A new method is proposed to classify ozone-rich layers observed in tropospheric profiles in terms of their origin using multivariate analysis. We combine principal component and discriminant analyses to quantify the respective ability of 21 measured physical parameters to describe the layers. Agglomerative hierarchical clustering shows the existence of clusters of air masses with specific physical characteristics. Quadratic discriminant analysis allows the definition of multidimensional borders between these clusters. The geophysical characteristics of the clusters are discussed and related to the origins of the layers: recently transported from the stratosphere (ST) or from the boundary layer (BL) or transported over long distances in the free troposphere. This clustering is compared to the results of a Lagrangian particle dispersion model for a 2-year period. The proportions of layers originating either from the BL or from the ST are highly consistent using both methods as well as the respective contribution of each reservoir to the total ozone mass. About 10% of the ozone measured in the tropospheric layers was exported recently from the BL and one-fifth has a recent stratospheric origin. The remaining proportion could not be attributed to any recent transport pathway. Season-dependent criteria allow very satisfactory reproduction of the seasonal variability of the layering as seen by the Lagrangian model. Analysis of the geographical origin of BL air masses suggests that the statistical clustering underestimates long-range transport, especially in fall.
Impact of vertical transport processes on the tropospheric ozone layering above Europe. Part I: Study of air mass origin using multivariate analysis, clustering and trajectories