PHOTOCHEMICAL CALCULATIONS ALONG AIR-MASS TRAJECTORIES DURING ASHOE MAESA/

The practicality of conducting photochemical calculations along trajec tories 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 wi th the Halogen Occultation Experiment (HALOE) data with coincident in situ observations from instruments onboard the ER-2 aircraft. Comparis ons are made for 10 ER-2 flights originating from Christchurch, New Ze aland, during the May to June and October 1994 Airborne Southern Hemis phere Ozone Experiment/Measurements for Assessing the Effects of Strat ospheric Aircraft (ASHOE/MAESA) deployments. Between 54 and 84 coincid ences are found, depending on the species measured. Correlations betwe en 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, a nd OH are found. Kolmogorov-Smirnov (KS) significance tests are used t o 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 dis tributions at significance levels greater than 0.80 (greater than 80% confidence that the predicted and observed variance are identical) for H2O, ClO, O-3, and NOy. The impact of computational errors in the tra jectory calculations and measurement uncertainty in the computed confi dence levels are investigated using Monte Carlo techniques. Computatio nal trajectory errors are found to play a small role in reducing confi dence levels. The error analysis shows that the HALOE trajectory/photo chemical model calculations reproduce the large-scale variability foun d 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 e ffective tool for interpreting in situ aircraft observations within th e global perspective provided by remote satellite measurements.