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The Etesians are the dominant synoptically driven winds observed in the Eastern Mediterranean, usually from late spring to late summer. Due to the complex topography, the Etesians can be very strong and pose significant environmental hazards, especially over wildfire incidents. This study assesses the impacts of climate change on future Etesians by analyzing the response of the most recent EURO-CORDEX regional climate simulations at the 12-km grid resolution over the twenty-first century. The mean model ensemble projects a significant increase of the Etesians' frequency and intensity under the two emission scenarios RCP4.5 and RCP8.5. This response is connected to an increase in the zonal wind at 200 hPa, a reinforcement of the midlatitude westerly flow, and a decrease in the wave amplitude. These circulation changes accelerate the mid-to-high latitude eastward propagation of the large-scale circulation systems which can favor enhanced ridges over the Balkans. A strengthening and poleward shift of the subtropical jet stream is also projected, connected with stronger subsidence over the Eastern Mediterranean. The projected changes will have profound environmental and societal implications, including the lengthening of the wildfire season and increasing air pollution risk in the region. On the other hand, the current estimate of future wind power potential in the Aegean Sea will be significantly increased by the end of the century, which might have positive impact in the regional economy.

The 10-year (1988–1997) surface ozone record at Jungfraujoch (JFJ; 46°33′N, 07°59′E) in the Swiss Alps is analyzed with regard to changes in the seasonal cycle. For comparison, the surface ozone time series (1988–1996) at Zugspitze (ZUG; 47°25′N, 10°59′E, 2960 m above sea level) on the northern fringe of the Alps in Germany is also examined. The applied structural model in state space form is flexible enough to isolate a statistically significant decrease in the estimated seasonal component for May ozone concentrations over the last 10 years, in both the original JFJ and ZUG ozone records. A negative trend in May is also displayed by ordinary linear regression but is not statistically significant, demonstrating the advantage of the employed methodology over simple regression approaches in the analysis of changing seasonal cycles of tropospheric ozone records. We attempt to isolate potential mechanisms underlying the observed changes in the seasonal ozone cycles at JFJ by analyzing filtered ozone records. Sieving was carried out with wind direction, wind speed, and time windows to exclude the confounding influences from the atmospheric boundary layer and outflow from southern Europe. Taking only nighttime (2400–0500 central European time (CET)) ozone values into account, the seasonal ozone cycle at JFJ is characterized by a broad spring-summer maximum. This filter is believed to represent the lower free troposphere over the Alps and also shows a significant decrease in May ozone concentrations. It is suggested that the results may reflect ozone changes in the free troposphere over continental Europe.

The second Free Tropospheric Experiment (FREETEX ’98) took place at the Jung-fraujoch Observatory (3580 m above sea level), Switzerland, in March/April 1998. A number of trace gases and photolysis parameters were measured, including peroxy radicals (HO2+RO2), the photolysis rate coefficients j(NO2) and j(O1D), O3, NOx, NOy, HCHO, peroxyacetylnitrate (PAN), CO, CN, and hydrocarbons. The average midday concentration of HO2+RO2 observed during the campaign is 13±3 parts per trillion by volume (pptv). The peroxy radical concentrations measured during FREETEX '98 are discussed in terms of the respective NO measurements and box model calculations. By screening out polluted and cloudy conditions during FREETEX ’98, 8 relatively clean days are selected. These 8 days represent near free tropospheric conditions. The average midday concentration of HO2+RO2 observed during these 8 days is 17±5 pptv and the modeled diurnal cycle of [HO2]+[CH3O2] simulated by a photochemical box model shows very good agreement with the mean diurnal cycle of the peroxy radicals over these 8 days. The in situ net ozone production rate is calculated for each day during FREETEX '98 using the peroxy radical, j(O1D), NO, H2O, and O3 measurements. The mean net ozone production rate during the 8 relatively clean selected days is 0.09 ppbv h−1. If more stringent criteria are applied, 3 free tropospheric days can be selected with a mean net ozone production rate of 0.05±0.01 ppbv h−1, indicating the positive role of in situ photochemistry during springtime in the free troposphere over the Alps and supporting the previous results of the first FREETEX campaign in April/May 1996. Using the experimental data, it is estimated that the ozone compensation point, that is, the point where photochemical ozone production equals destruction, occurs at 24±9 pptv of NO in good agreement with theoretical calculations.

Continuous measurements of peroxy radicals (HO2+RO2), the photolysis rate coefficients j(NO2) and j(O1D), and a number of other trace gases including O3, NOx, and CO, were carried out at the alpine site at Jungfraujoch (3580 m asl), Switzerland, in April/May 1996 during the Free Tropospheric Experiment (FREETEX '96). In spring, the Jungfraujoch Observatory is very often located in the lower free troposphere. A mean midday concentration of 17.1±2.2 parts per trillion by volume (pptv) was calculated from continuous measurements of the sum of inorganic and organic peroxy radicals using the chemical amplification technique. The average diurnal cycles for peroxy radicals and the photolysis rate coefficient j)O1D) during FREETEX '96 showed a statistically more significant square root than first-order relationship, indicative of a relatively clean atmosphere at the mountain site, where peroxy radical self- and cross-reactions were the dominant peroxy radical loss processes. The average ozone diurnal cycle during FREETEX '96 displayed an increase of about 2 ppbv during the daytime despite the square root relationship. Similar results were derived from individual days during the campaign. A simple photochemical box model indicated that there is a regime in NO concentration from about 20 pptv to 100 pptv where a better square root (than first-order) correlation between peroxy radicals and j(O1D) is accompanied by net ozone production. Taking into account that median daytime concentrations of NO at Jungfraujoch range from 51 pptv in March to 93 pptv in April, it is suggested that Jungfraujoch, during FREETEX '96, has been in the net ozone production regime in spite of the observed square root dependence of peroxy radicals on j(O1D)

[1] Regional climate-air quality simulations were performed over Europe for two future decades, 2041–2050 and 2091–2100 under IPCC A1B scenario and the control decade 1991–2000. The simulations serve as a theoretical experiment investigating the impact of changing climate on summer surface ozone. Our simulations suggest that changes in summer surface ozone imposed by climate change until the 2040s are below 1 ppbv and more pronounced until the 2090s. The median of summer near surface temperature for whole Europe is 2.7 K higher at the end of the 21st century than to the end of the 20th century with more intense temperature increase simulated for southern Europe. A prominent feature is the decrease of cloudiness mostly over western Europe at the end of the 21st century associated with an anticyclonic anomaly which favors more stagnant conditions and weakening of the westerly winds for the larger part Europe southern of 50°N. Biogenic emissions double in the simulation at the end of the 21st century for latitudes below 50° and together with changes in circulation patterns, temperature, and solar radiation, contribute to the enhanced average ozone concentrations at the end of the 21st century. The change is more intense over southwest Europe, where the median of ozone increases by 6.2 ppbv. Sensitivity simulations suggest that biogenic emissions, temperature and solar radiation have a comparable impact on average surface ozone in the examined range of perturbations. The maximum response of the imposed perturbations was seen over southern Europe.

[1] In this work, we investigate the issue of the turnaround in ozone trends of the recently homogenized Umkehr ozone record of Arosa, Switzerland, which is the longest Umkehr data set, extending from 1956 to date, using different statistical methods. All methods show statistically significant negative ozone trends from 1970 to 1995 in the upper stratosphere (above 32.6 km) throughout the course of the year as well as in the lower stratosphere (below 23.5 km) mainly during winter to spring, which can be partially attributed to dynamical changes. Over the recent period (1996–2004) the year-round trends in the lower stratosphere become positive and are more positive during the winter to spring period. The results also show changes in upper stratospheric ozone trends after 1996, which are, however, not statistically significant at 95% if aerosol correction is applied on the retrieved data. This lack of significant trend changes during the recent period in the upper stratosphere is regionally coherent with recent results derived from upper stratospheric ozone data recorded by lidars, microwave radiometers, and satellite instruments at an adjacent location. Although the positive change in trends after 1996 both for upper and lower stratospheric ozone is in line with the reduction of the emissions of ozone-depleting substances from the successful implementation of the Montreal Protocol and its amendments, we recommend, because of lack of significance for the upper stratospheric trends, repeating this analysis in a few years in order to overcome ambiguous results for documentation of the turnaround of upper stratospheric ozone.

[1] Model studies show that changes in photodissociation rates resulting from changes in ozone column densities induce changes in lower tropospheric ozone, which vary significantly with location and time of the year. The validity of the model results is tested against daily total ozone and ground level ozone at three selected stations (Samoa, Mauna Loa, and Hohenpeissenberg). Observational data for a period of more than 1 decade have been analyzed. Comparisons are made of model-simulated distribution of ozone and its precursors (NOx and CO) at the three stations. Further comparisons are made of observed and model-calculated sensitivity in surface ozone to reduction in ozone column densities. Calculations performed with a global-scale chemical transport model (CTM) with extensive ozone chemistry reproduce well the observed levels and seasonal distribution of NOx, CO, and ozone at remote background stations (Samoa and Mauna Loa) and at stations in more polluted regions (Hohenpeissenberg). A chemical box model is used to demonstrate the chemical link between surface ozone changes and changes in total ozone for different NOx levels. Model studies and analysis of the observational data show that ground level ozone at the remote, low-NOx stations of Mauna Loa and Samoa is correlated positively with total ozone, with an exception at Mauna Loa during winter months. A reduction in ozone column densities, which leads to enhanced photochemical activity, reduces ozone levels at ground level. The sensitivity of surface ozone to changes in total ozone is particularly large in the low-NOx regime at Samoa. An anticorrelation between ground level ozone and total ozone is found at the Hohenpeissenberg station both in the observational data and in the model results during wintertime with high NOx levels. Enhanced photochemical activity leads to enhanced ozone production. There is, however, a disagreement between the observed and CTM-modeled sensitivity in surface ozone to ozone column density during the summer months at Hohenpeissenberg. The strong anticorrelation found in the observations, giving increases in surface ozone at low ozone column densities, is not present in the CTM model studies. It is suggested that a correlation between low ozone column densities and stagnant high-pressure systems is an important cause for the observed anticorrelation.

[1] Continuous measurements of peroxy radicals were carried out using the chemical amplification technique during three intensive FREE Tropospheric EXperiments, (FREETEX) in 1996, 1998 and 2001 at the Jungfraujoch (3,580 m asl) in the Swiss Alps, in combination covering the period of ozone build-up from late winter to late spring. The monthly medians of the peroxy radical measurements from 10:00 to 13:00 UT in February, March, April, and May, after filtering out the more cloudy conditions, show a clear seasonal behaviour with a gradual increase of the peroxy radical mixing ratios from around 12 pptv in February to around 20 pptv in May. This data represents among the first semi-continuous seasonal observations of peroxy radical concentrations, which indicates the important and possibly dominant role of in-situ photochemistry in the observed ozone build up from winter to spring in the lower free-troposphere over the Alps.

Jungfraujoch (JUN), Switzerland and Zugspitze (ZUG), Germany. Inspection of the variability of the ratio 10 Be/ 7 Be in relation to 10 Be, 7 Be, and relative humidity (RH) reveals that the ratio is independent from processes that have a clear effect on both radionuclides, such as wet scavenging. High ratio values are generally met under cyclonic or northerly advective conditions, which are the synoptic situations mostly related to stratosphere-to-troposphere transport (STT) events over central Europe, while the 10-day back trajectories indicate a stratospheric source for the majority of the cases within the upper 10% quantile of 10 Be/ 7 Be ratios. The monthly 10 Be/ 7 Be ratios show a clear May and June peak at JUN and a much weaker seasonality at ZUG. A simple mixing model is used for an independent estimate of the strength of STT throughout the year based on the 7 Be and 10 Be measurements. In spite of the various uncertainties, the results indicate a seasonal cycle of stratospheric ozone percentage contribution with an early spring maximum (3– 11%) and autumn minimum (1–2%) at ZUG, while at JUN, a primary maximum in May and June (6–18%), a secondary maximum in March (4–13%), and a minimum again in autumn (1–4%) are revealed. Although the simple method applied here provides an independent estimate for the impact of STT to the lower troposphere, it nevertheless shows relatively good agreement with Lagrangian model calculations, especially for ZUG. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3362 Meteorology and Atmospheric Dynamics: Stratosphere/troposphere interactions; KEYWORDS: stratospheric intrusions, cosmogenic radionuclides, tropospheric ozone, 7 Be, 10 Be, Alps Citation: Zanis, P., et al., An estimate of the impact of stratosphere-to-troposphere transport (STT) on the lower free tropospheric ozone over the Alps using 10 Be and 7 Be measurements, J. Geophys. Res., 108(D12), 8520, doi:10.1029/2002JD002604, 2003.

[1] A variety of surface chemical measurements, including surface ozone, NO, NO2, NOx, peroxyacetyl nitrate, and nonmethane hydrocarbon, were carried out at a rural coastal site, Nopigia, Crete, and at a rural elevated site, Prases, Crete, within the framework of the Photochemical Activity and Ultraviolet Radiation (PAUR) II experiment, which took place in Greece and Italy in early summer (1–31 May 1999). The total ozone exhibited very large fluctuations (larger than 90 Dobson units, 1 DU = 2.69 × 1016 molecules cm−2) during the campaign with maximum values of around 400 DU during 10 May and minimum values of

[1] The Photochemical Activity and Ultraviolet Radiation (PAUR) II project (a continuation of an earlier PAUR I project) had the purpose of studying the interrelationships between changes in total ozone, tropospheric aerosols, UV radiation and photochemical activity. As part of PAUR II project, a campaign took place in Greece and Italy during May–June 1999, with the participation of 15 European and 3 American research institutions. A variety of radiation and gaseous and aerosol atmospheric composition and optical characteristics measurements were made during the campaign. Radiative transfer models and three-dimensional (3-D) regional chemistry transport models (CTM) were applied and compared to the available data set of PAUR II. The present overview paper gives an introduction to the project and to the meteorological and environmental conditions that prevailed and outlines some results that are extensively described in the subsequent papers which form this special section. The modulation of the UVB field in the presence of different types of aerosols, its transmittance and role in the photochemistry of the particular eastern Mediterranean environment is overviewed. Using a 3-D CTM, it is shown that even a 50% reduction in Greek anthropogenic emissions has only a small effect in reducing the ozone levels over the eastern Mediterranean in summer. The environmental conditions, which prevailed during the PAUR I and PAUR II campaigns, offered cases of background conditions over the Aegean Sea as well as conditions with Saharan dust episodes and extremes in total ozone.