An unusual springtime ozone episode at high elevation in the Swiss Alps: contributions both from cross-tropopause exchange and from the boundary layer

An unusually “smooth” springtime ozone episode (maximum 97 ppb(v)) at the Jungfraujoch Observatory (3580 m asl) in the Swiss Alps is described. Analysis of meteorological variables is combined with back-trajectory analysis, and numerical simulations from a mesoscale model nested within a limited area model to examine the origin of the episode. Cross-tropopause exchange on the western flank of an upper trough, deep convection in a Gulf of Genoa cyclone, and the northwards advection of boundary-layer air (with possible in situ photochemistry) all combined to produce the highest ozone concentration of the year (1987) at the mountain site.