The ozone situation in the Eastern Alps was investigated during the south foehn period from 4 - 6 May 1997. The event was studied in detail using surface measurements, soundings and aircraft measurements of meteorological and chemical parameters. A numerical simulation with a prognostic meteorological model (MM5) and a chemistry-transport model with 6 km resolution provided additional insight. The case study was supplemented by a climatological evaluation of a three-year data set. The foehn period was preceeded by an ozone episode in the Po Basin south of the Alps. Advection of residual-layer air masses from that area caused a maximum of the ozone concentration in the beginning. Later on, the ozone concentration in the foehn area was determined by a mixture of regional-scale advection from the lower free troposphere and boundary-layer air form the south. The contribution o boundary-layer air was especially strong in the lee of the deep gap formed by the Brenner Pass and visiblein many parameters. The climatological evaluation showed that during south foehn, ozone concentrations are elevated in the foehn area, especially in the valleys and during nighttime where the usual nocturnal minimum is suppressed.

This paper presents studies of stratospheric intrusions in the Alps and northern Apennines, their seasonal variations, and their effect on ozone concentrations. The results are based on experimental data and on simulations with a Lagrangian tracer model. The model, employing analyzed meteorological data, advects a passive stratospheric ozone tracer through the calculation of a large number of three-dimensional trajectories. In two case studies, the model is evaluated using a comprehensive set of observation data, consisting of water vapor satellite images, total column ozone measurements, ozone soundings, and measurements of ozone, beryllium 7 and meteorological parameters at three high Alpine sites and at the highest peak in the northern Apennines. During the two episodes considered, stratospheric air was detectable in the whole Alpine area with peak ozone mixing ratios in the 70–90 ppb range and even penetrated into some valleys. During one episode, stratospheric air also reached the northern Apennines, which highlights the large extension of the affected region. At the end of this episode, as shown by the model, the air was a mixture of tropospheric air with air originating from three different stratospheric intrusions. For three high Alpine sites, the frequency of stratospheric intrusions and its seasonal variation is derived using ozone, beryllium 7 and humidity measurements. The periods covered by this climatology are 1991 to 1997 for Zugspitze, and 1996 to 1998 for Jungfraujoch and Sonnblick. Another short climatology was established from a three-year (1995–1997) model simulation. Good agreement between the two approaches is found for Zugspitze and Sonnblick: the simulated ozone tracer mixing ratios are significantly higher on “intrusion days”, identified from the observations, than on “non-intrusion days”. For Jungfraujoch, the agreement is less good, which could partly be due to the coarser time resolution of the beryllium 7 measurements at this site. The absolute frequency of stratospheric air intrusions as identified from the observations depends critically on the specification of threshold values for ozone, beryllium 7 and humidity, while the relative shape of the annual cycle is rather insensitive to threshold variations. At Zugspitze and Sonnblick, it shows a maximum in October, a secondary maximum in January and February, and a deep summer minimum. For Jungfraujoch, where the frequency of intrusions is higher than at Zugspitze and Sonnblick throughout most of the year, no clear seasonal variation is found. Simulated ozone tracer mixing ratios in the Alps are found to peak in late-winter/early-spring, when ozone concentrations are at a maximum in the stratosphere, but are almost at the same level in autumn, due to somewhat higher frequency of stratospheric intrusions in that season. Similar to the observations, there is a deep minimum in summer, when the model showed practically no intrusions with a tropospheric age of less than four days.

In order to point out and study transports of ozone rich air masses in the lower troposphere from the stratosphere/upper troposphere, continuous measurements of several parameters have been undertaken at Mt. Cimone during the European Community VOTALP project (Vertical Ozone Transport in the Alps). Several high values of surface ozone concentration due to vertical stratospheric-tropospheric exchanges have been recorded in the four mountain peak stations involved in this project (Jungfraujoch, Sonnblick, Zugspitze and Mt. Cimone) in 1996–1997. This paper presents and analyses data concerning the Mt. Cimone ground-based station, which is the highest peak of the Italian Northern Apennines and the most representative WMO-GAW site in Italy. Episodes of vertical exchange in the lower stratosphere, as tropopause folding, or in the upper troposphere, as down draft transport, have been registered at Mt. Cimone since March 1996 and subsequently studied. In fact, the comparison between the behaviours of different background trace gases at a mountain baseline station, the weather situations and the backward trajectory analyses can bring to light these events and be very useful for a better knowledge of transport phenomena. Correlation between high level of ozone concentration, chemical and meteorological parameters and three-dimensional backward trajectories relative to two particular events are herein presented.