Atmospheric Environment (v.34, #9)

The research project VOTALP – general objectives and main results by Gerhard Wotawa; Helga Kromp-Kolb (1319-1322).
The research project “Vertical ozone transports in the Alps” (VOTALP), conducted between 1996 and 1998, investigated mechanisms that cause increased ozone concentrations in the Alps, focusing on vertical transport processes. The major aim of the project was to deepen our understanding of dynamics and chemistry of ozone in mountainous areas.
Keywords: Ozone; Photochemistry; Transport; Alps; Mountain; Measurement; Modelling;

The influence of stratospheric intrusions on alpine ozone concentrations by A Stohl; N Spichtinger-Rakowsky; P Bonasoni; H Feldmann; M Memmesheimer; H.E Scheel; T Trickl; S Hübener; W Ringer; M Mandl (1323-1354).
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.
Keywords: Stratospheric intrusions; Ozone; Beryllium 7; Alps; Lagrangian model;

Stratospheric ozone intrusion episodes recorded at Mt. Cimone during the VOTALP project: case studies by P Bonasoni; F Evangelisti; U Bonafe; F Ravegnani; F Calzolari; A Stohl; L Tositti; O Tubertini; T Colombo (1355-1365).
Continuous measurements of ozone concentrations performed at baseline stations on high mountain summits allow stratosphere–troposphere exchange events to be monitored and their variability over small scales to be studied. In this paper we present two characteristic episodes related to lower stratosphere/upper troposphere vertical transport registered at Mt. Cimone during the VOTALP project and we highlight the use of carbon monoxide (CO) and carbon dioxide (CO2) as non-conventional tracers of lower stratospheric air masses. In this way, stratosphere–troposphere exchange events transporting high ozone concentration into the troposphere have been identified by an accurate analysis of the CO and CO2 behaviour. Moreover, three-dimensional ensemble trajectories, satellite maps (water vapour and total ozone), ground-based ozone measurements and ozone-soundings have been analysed together with conventional tracers as 7 Be and relative humidity as well as the normalised fraction f ( 7 Be , 210 Pb ) in order to fully characterise the examined events.
Keywords: Surface ozone; Carbon dioxide; Carbon monoxide; Tropopause folding; Cut-off low; Stratospheric tracers;

We present results of statistical trajectory source analyses applied on ozone concentrations measured at high mountain peaks within and at the fringes of the Alps supported by Lagrangian photochemical box model calculations. These analyses yielded coherent pictures of transport processes causing elevated ozone concentrations in the Alps, and of the amount of ozone produced during transport over high-emission areas. Using measurement data, specific emission areas like the Po Basin, southern Germany, the “Black Triangle” region and some areas in eastern Europe were identified as important source regions, causing elevated ozone concentrations in the Alps. These statistics were supported by model calculations of transport and formation of ozone, giving similar results. Mesoscale transport processes and ozone formation in the boundary layer along the pathways were found to play an important role in determining Alpine ozone concentration levels. Ozone concentration tendencies along transport pathways were quantified climatologically using the box model. During the last 24 h of transport, concentration increases of 6–13 ppb, on the average, were found along 60–80% of all trajectories reaching the Alps, depending on the specific location. These estimates were confirmed by a measurement-based analysis of ozone formation during transport over the Po Basin, obtaining values of similar order of magnitude.
Keywords: Trajectories; Lagrangian transport; Box models; Ozone; Photochemistry;

South foehn and ozone in the Eastern Alps – case study and climatological aspects by Petra Seibert; Hendrik Feldmann; Bruno Neininger; Martin Bäumle; Thomas Trickl (1379-1394).
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 from the south. The contribution of boundary-layer air was especially strong in the lee of the deep gap formed by the Brenner Pass and visible in 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.
Keywords: Mountains; Aircraft; Lidar; Trajectories; Mesoscale meteorological model; Chemistry-transport model;

The VOTALP Mesolcina Valley Campaign 1996 – concept, background and some highlights by Markus Furger; Josef Dommen; Werner K Graber; Lionel Poggio; André S.H Prévôt; Stefan Emeis; Georg Grell; Thomas Trickl; Bostjan Gomiscek; Bruno Neininger; Gerhard Wotawa (1395-1412).
The Mesolcina Valley campaign was an important part of the VOTALP project. Its main goals were the study of the effects of thermal wind systems on horizontal and vertical ozone transport over various distances, and on local production of ozone in an Alpine valley. The field measurements took place in the Mesolcina Valley in southern Switzerland in July and August 1996. The Mesolcina Valley is typical for the Alps for its size and its rural character, and contains an important traffic route. Ground-based and airborne instruments were deployed to obtain a four-dimensional chemical and physical picture of the valley atmosphere and the surroundings. Field measurements were complemented by numerical modeling studies. An overview of the campaign, its geographic background, and the measurement plan is given together with selected highlights of the results obtained so far. A synoptic-climatological approach tries to evaluate the representativeness of the observation days. It was found that during the campaign representative data for typical ozone days were collected, while extreme events did not occur. The main results of the campaign are that very high pollutant concentrations at the Alpine crests are the effect of advective, larger-scale transport, mainly from the Alpine forelands. The valley is very effective in pumping air into elevated layers during the day, with the slope winds probably carrying the bulk of the air volume to higher levels. Up to five times the valley volume may be exported during one upwind phase. Net local production is estimated to be approximately 3–8 ppb h−1 of ozone, but seems to be compensated by dry deposition. This indicates that the Alpine valleys may be important net sinks of air pollutants, which may lead to negative impacts on the Alpine ecosystems.
Keywords: Alps; Alpine boundary layer; Mountain meteorology; Ozone; Photochemistry;

Volatile organic compounds (VOCs) were measured during one summer month on the floor of the deep Swiss Alpine Mesolcina valley, accompanied by aircraft-based VOC measurements during six intensive observation days. In the late morning, a strong decrease of VOC concentrations despite an increasing traffic intensity was observed on the valley floor. This occurred when clean air masses were advected during the preceding night from high altitudes north of the Alps. During these nights the average VOC concentration was 40% lower in the nocturnal surface layer than in cases of southerly advection. Nights with southerly advection led to high VOC concentrations in the valley on the following morning and no decrease of VOC concentrations between 9 and 12 h. Only during these nights high ozone concentrations produced in the Po Basin of northern Italy could reach the Alpine crest and yielded ozone concentrations above 100 ppb there. The diurnal cycle of VOCs, influenced by the various meteorological features characteristic for deep Alpine valleys, is discussed in detail.
Keywords: VOC; Ozone; Mountain meteorology; Transport of air pollutants; Vertical mixing; Alp;

The IFU three-wavelength lidar was operated during part 1 of the VOTALP valley campaign in the Swiss Mesolcina valley near Grono. The measurements yielded information on both the three-dimensional distribution of the aerosol and the pollution transport in and above the valley. In the afternoon, a pronounced asymmetry of the transverse aerosol distribution was found which may be due to a difference in heating by the sun or to the bending of the valley near Grono. With very short delay with respect to the arrival time of the air pollution at the bottom of the valley a second step with enhanced aerosol formed every day which is ascribed to the build-up of a reverse (anti-valley) wind. During the second half of the investigations this reverse flow is expected to have been influenced by the almost parallel synoptic wind. For the final day of the lidar measurements wind data are available obtained during aircraft flights. This allows an estimate of the upward transport efficiency to be made. The flux ratio of the flow above and in the valley in the afternoon of that day was found to be roughly 90%. This high efficiency demonstrates the importance of the vertical exchange processes in the Alps for the pollution export out of the boundary layer over Central Europe.
Keywords: Vertical transport; Aerosol; Lidar; VOTALP; Alps; Valley; Mesolcina;

Application of a multiscale, coupled MM5/chemistry model to the complex terrain of the VOTALP valley campaign by Georg A Grell; Stefan Emeis; William R Stockwell; Thomas Schoenemeyer; Renate Forkel; John Michalakes; Richard Knoche; Winfried Seidl (1435-1453).
A coupled complex meteorology/chemistry model has been used to simulate the flow field and the concentration fields of atmospheric pollutants in Alpine valleys during the VOTALP (Vertical Ozone Transports in the ALPs) Valley Campaign in August 1996 in southern Switzerland. This paper starts with a description of a coupled numerical model (MCCM, Multiscale Climate Chemistry Model), which is based on the Penn State/NCAR nonhydrostatic mesoscale model (MM5) and the RADM2 gas-phase chemical reaction scheme. The second part of the paper presents a simulation for the Mesolcina Valley, the core region of the VOTALP Valley Campaign, and adjacent regions. The simulation was done using the nesting facility of the coupled meteorology/chemistry model. The horizontal resolution for the innermost nest was 1 km. The simulations depict the daily thermally induced valley and mountain wind system and the advection of pollutants with this wind system. It becomes obvious that in the model simulation highly polluted air from the Po Basin is transported into the Alpine valleys during the day. During the night cleaner air is brought downward with the mountain winds. Cross sections from the high-resolution model results give a closer look at the inflow and outflow of pollutants into and from the Mesolcina Valley.

Wet and dry atmospheric deposition at prealpine and alpine sites in northern Italy by Raffaella Balestrini; Laura Galli; Gianni Tartari (1455-1470).
Modified wet and dry samplers that use a water surface to collect dry deposition were used in a pluriannual field campaign in five sites in alpine and prealpine areas of Lombardy (Italy). SO4 2−, NO3 , Cl and NH4 + concentrations in rainfall increased from north to south; the southern area being closest to industrialised areas of the Po Valley. In dry deposition the spatial pattern of sulphur and nitrogen species was ostensibly linked to local pollution sources. In winter, the SO4 2− concentration in dry deposition samples from Morbegno (urban) and Longone (semi-urban) was about 3 times that in summer in association with low pH, explained as due to a higher rate of SO2 dissolution in winter followed by oxidation to sulphuric acid. The dry deposition of Ca2+, Mg2+ and K+ formed 41–60, 35–64 and 57–67% of the total flux, respectively; furthermore 25–50% of total SO4 2−, 21–37% of total NH4 +, and 23–31% of total NO3 came from dry deposition. Only at the remote high-altitude site was the dry deposition of these species insignificant. The sum of the wet and dry deposition rates indicated that current nitrogen loads exceed the nutrient critical loads by several kg N ha−1 at all investigated sites. Measured nitrogen loads at Longone and Castione (rural) were about 4 times higher than critical values.
Keywords: Wet deposition; Dry deposition; Water surface; Ammonium sulphate; Sulphur dioxide; Atmospheric fluxes; Critical loads;

Benzene and toluene levels measured with a commercial DOAS system in Thessaloniki, Greece by Kostas Kourtidis; Ioannis Ziomas; Christos Zerefos; Achilleas Gousopoulos; Dimitris Balis; Paraskevi Tzoumaka (1471-1480).
Benzene, toluene and p-xylene were measured above the buildings level in Thessaloniki, Greece for around eight months during the period December 1993–August 1994 by means of a commercial differential optical absorption spectrometer (OPSIS DOAS). Daily mean mixing ratios for benzene and toluene varied between 1–6 and 1–32 ppb, respectively. The data indicate that the annual mean benzene concentrations most probably lie in the range 2–3 ppb, which is below the guide values but above the target values of European countries that have set limits for benzene. Mean diurnal variations of benzene and toluene during summer and winter months reflect the effects of emission, photochemical degradation and mixing. Benzene and toluene hourly values correlated well with each other and with NO2. Toluene and benzene hourly values were negatively correlated with ozone during summertime. During wintertime, considerable enhancements of benzene and toluene mixing ratios might be associated with the passage of synoptic weather systems of fair weather. Although the measurement of p-xylene is, in principle, efficiently performed with the DOAS technique, the OPSIS instrument p-xylene measurements contradict current understanding of its sources and sinks and are thus attributed to errors of the instrument in the evaluation of this substance.
Keywords: Urban air pollution; Aromatic hydrocarbons;

Polychlorinated biphenyls in the lower atmosphere were studied over a regional area covering approximately 11 000 km2 in southern Sweden. Sampling were performed in 1992–1993, continuously during one year, where samples from 11 sampling-sites (in all 260 samples) were analysed. PCB concentrations ranged over two order of magnitudes, 7–983 pg m−3. Differences in PCB concentrations among the sampling sites revealed a large number of high-concentration outliers, mainly originating from one suburban site. Smaller differences in PCB concentrations between sites probably originated from varying geographical and meteorological conditions, that affected exchange processes between air and surfaces differently at the sampling sites.
Keywords: Polychlorinated biphenyls; Atmosphere; Regional scale; Urban- and rural areas;

Summertime fog chemistry at a mountainous site in central Europe by Thomas Wrzesinsky; Otto Klemm (1487-1496).
In this paper, 1997 atmospheric visibility data as an indicator for the presence and density of fog at our ecosystem research site in the Fichtelgebirge mountains, Central Europe, are presented. At this location (about 800 m a.s.l.), fog occurred on 50% of the days and added up to a total fog duration of 1158 h in 1997. Using the visibility as a determining parameter, an automatic system for the collection of fog water was developed, including an automatic sampler, and sampling on an event basis. The data from the summer 1997 season exhibit a large variability in the chemical composition of fog. Easterly winds were associated with fog with higher concentrations of the major ions (NH4 +, NO3 , SO4 2−) and metals than westerly winds. Although the H+ concentrations are lower than concentrations measured between 1983 and 1988 in NE Bavaria, it is not clear from our data alone if the negative trend over time is significant. A simple parameterization of fog deposition shows that fog plays an important role in the water balance of our forests. However, more detailed studies are needed to quantify this input, and the input of pollutants and nutrients to the ecosystems through fog deposition.
Keywords: Fog; Fog chemistry; Fog frequency; Air pollution;

Atmospheric black carbon in Helsinki by Tuomo A Pakkanen; Veli-Matti Kerminen; Christina H Ojanen; Risto E Hillamo; Päivi Aarnio; Tarja Koskentalo (1497-1506).
Atmospheric fine particle black carbon (BC) was measured close to downtown of Helsinki, during November 1996–June 1997. The average BC concentrations were 1.5 μg m−3 for working days, 1.2 μg m−3 for Saturdays, and about 1 μg m−3 for Sundays and public holidays. The overall average BC concentration was equal to 1.38 μg m−3 and its average contribution to fine particle mass equal to 19%. On working days BC concentrations showed a clear diurnal variation with the highest hourly average values occurring during the morning (2.7 μg m−3) and evening (1.9 μg m−3) rush hours. Contribution of the long-range transport to the BC was estimated to be on average about 0.4 μg m−3, which is believed to represent the average regional background over southern Finland. However, this transport was very different from different geographical regions: from the densely populated areas of East- and Central-Europe it was about 0.8 μg m−3, while in northerly and northwesterly air masses it was only about 0.15 μg m−3. Local traffic was by far the most important local BC source contributing about 63, 54 and 44% on working days, Saturdays and Sundays, respectively. Other local sources were largely masked by the traffic and their contribution was estimated roughly to be of the order of 10%. The rest of BC was attributed to long-range transport.
Keywords: Atmosphere; Traffic; Black carbon; City; Sources; Background;