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 this paper we present a study on stratospheric intrusion (SI) events recorded at a high mountain station in the Italian northern Apennines. Six years (1998–2003) of surface ozone and beryllium-7 concentration measurements as well as relative humidity values recorded at the GAW Mt. Cimone research station (44°11'N, 10°42'E; 2165 m asl) were analyzed. Moreover, three-dimensional backward trajectories calculated by the FLEXTRA model and potential vorticity values along these trajectories were used. In order to identify SI and evaluate their contribution to the tropospheric ozone at Mt. Cimone, a statistical methodology was developed. This methodology consists of different selection criteria based on observed and modeled stratospheric tracers as well as on tropopause height values recorded by radio soundings. On average, SI effects affected Mt. Cimone for about 36 days/year. The obtained 6-year SI climatology showed a clear seasonal cycle with a winter maximum and a spring-summer minimum. The seasonal cycle was also characterized by an interannual variation. In particular, during winter (autumn), SI frequency could be related to the intensity of the positive (negative) NAO phase. In order to separate direct SI from indirect SI, a restrictive selection criterion was set. This criterion, named Direct Intrusion Criterion (DIC), requested that all the analyzed tracers were characterized by stratospheric values. Direct SI affected Mt. Cimone for about 6 days/year, with frequency peaks in winter and early summer. At Mt. Cimone, SI contribution to background ozone concentrations was largest in winter. On average, an ozone increase of 8% (3%) with respect to the monthly running mean was found during direct (indirect) SI. Finally, the typical variations of stratospheric tracers during SI events were analyzed. The analysis of in situ atmospheric pressure values suggested that direct SI were connected with intense fronts affecting the region, while indirect SI were possibly connected with subsiding structures related with anticyclonic areas.

At the Nepal Climate Observatory - Pyramid a UV-absorption analyser (Thermo TEI 49C UV analyzer) is used for measurement of surface ozone adopting the sampling procedures suggested within the GAW-WMO (GAW, 1992).

In the Khumbu Valley, located in the central part of the Himalayan range and including the area of Sagarmatha National Park, a network of 7 Automatic Weather Station (AWSs) has been installed since 1994. These weather stations are located at different altitudes: Lukla (2,660 m a.s.l), Namche (3,570 m a.s.l), Periche (4,260 m a.s.l), Lobuche (5,050 m a.s.l) near the Pyramid Laboratory - Observatory, Kala Patthar (5,600 m a.s.l), Changri Nup Glacier (5,700 m a.s.l) and Mt. Everest - South Col (8.000 m a.s.l). The network mainly takes measurements of 7 standards parameters: air temperature, relative humidity, atmospheric pressure, wind speed and direction, global radiation and total precipitation. After a test period during summer 2008, the South Col AWS station has been re-installed on May 2011. The South Col station is equipped with technologically-advanced sensors for measuring temperature, humidity, wind speed and direction, atmospheric pressure and solar radiation. Support, energy and data transmission systems are optimized for functioning in adverse weather conditions. South Col AWS observations can provide useful information for investigating: (i) the variability of the subtropical jet stream analysis, (ii) the summer monsoon onset and decay, (iii) the stratospheric intrusion events, (iv) the occurrence of severe weather conditions on Mt. Everest. Geographical coordinates: - Latitude: 27° 58' N - Longitude: 86° 56' E

At the Nepal Climate Observatory - Pyramid a UV-absorption analyser (Thermo TEI 49C UV analyzer) is used for measurement of surface ozone adopting the sampling procedures suggested within the GAW-WMO (GAW, 1992).

The Vaisala WXT520 multi-parameter weather transmitter installed at Nepal Climate Observatory -Pyramid is a compact and lightweight multi-sensor instrument that measures the most essential weather parameters. The meteorological parameters measured are air temperature, atmospheric pressure, relative humidity, wind speed and direction, precipitation.

In 2008 expedition, at South Col was installed the DMA572 termohygrometer, that was replaced during the 2011 expedition.

Both in 2008 and in 2011 expedition a PTB330 barometer was installed at South Col. This sensor is manufactured by Vaisala company. Vaisala's Barometer measure Atmospheric Pressure in weather station.

Surface wind measurements using an ultrasonic anemometer WMT700. The WMT700 series uses ultrasound to determine horizontal wind speed and direction.

Both in 2008 and in 2011 expedition a DPA 007, manufactured by LSI-Lastem was installed at South Col. This broad band radiometers measure electromagnetic radiation intensity in a determined interval of wavelength. Applications often need information about emission in ultraviolet, infrared, and visible spectral band, demanding a more specialized instrumentation.