2 observations of radical and long-lived species. Largest summertime photochemical ozone losses (over 1.4 ppmv/month) are found to occur poleward of 60 ¿N above 30 mbar, in good agreement with steady state photochemical calculations constrained with observations from the balloon-borne Fourier transform infrared solar absorption spectrometer (MkIV) instrument. Summertime polar photochemical ozone losses are driven largely by NOx chemistry and are largest for air parcels with high NOx/NOy ratios that have experienced continuous sunlight for several days. Differences between predicted net changes in ozone and changes due to photochemistry are used to estimate residual changes due to transport processes. Photochemical and residual transport tendencies tend to be of similar magnitude but opposite sign. Photochemical loss of ozone tends to outweigh positive transport tendencies in high latitudes, while upwelling of low ozone below the tropical ozone maximum moderates photochemical production there. The estimated transport tendencies are generally consistent with expectations based on transformed Eulerian circulation derived from the DAO assimilated data and the mean ozone distribution. A net (photochemical plus transport) ozone decrease of over 0.2 ppmv/month is predicted throughout the middle and lower stratosphere poleward of 70 ¿N during the summer months. ¿ 1999 American Geophysical Union