Episodes of high ozone concentrations at the earth's surface resulting from transport down from the upper troposphere/lower stratosphere: a review and case studies

Abstract

When compared with photochemically induced ozone episodes produced in situ near the Earth's surface, relatively little attention has been paid to those surface episodes (including mountain sites) which have their origin in transport down from the upper troposphere/lower stratosphere. Although these episodes may be relatively uncommon, they can produce transient peak ozone concentrations of around 100 ppb at sea-level and concentrations in excess of 250 ppb have been reported in mountain regions. The published literature covering such episodes is reviewed, and those synoptic/dynamic features which are common to them are identified. These features are examined in detail through two case studies. The first is a winter 10-day period at a station on the Dutch coast when mean hourly concentrations exceeded 70 ppb on several occasions. The high concentrations over the period appeared to be a consequence of several processes. It is possible that photochemical production from precursors transported from the Greece/Italy region and from central-west Europe made important contributions, although the timing was early in the year for NW Europe. Other components of the episode are more readily explained in terms of mixing down of ozone-rich air from stratospheric intrusions and transport into the planetary boundarylayer, either in association with vigorous cold fronts, or in immediately adjacent high pressure ridges; or in more extensive following anticyclones. Stratospheric intrusions, associated with pronounced cut-off circulations, acted over a number of days to provide a large reservior of ozone-rich air in subsiding, non-dispersive, circulations in the middle/lower troposphere, for eventual transport to the ground. This transport depended on the diurnal cycle of vertical exchange in the lower atmosphere, and so may be in phase with any ozone produced by photochemical reactions near the surface. The second case study was a short-lived ozone “spike” (92 ppb hourly concentration) at a mountain site (3580 m elevation) in Switzerland. In this case, potential vorticity distributions and three-dimensional back-trajectories confirmed that the rapid descent of ozone-rich air, from a tropopause fold, behind a cold front was the mechanism of transfer to the mountain. In both case studies the stratospheric intrusions were associated with tropopause folds on the Western side of upper troughs/cut-off lows.