The practicality of conducting photochemical calculations along trajectories of air masses is investigated. An isentropic trajectory package is used in conjunction with a detailed photochemical model to compare predictions of the mean chemical content of air masses initialized with the Halogen Occultation Experiment (HALOE) data with coincident in situ observations from instruments onboard the ER-2 aircraft. Comparisons are made for 10 ER-2 flights originating from Christchurch, New Zealand, during the May to June and October 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) deployments. Between 54 and 84 coincidences are found, depending on the species measured. Correlations between the ER-2 and HALOE air mass/box model calculations are high (0.56--0.90) for most species considered except for H2O (0.14) and HCl (0.24). Statistically significant low biases in the prediction of HCl, H2O, and OH are found. Kolmogorov-Smirnov (KS) significance tests are used to quantify the agreement between the distribution of species observed by the ER-2 and predicted by the HALOE trajectory/photochemical model. The model predictions agree with the observed variance within the distributions at significance levels greater than 0.80 (greater than 80% confidence that the predicted and observed variance are identical) for H2O, ClO, O3, and NOy. The impact of computational errors in the trajectory calculations and measurement uncertainty in the computed confidence levels are investigated using Monte Carlo techniques. Computational trajectory errors are found to play a small role in reducing confidence levels. The error analysis shows that the HALOE trajectory/photochemical model calculations reproduce the large-scale variability found in the in situ ER-2 constituent measurements to within the expected uncertainties in the HALOE observations for all species considered. It is concluded that the combined trajectory/photochemical model is an effective tool for interpreting in situ aircraft observations within the global perspective provided by remote satellite measurements.¿ 1997 American Geophysical Union