Estimating relationships between air mass origin and chemical composition

Observations of a chemical at a point in the atmosphere typically show sudd en transitions between episodes of high and low concentration. Often these are associated with a rapid change in the origin of air arriving at the sit e. Lagrangian chemical models riding along trajectories can reproduce such transitions, but small timing errors from trajectory phase errors dramatica lly reduce the correlation between modeled concentrations and observations, Here the origin averaging technique is introduced to obtain maps of averag e concentration as a function of air mass origin for the East Atlantic Summ er Experiment 1996 (EASE96, a groundbased chemistry campaign). These maps a re used to construct origin averaged time series which enable comparison be tween a chemistry model and observations with phase errors factored out. Th e amount of the observed signal explained by trajectory changes can be quan tified, as can the systematic model errors as a function of air mass origin . The Cambridge Tropospheric Trajectory model of Chemistry and Transport (C iTTyCAT) can account for over 70% of the observed ozone signal variance dur ing EASE96 when phase errors are side-stepped by origin averaging. The dram atic increase in correlation (from 23% without averaging) cannot be achieve d by time averaging. The success of the model is attributed to the strong r elationship between changes in ozone along trajectories and their origin an d its ability to simulate those changes. The model performs less well for l onger-lived chemical constituents because the initial conditions 5 days bef ore arrival are insufficiently well known.