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The influence of south foehn on the ozone mixing ratios at the high alpine site Arosa
ARTICLE IN PRESS
Atmospheric Environment 39 (2005) 2945–2955 www.elsevier.com/locate/atmosenv
The influence of south foehn on the ozone mixing ratios at the high alpine site Arosa Mike Campanaa, Yingshi Lia, Johannes Staehelina,, Andre S.H. Prevotb, Paolo Bonasonic, Hanspeter Loetscherd, Thomas Petera a Institute for Atmospheric and Climate Sciences, ETH Zurich, CH-8093 Zurich, Switzerland Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, CH-5232 Villigen PSI, Switzerland c Institute of Atmospheric Sciences and Climate, via Godetti 101, I-40129 Bologna, Switzerland d Amt fu¨r Umwelt, Gu¨rtelstrasse 89, CH-7000 Chur, Switzerland
b
Received 19 December 2003; received in revised form 23 December 2004; accepted 6 January 2005
Abstract Within 2 years of trace gas measurements performed at Arosa (Switzerland, 2030 m above sea level), enhanced ozone mixing ratios were observed during south foehn events during summer and spring (5–10 ppb above the median value). The enhancements can be traced back to ozone produced in the strongly industrialized Po basin as confirmed by various analyses. Backward trajectories clearly show advection from this region during foehn. NOy versus O3 correlation and comparison of O3 mixing ratios between Arosa and Mt. Cimone (Italy, 2165 m asl) suggest that ozone is the result of recent photochemical production (+5.6 ppb on average), either directly formed during the transport or via mixing of air processed in the Po basin boundary layer. The absence of a correlation between air parcel residence times over Europe and ozone mixing ratios at Arosa during foehn events is in contrast to a previous analysis, which suggested such correlation without reference to the origin of the air. In the case of south foehn, the continental scale influence of pollutants emission on ozone at Arosa appears to be far less important than the direct influence of the Po basin emissions. In contrast, winter time displays a different situation, with mean ozone reductions of about 4 ppb for air parcels passing the Po basin, probably caused by mixing with ozone-poor air from the Po basin boundary layer. r 2005 Elsevier Ltd. All rights reserved. Keywords: Mountains; Ozone; Case study; Trajectories; Trace gases
1. Introduction South foehn is a typical Alpine meteorological phenomenon characterized by south to north advection, which is caused by a strong pressure gradient across the Corresponding author. Tel.: +41 1 633 2748;
fax: +41 1 633 1058. E-mail address: [email protected] (J. Staehelin).
Alps leading to a descending air stream north of the Alps (Brinkmann, 1971; Hoinka, 1980; Seibert, 1990). South foehn may lead to high wind speed (10 m s1) at measurement stations located on exposed mountain sites. Piringer et al. (2001) described the influence of a strong foehn event on the meteorological structure of the boundary layer at a station located north of the Alps at low altitude and highlighted the typical changes in temperature, relative humidity, pressure and wind, which can be used to detect south foehn events. Seibert
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.01.037
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(1990) showed that foehn air does not necessarily rise from low altitude at the southern edge of the Alps to the crest. Often, air reaching the ground at the northern edge of the Alps originates from about 2000 m altitude in the south. In the southern Alpine valleys close to the Alpine crest, cloudiness usually develops and may be accompanied by precipitation. However, the bulk of the air remains in the south and is usually not part of the foehn flow (Seibert, 1990). Precipitation on the windward side is not a necessary condition for foehn. Trace gas measurements at different sites show that foehn is often accompanied by a specific pattern in trace gas mixing ratios. Already, Go¨tz (1951) measured enhanced ozone mixing ratio in June during south foehn in Arosa. However, the overall chemical conditions at that time differed considerably from today: the mean surface ozone mixing ratio in Arosa was lower by more than a factor of two compared to that in the early 1990s (Staehelin et al., 1994) and photochemical activity in the Po Basin (Thielmann et al., 2002) was not as large as today. Baumann et al. (2001) investigated the ozone mixing ratios at stations located in the Rhine valley north of the Alps on single events during the Mesoscale Alpine Programme. Several foehn-induced ozone episodes were observed at several low altitude stations. Forrer et al. (2000) described the south foehn influence at the remote Alpine site Jungfraujoch (3580 m asl) showing a large variability in trace gas mixing ratios. In some foehn cases, the carbon monoxide (CO) mixing ratios were found to be twice as high as the monthly median value and nitrogen oxides (NOx) could exhibit even larger increases. Depending on the season, the ozone mixing ratio during south foehn at Jungfraujoch usually shows an increase in summer and a decrease in winter when compared to the median value. The climatology presented by Zellweger et al. (2003) confirmed that the station at Jungfraujoch is influenced by transport due to south foehn for about 2% of the time in winter and 6% in summer. Seibert et al. (2000) described one south foehn episode during spring showing ozone mixing ratio in the foehn area north of the Alps higher than at Mt. Cimone (2165 m asl), located south of the Po basin. The difference of 10–20 ppb was attributed to ozone production in the Po basin and in the southern Alpine foothills. In this paper, the influence of the south foehn on the measurements at the high alpine site Arosa is investigated. The potential influence of foehn events is not that obvious for Arosa, because the site is fairly well shielded against a direct inflow from the south due to the surrounding high mountains. This study includes ozone measurements performed at Mt. Cimone, which is located in the northern Apennines at approximately the same altitude as Arosa. During south foehn events Mt. Cimone is upwind of the Po basin. In addition, measurements of this study are
compared with a paper that described a positive correlation between surface ozone mixing ratios of air parcels arriving at Arosa and residence times of these air parcels over Europe (Pochanart et al., 2001). In the following, the term ‘‘foehn’’ designates always ‘‘south foehn’’, i.e. strong advection from south to north.
2. Methods and data 2.1. Trace gas measurements and sites The measurement station in Arosa (Lon. 09.68E, Lat. 46.78N) is located within the eastern part of the Swiss Alps at the end of the Plessur Valley at about 2030 m asl (Fig. 1). The station is located in a relatively wide basin at the upper end of the valley. The basin is surrounded by several mountain peaks reaching 2500–3000 m asl. The measurement site is located a few hundred meters above the village. The city of Chur (Lon. 09.32E, Lat 46.51N), at 500 m asl, with about 40 000 inhabitants and small industries is located at the entrance of the Plessur Valley leading to Arosa. We measured ozone (O3), nitrogen oxides (NOx), total reactive nitrogen (NOy), carbon monoxide (CO) and volatile organic compounds (VOC) at Arosa during 2 years (September 2000–August 2002). For the measurements standard instruments were used. The full description of the measurement instruments and the calibration procedures are contained in Campana (2003). We included ozone data from Mt. Cimone (Italy) obtained from CNR Research Station, Mt. Cimone, from Chur provided by the Cantonal Authority of Grisons and from Jungfraujoch measured within the network NABEL operated by EMPA. Mt. Cimone (Lon. 10.42E, Lat. 44.11N, 2165 m asl) is the highest peak of the northern Apennine, thus at an altitude comparable to Arosa. Similar to observations at the high mountain station Jungfraujoch (3578 m) (Zellweger et al., 2000) or Zugspitze (2960 m) (Lugauer and Winkler 2005), one might expect an influence of locally emitted and vertically transported pollutants during convection or frontal lifting events at Mt. Cimone. Nevertheless ozone measurements at this remote station represent under certain circumstances a good estimate of tropospheric ozone background in southern Europe at this altitude (Bonasoni et al., 2000a) similar to Jungfraujoch, which is often located in the undisturbed free troposphere (Zellweger et al., 2003). 2.2. Meteorological data for foehn identification Meteorological filters were defined in order to delimit periods characterized by strong south advection over the Alps and particularly in the area over Arosa (Table 1).
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Fig. 1. Measurement sites of chemical and meteorological data used in this study. The station at Mt. Cimone, located south west of the Po basin, is shown in Fig. 2.
Table 1 Criteria used to define periods with foehn at Arosa and in Chur
Station Foehn at Arosa Foehn in Chur
Pressure gradient
Wind direction
Wind speed I
Wind speed II
Relative humidity
Number of hours
Lugano—Zurich D42:1 hPa=100 km D42:1 hPa=100 km
Weissfluhjoch [1501–2501] [1501–2501]
Gu¨tsch 410 m/s 410 m/s
Arosa 44 m/s 44 m/s
Chur o65% o45%
— 882 h 486 h
The pressure gradient was calculated from the ANETZ stations Zurich and Lugano using pressure measurements reduced to sea level. The value of 2.1 hPa 100 km typical for foehn was cited in Forrer et al. (2000) and reported in a climatological study on foehn by Hoinka (1980) as a mean pressure gradient for foehn occurrence. Stable synoptic condition with southerly flows is ensured using the wind criterion at the high mountain station Weissfluhjoch. The typical high wind speed for foehn was checked according to Burri et al. (1993) at the well exposed station at Guetsch. The parameter relative humidity was considered, especially to detect strong foehn events reaching the valley floor.
These do not only include meteorological measurements performed at Arosa, but also consider measurements of other stations (Fig. 1). The criteria listed in Table 1 need to be fulfilled simultaneously. We identified 882 h of south foehn in the period between September 2000 and August 2002 (about 5% of the time), which were distributed over the 4 seasons as follows: spring 300 h, winter 210 h, fall 279 h, whereas a minimum was found in summer (103 h). Other than at high-altitude sites,
foehn occurrence in an inner valley is very often characterized by a striking drop in humidity (Piringer et al., 2001). To detect foehn events in Chur relative humidity in Table 1 was set to 45% in accordance with other observations made at locations in the Rhine Valley (Burri et al., 1993). The amount of hours selected dropped to 486 h, thus only 55% of the selected foehn events at Arosa reached simultaneously the valley floor in Chur.
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2.3. Trajectories The three-dimensional 10-days backwards trajectories used in this study were calculated by applying the model LAGRANTO (Wernli and Davies, 1997) to ECMWF wind fields with a horizontal resolution of 11 11 geographical grid covering the whole northern hemisphere and temporal resolution of 6 h. Trajectories were available for every day during the whole measurement campaign. During day time, the local influence due to polluted air masses is generally largest, especially when strong up slope winds develop. To reduce these possible influences trajectories were chosen to arrive at Arosa at 06 UTC at a model altitude of 800 hPa. Besides the trajectory starting at the position of the measurement site four additional trajectories were calculated with a displacement of 70.51 of latitude and longitude from the monitoring site. This procedure was applied in order to assess the reliability of the trajectories arriving at Arosa. For trajectories arriving at Arosa within a foehn period the dispersion of the five trajectories was low, i.e. the trajectories were showing a consistent and distinct flow in the last few days. The presented results of the trajectory analyses always refer to a mean trajectory calculated out of the bundle. Fig. 2 shows trajectories selected during foehn events (registered for the period 76 h relative to their arrival at Arosa), the majority of air parcels crossed the Po basin and none of the trajectories indicates a passage over the Adriatic area. According to the altitude of the trajectories air masses during foehn events did not necessarily originate from low altitude in the boundary layer South of the Alps (o900 hPa). Note that the real downward motion into the Arosa basin cannot be resolved by ECMWF trajectories because the complex topography at the receptor site is only coarsely described by this model. From these trajectories we calculated the residence time of air parcels over the ‘‘polluted’’ European continent that was defined as the latitude region from 351N to 601N and the longitude region from 101W to 301E. Within this region we considered only a passage over the land masses in order to better account for the distribution of emission sources. Thereafter, ozone mixing ratios at Arosa during foehn events were compared with air parcels residence time over Europe. Additionally, trajectories allowed a comparison between Arosa and Mt. Cimone. Ozone measurements at Mt. Cimone were available for the period from September 2000 to December 2001. We averaged ozone mixing ratios at both sites over 3 h and considered matches at the time of the closest passage of the trajectory, i.e. when the trajectory crossed a circular area with a radius of 2.71 around Mt. Cimone.
FOEHN DAYS
49.2N
latitude (deg)
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Mt. Cimone
32.9N 5.7 W
24.2E longitude (deg) FOEHN DAYS pressure
500 600 700 800 900 1000 -50
-40
-30
-20 hours
-10
0
Fig. 2. 10-d backward trajectories arriving at Arosa during foehn events (see Table 1). For clarity only 53 trajectories are depicted, corresponding to about half of the trajectories available within the foehn periods. Left panel: geographical distribution. Right panel: altitude distribution. Shown are only the last 48 hours before arriving at Arosa and only one trajectory per event is depicted. The Mt. Cimone is located in the Apennines (triangle).
3. Results and discussion 3.1. Ozone seasonal variation at Arosa and at Mt. Cimone Fig. 3 shows the seasonal variation of ozone mixing ratios at Arosa and Mt. Cimone. The typical ozone variation at remote sites in central Europe (Scheel et al., 1997) is characterized by a spring maximum, which is still recognizable at Arosa (maximum mean values in April–May). At Mt. Cimone the high spring ozone concentrations are followed by likewise high concentrations in summer. Mean ozone concentrations are clearly higher at Mt. Cimone than at Arosa during the warm season. The location of Mt. Cimone in the south and its close proximity to the Po basin are most probably responsible for the difference to Arosa. Bonasoni et al.
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cold season
cold season
O3 [ppb]
80 60 40 20 0
warm season
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean Mt. Cimone Min Mt. Cimone Max Mt. Cimone
Mean in Arosa Min Arosa Max Arosa
Fig. 3. Seasonal variations in ozone concentrations at Arosa (2035 m, gray symbols) and at Monte Cimone (2165 m, black symbols), from hourly averages of the 2001 data.
(2000a) explained high O3 mixing ratios at Mt. Cimone by the formation of mixing heights over the Po basin that can reach very high altitudes. In the cold season (from October to March) ozone concentrations at both sites become much more similar. Ozone mixing ratios at Mt. Cimone are slightly higher than at Arosa. Bonasoni et al. (2000b) similarly found at Zugspitze, in the Alps (2962 m asl), lower ozone concentrations than at Mt. Cimone in winter and explained the result by events of pollution advection from the northerly sector, which represent for the Alpine region an additional pollution source. During winter an inversion layer frequently forms over the Po basin (Baumann et al., 2001; Seibert et al., 1998) and the Mt. Cimone can be considered as decoupled from the polluted low boundary layer over the Po basin.
3.2. Case study of July 2002 A period with three strong South foehn events occurred in July 2002 as depicted in Fig. 4. These events were interrupted by humid and cold northerly advection. Abrupt changes in the meteorological parameters are typical for the onset and the breakdown of foehn. Ozone mixing ratios at Arosa during this extended foehn event strongly increased at each onset of the foehn. The maxima in ozone mixing ratios during these three events (gray shading) were similar to each other (76, 68 and 70 ppb, respectively) and much higher than the median in July 2002 (50 ppb). The NO and NOx peaks, which are regularly observed in the morning hours during days with weak synoptic flow, are caused by advection of local emissions by
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upslope winds (Campana, 2003). They disappeared during the periods with strong southerly advection (most pronounced for NO in Fig. 4). Conversely, the mixing ratios of CO and NOy did not show a similarly regular pattern as ozone. Nevertheless a clear increase in their mixing ratios was found at the onsets of the first two events. The ratios of NOx/NOy were similar during all the three events, 0.22, 0.18 and 0.25. The ratios of NOy/CO were 0.023, 0.022 and 0.013. This clearly indicates that photochemically well-aged air masses reached Arosa on these particular periods (Olszyna et al., 1994). Zellweger et al. (2003) reports for summer south foehn events at the Jungfraujoch a climatological mean NOx/NOy of 0.25 that agrees well with the ratio calculated for Arosa. The averaged NOy/CO is instead much lower (0.005 at Jungfraujoch) due to the generally lower NOy concentration at this high Alpine site. This result is further related to the much stronger influence of air with free tropospheric origin at Jungfraujoch than at Arosa and the shorter life time of NOy compared to CO. 3.3. Ozone mixing ratio at Arosa during foehn The difference DO3 between ozone mixing ratios during foehn and median values (OFoehn –OMedian ) for 3 3 the entire period September 2000–August 2002 was generally less pronounced than in the strong events in July 2002. The result shows high variability (Fig. 5). In each season, foehn can induce higher as well as lower ozone concentrations than the ozone median value. However, in most cases during summer and spring and to a lesser extent during fall, foehn causes enhanced ozone mixing ratios. Fig. 6 shows the difference DO3 versus the difference DNOy (NOyFoehn–NOyMedian) during the foehn events. In the case of precipitation south of the Alps (during about 332 h out of the 882 h with foehn occurrence), the occurrence of cases with a positive NOy difference strongly decreases. This is most pronounced in spring and summer. This observation reflects, probably, the high solubility of HNO3, i.e. the high efficiency of wet deposition (Munger et al., 1996). This result agrees with previous observation at Sonnblick showing that during advection from the Po basin high concentration of sulfur dioxide are often limited to ‘‘dry events’’ (Seibert et al., 1998). For foehn cases without precipitation south of the Alps in summer the scatter plot in Fig. 6 shows a positive correlation. This observation suggests a recent photochemical production of a part of the ozone transported to Arosa during foehn in spring and summer. This result combined with the trajectories in Fig. 2 provides evidence that photochemical air pollution of the Po basin is a significant source for ozone observed at Arosa during foehn in summer. In spring the
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wind speed Guetsch 270 wind direction Weissfluhjoch
8
180
4
90
0
0
200
8 6
150 100 50
CO NOx
4 2 0
0
O3 [ppb]
NOx [ppb]
360
12
80 70 60 50 40 30 20
O3 Arosa NOy NO
6 5 4 3 2 1 0
NO, NO y [ppb]
-1
wind speed [ms ] CO [ppb]
16
wind direction [˚]
M. Campana et al. / Atmospheric Environment 39 (2005) 2945–2955
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RH [%]
100 75 50
rel. humidity Arosa rel. humidity Chur
360
10 8 6 4 2 0 23.06.02
wind speed Arosa wind direction Arosa
270 180 90 0
27.06.02
01.07.02
05.07.02
09.07.02
13.07.02
wind direction [˚]
wind speed [ms-1]
25
17.07.02
Fig. 4. Time series of 1 h averages of chemical and meteorological measurements at Arosa during three foehn events (gray shading). Relative humidity in Chur and wind at Guetsch and Weissfluhjoch are also shown. Labels on the time axis refer to 00 CET.
30
Chur
Arosa
∆O3 [ppb]
20
10
0
-10 spring
summer
fall
winter
spring
summer
fall
winter
Fig. 5. Boxplot of the difference between ozone mixing ratios during foehn events and the corresponding monthly median at Arosa (left panel) and in Chur (right panel). Boxes indicate the median (central horizontal line) and the 75–25% percentiles, whiskers show the 95–5% range. Symbols: & mean value, ; 99% or (1%) value, — maximum or (minimum) value.
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chemically more active period of the year ozone mixing ratios south of the Po basin were on average lower than at Arosa (Table 2). Out of the selected 23 cases only three showed ozone values higher at Mt. Cimone than in Arosa. The ozone excess in Arosa was on average 5.6 ppb. Considering that during the warm season, ozone mixing ratios are usually higher at Mt. Cimone (Fig. 3), i.e. in strong contrast to the foehn cases, this result becomes even more convincing. During the foehn cases selected air parcels originated mostly from the Tyrrhenian Sea region and had not been in recent
correlation is slightly lower and in winter no correlation is found. This behavior of the correlation coefficients can be linked to the seasonality of photochemical activity in Europe which is much lower in the cold season. In the next step, the ozone values measured at Mt. Cimone during foehn were subtracted from those measured in Arosa in cases when trajectory calculations indicate that both stations were related to each other. This difference can be regarded, in a simple approach, as variation in ozone concentration in the air parcel along the trajectory. During foehn occurrence in the photo30
∆O3 [ppb]
20
10
0
-10
-2
0
Spring
R2=0.24
Summer
R2=0.43
Fall
R2=0.26
Winter
R2=0.00
2 ∆ NOy [ppb]
4
Spring Summer Fall Winter
6
-2
0
2 ∆ NOy [ppb]
4
6
Fig. 6. Variation of O3 and NOy concentrations at Arosa during foehn from the monthly median. Left: only cases without precipitation in Magadino and in Lugano (r ¼ 0 mm). Right: only cases with precipitation in Magadino or in Lugano (r40 mm). The small availability of NOy measurement in fall explains the small number of hours with foehn displayed in this plot. To discriminate events with and without precipitations data at the ANETZ stations Lugano and Magadino hourly averages were used and a time shift of 3 h with respect to the ozone measurements at Arosa was taken into account as a simple estimate of the traveling time between the southern Alpine foothills and Arosa.
Table 2 Averaged O3 mixing ratios and standard deviations south and north of the Po basin (i.e. at Arosa and at Mt. Cimone, respectively) during south foehn (data from September 2000 to December 2001) Season (and residence times t)
O3 Arosa (ppb)
O3 Mt. Cimone (ppb)
Difference in O3 (ppb)
No. foehn cases
Apr–Sep Oct–Mar Apr–Sep Dto12 h Apr–Sep Dt412 h
61.476.7 40.275.5 54.7714.3 65.075.6
55.875.1 44.274.2 50.9713.5 57.574.7
+5.674.7 4.075.1 3.775.4 7.572.8
23 37 12 11
The data are subdivided into a warm season (from April to September) and into a cold season (from October to March). Residence times over the Po basin have been estimated from trajectories between Arosa and Mt. Cimone.
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contact with a plume of a big agglomeration before passing over the Po basin. After having passed the mountainous Apennine region, air parcels were transported over the Po basin towards the Swiss Alpine region. The passage over the Po basin took place rather fast, according to trajectory the mean duration amounted to less than 1 day (17 h713 h). This increase in ozone between both stations can be interpreted either as photochemical production, occurring during the transport, or by mixing with ozone-rich air present over the Po basin. Due to strong solar insolation mixing heights over the Po basin can be rather high. The foehn flow can most probably incorporate pollutants even though the air parcels do not pass close to the surface of the Po basin. Bonasoni et al. (2000b), comparing ozone concentrations at Mt. Cimone to those at Zugspitze, found similar results for air parcels moving southwards. In the period from March to September the enhancement in ozone concentration ranged between 4.6 and 8.0 ppb. A more detailed analysis of these summer foehn events showed that for those 12 cases, in which trajectories were traveling rapidly over the Po basin (residence time between Mt. Cimone and Arosa Dto12 h), the increase was on average lower than in the 12 cases in which air parcels were residing longer over the Po basin (Table 2). This result suggests that the residence time over the Po basin could be an estimate of the exposure time to pollutant emissions in this region. 3.4. Comparison with air parcels residence times over Europe In the last part of Section 2.3, we discussed the possible role played by the residence time of an air parcel over the Po basin. Here we investigate the importance of the longer term history of an air parcel for the ozone mixing ratios. It turned out that this is not an important factor during foehn. Limiting the dataset to foehn events the ozone mixing ratio at Arosa is indeed independent of the total residence time over Europe, i.e. the history including also the time before the crossing of the Po basin. This specific behavior found for foehn events is in contrast with the results of the study presented by Pochanart et al. (2001). From the positive dependence between ozone mixing ratios at Arosa and residence times of air parcels over Europe, Pochanart et al. (2001) postulated an accumulation of regionally produced ozone taking place by increasing residence time. They assumed a ‘‘linear dependence’’ to represent the mean production of ozone due to emissions on the regional to continental scale. The result of the present analysis shows that in case of foehn events the passage over the strongly polluted Po basin is more important for the ozone mixing ratio than the long-term history.
3.5. Influence of foehn events on air quality in Chur Information contained in Section 3.4 and in Table 2 describes the source-receptor relationship on a regional and continental basis and identifies the Po basin as an important contributor to enhanced ozone concentrations at Arosa during foehn events. This may suggest that part of the enhanced ozone concentrations during foehn observed in Chur (Fig. 5, right panel), could also be caused by recent photochemical activity over the Po basin. However, for the understanding of these increases in ozone concentration one should also consider the development of strong temperature inversions, especially during winter and fall, which cause low median ozone mixing ratios at lower altitude. Low concentrations are due to dry deposition and titration with freshly emitted NO. Therefore, to a large extent the increase in ozone mixing ratio during foehn is caused by the strong winds which supply ozone-rich air from above and reduce the effectiveness of deposition and titration on the valley floor. 3.6. Influence of foehn on air pollutant concentrations in winter In comparison to Chur the ozone concentration increase at Arosa during south foehn in winter is much smaller (Fig. 5 left panel). The right panel of Fig. 5 depicts Arosa, Chur and Mt. Cimone in an averaged foehn situation in winter. During periods of persistent temperature inversion in winter Arosa is located in relatively ozone-rich air, well above the pool of cold and ozone-poor air which is building up in the lowest boundary layer (Campana, 2003). The lack of large regional photochemical ozone production in the cold season suggests that during advection of highly polluted air masses, as is the case for air advected from the boundary layer of the Po basin, the ozone concentration drops. At the high Alpine station Jungfraujoch Forrer et al. (2000) found in several episodes during foehn events in winter (1996 and 1997) a relatively strong and negative correlation between DCO and DO3 (D being defined as difference between actual measurement and median value) together with ozone concentration lower than 30 ppb. This was never the case for Arosa measurements (in the two years 2000–2002); low values (o30 ppb) during foehn were reached only once. This may suggest that Jungfraujoch is more sensitive and exposed to polluted air transport from the lower boundary layer over the Po basin than Arosa. However, the interpretation may be more difficult. The Jungfraujoch is in winter most of the time exposed to free tropospheric air (Zellweger et al., 2003) while Arosa is more exposed to transport of ozone poor air from the boundary layer North of the Alps and to local pollution. This fact results in lower ozone mean values at Arosa
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than at Jungfraujoch (Table 3), in other words the changes measured in DO3 are more discernible at Jungfraujoch than at Arosa. A comparison of absolute values of ozone concentrations during foehn did not show a significant difference between Arosa and Jungfraujoch (Table 3). Also minimal ozone mixing ratio measured during foehn did not differ much at these two Alpine sites (in the two years 2000–2002), which does not support the previous idea of the Jungfraujoch being more exposed to the Po basin during foehn than Arosa. The comparison between Arosa and Mt. Cimone for foehn events during the cold season shows that ozone mixing ratios at Arosa during foehn were on average 4.0 ppb lower than at Mt. Cimone, with only five cases out of 37 showing a higher concentration downstream (Table 2). Since both stations are located at the same altitude this difference in ozone can be understood as averaged ozone destruction due to primary pollutants emitted in the Po basin. The chemical signature in the air parcel depends on the mixing between higher altitude air and polluted boundary layer air. In a foehn case monitored in October 1999 Baumann et al. (2001) provided evidence that the foehn flow was crossing the Po basin above a capping inversion located at about 1000 m asl. Below this inversion layer the ozone content was less than 25 ppb. Ozone concentration measured in the foehn flow air above this inversion corresponded in this episode to values measured in the foehn air at Jungfraujoch and north of the Alps. In this case no important interaction took place between foehn flow and the Po basin boundary layer air. However, there are cases when such an interaction takes place. Such a foehn case occurred at the beginning of February 2001 when ozone concentrations at Jungfraujoch abruptly dropped to values of around 15 ppb lower than the median (30 ppb). CO and NOy increased from 100–120 ppb to 300–320 ppb and from 0.1–0.2 ppb to more than 10 ppb, respectively. During this event we measured similar
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ozone and NOy mixing ratios at Arosa as well (35 ppb, 7.5 ppb, respectively).
4. Conclusions Enhanced ozone concentrations during spring and summer foehn events showed that foehn can descend at Arosa when passing the Alps (about 5% of the time). The foehn flow can obviously penetrate into an inner Alpine area such as the surroundings of Arosa, which are normally not well exposed to the synoptic circulation. This is the case for strong advection events. Trajectories based on ECMWF analyses describe the motion from the south during foehn in reasonable accordance with the meteorological parameter used to detect foehn. The measurements in summer and spring together with the specific comparison of ozone mixing ratios upstream and downstream of the Po basin strongly point to transport of recently produced ozone towards and across the Alps. Compared to median values in spring and summer ozone mixing ratios were on average 5–10 ppb higher and the ozone increase in air crossing the Po basin amounted on average 5.6 ppb when using measurements at Arosa and at Mt. Cimone. The comparison between residence time over Europe and ozone mixing ratios at Arosa for south foehn events showed that these two parameters are uncorrelated, contradicting for these special periods the previous general analysis by Pochanart et al. (2001). This is interpreted as a stronger importance of pollutant emissions in the Po basin during foehn as compared to the large scale European influence. In contrast, the residence time over the Po basin was found to be more relevant for ozone mixing ratio at Arosa during summer. Foehn events showing high residence times over the Po basin (Dt412 h) were related on average to the highest ozone mixing ratios at Arosa.
Table 3 Mean ozone mixing ratios at Arosa and at Jungfraujoch in the cold season O3 [ppb]
Foehn at Arosaa Mean at Arosab Min during Foehn Arosac
Foehn at JFJa
Mean at JFJb
Min during Foehn JFJc
January February March October November December
42.4 42.8 49.7 42.0 39.1 39.9
41.8 44.2 51.1 45.8 41.0 42.8
46.6 46.5 51.9 47.6 42.7 45.0
33.1 35.4 34.9 33.0 31.6 35.5
42.2 42.1 48.1 40.0 37.3 39.1
35.0 35.3 33.1 33.5 31.0 34.5
Foehn period at Jungfraujoch was equated to foehn period at Arosa (data from September 2000 to August 2002). a During foehn. b Monthly average. c Minimum ozone mixing ratio during foehn period.
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In the cold season the picture is more complex than during the summer. In winter, although meteorological measurements reveal foehn at Arosa, the ozone mixing ratio does not indicate a clear pattern when comparing to all other situations. This observation is in contrast to results found at Jungfraujoch and is due to the comparably stronger exposition of Arosa to the boundary layer north of the Alps. In winter, comparison between ozone mixing ratios at Arosa and at Mt. Cimone allows to estimate an averaged ozone reduction of 4 ppb during the passage over the Po basin. The formation of a persistent inversion layer over the Po basin suggests that the interaction between foehn flow and the boundary layer over the Po basin, i.e. the stability of the atmosphere in the Po basin, are relevant for understanding of the trace gas behavior at Alpine sites during foehn events.
Acknowledgments This work was supported by research grants from ETHZ and from the Swiss National Science Foundation. We particularly thank MeteoSwiss for the access to the infrastructure at Arosa (Tschuggen) and for providing meteorological data. We also thank the staff at the CNR research station at Mt. Cimone, the Cantonal Authority of Grison in Chur, as well as the Swiss Agency for Environment in Bern for providing trace gas measurement data used in this study.
References Baumann, K., Maurer, H., Rau, G., Piringer, M., Pechinger, U., Prevot, A., Furger, M., Neininger, B., Pellegrini, U., 2001. The influence of south Foehn on the ozone distribution in the Alpine Rhine valley—results from the MAP field phase. Atmospheric Environment 35, 6379–6390. Bonasoni, P., Scheel, H.E., Stohl, A., Trickl, T., Colombo, T., 2000b. Ozone transport between Apennines and Alps: influence of Po-basin-polluted air masses. In: NASDA (Ed.), XIX Quadrennial Ozone Symposium, Sapporo, pp.319–320. Bonasoni, P., Stohl, A., Cristofanelli, P., Calzolari, F., Colombo, T., Evangelisti, F., 2000a. Background ozone variations at Mt. Cimone station. Atmospheric Environment 34, 5183–5189. Brinkmann, W.A.R., 1971. What is foehn? Weather 26, 230–239. Burri, K., Ha¨chler, P., Schu¨epp, M., Werner, R., 1993. Arbeitsberichte: Der Foehnfall von. MeteoSchweiz, Zurich, pp. 85. Campana, M., 2003. Variability of trace gases at the Alpine site Arosa in relation to meteorological processes. Ph.D. Thesis. Natural Sciences Departement, ETH Zurich, pp. 135.
Forrer, J., Ruttimann, R., Schneiter, D., Fischer, A., Buchmann, B., Hofer, P., 2000. Variability of trace gases at the high-Alpine site Jungfraujoch caused by meteorological transport processes. Journal of Geophysical ResearchAtmospheres 105, 12241–12251. Go¨tz, F.W.P., 1951. Aroser Messungen des Ozongehalts der unteren Tropospha¨re und sein Jahresgang. Zeitschrift Naturforschung 6a, 634–639. Hoinka, K.P., 1980. Synoptic scale atmospheric features and foehn. Contributions Atmospheric Physics 53, 485–507. Lugauer, M., Winkler, P., 2005. Thermal circulation in South Bavaria–climatology and synoptic aspects. Meteorologische Zeitschrift 14, 15–30. Munger, J.W., Wofsy, S.C., Bakwin, P.S., Fan, S.M., Goulden, M.L., Daube, B.C., Goldstein, A.H., Moore, K.E., Fitzjarrald, D.R., 1996. Atmospheric deposition of reactive nitrogen oxides and ozone in a temperate deciduous forest and a subarctic woodland. 1. Measurements and mechanisms. Journal of Geophysical Research-Atmospheres 101, 12639–12657. Olszyna, K.J., Bailey, E.M., Simonaitis, R., Meagher, J.F., 1994. O3 and NOy relationships at a rural site. Journal of Geophysical Research-Atmospheres 99, 14557–14563. Piringer, M., Baumann, K., Pechinger, U., Vogt, S., 2001. Meteorological and ozone sounding experience during a strong foehn event—a MAP case study. Meteorologische Zeitschrift 10, 445–455. Pochanart, P., Akimoto, H., Maksyutov, S., Staehelin, J., 2001. Surface ozone at the Swiss Alpine site Arosa: the hemispheric background and the influence of large-scale anthropogenic emissions. Atmospheric Environment 35, 5553–5566. Scheel, H.E., Areskoug, H., Geiss, H., Gomiscek, B., Granby, K., Haszpra, L., Klasinc, L., Kley, D., Laurila, T., Lindskog, A., Roemer, M., Schmitt, R., Simmonds, P., Solberg, S., Toupance, G., 1997. On the spatial distribution and seasonal variation of lower-troposphere ozone over Europe. Journal of Atmospheric Chemistry 28, 11–28. Seibert, P., 1990. South foehn studies since the Alpex experiment. Meteorology and Atmospheric Physics 43, 91–103. Seibert, P., Kromp-Kolb, H., Kasper, A., Kalina, M., Puxbaum, H., Jost, D.T., Schwikowski, M., Baltensperger, U., 1998. Transport of polluted boundary layer air from the Po Valley to high-alpine sites. Atmospheric Environment 32, 3953–3965. Seibert, P., Feldmann, H., Neininger, B., Baumle, M., Trickl, T., 2000. South foehn and ozone in the Eastern Alps—case study and climatological aspects. Atmospheric Environment 34, 1379–1394. Staehelin, J., Thudium, J., Buehler, R., Volz-Thomas, A., Graber, W., 1994. Trends in surface ozone concentrations at Arosa (Switzerland). Atmospheric Environment 28, 75–87. Thielmann, A., Prevot, A.S.H., Staehelin, J., 2002. Sensitivity of ozone production derived from field measurements in the Italian Po basin. Journal of Geophysical Research-Atmospheres 107 Article No.-8194. Wernli, H., Davies, H.C., 1997. A Lagrangian-based analysis of extratropical cyclones. 1. The method and some applications. Quarterly Journal of the Royal Meteorological Society 123, 467–489.
ARTICLE IN PRESS M. Campana et al. / Atmospheric Environment 39 (2005) 2945–2955 Zellweger, C., Ammann, M., Buchmann, B., Hofer, P., Lugauer, M., Ruttimann, R., Streit, N., Weingartner, E., Baltensperger, U., 2000. Summertime NOy speciation at the Jungfraujoch, 3580 m above sea level. Switzerland. Journal of Geophysical Research-Atmospheres 105, 6655–6667.
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Zellweger, C., Forrer, J., Hofer, P., Nyeki, S., Schwarzenbach, B., Weingartner, E., Ammann, M., Baltensperger, U., 2003. Partitioning of reactive nitrogen (NOy) and dependence on meteorological conditions in the lower free troposphere. Atmospheric Chemistry and Physics 3, 779–796.
Atmospheric Environment 39 (2005) 2945–2955 www.elsevier.com/locate/atmosenv
The influence of south foehn on the ozone mixing ratios at the high alpine site Arosa Mike Campanaa, Yingshi Lia, Johannes Staehelina,, Andre S.H. Prevotb, Paolo Bonasonic, Hanspeter Loetscherd, Thomas Petera a Institute for Atmospheric and Climate Sciences, ETH Zurich, CH-8093 Zurich, Switzerland Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, CH-5232 Villigen PSI, Switzerland c Institute of Atmospheric Sciences and Climate, via Godetti 101, I-40129 Bologna, Switzerland d Amt fu¨r Umwelt, Gu¨rtelstrasse 89, CH-7000 Chur, Switzerland
b
Received 19 December 2003; received in revised form 23 December 2004; accepted 6 January 2005
Abstract Within 2 years of trace gas measurements performed at Arosa (Switzerland, 2030 m above sea level), enhanced ozone mixing ratios were observed during south foehn events during summer and spring (5–10 ppb above the median value). The enhancements can be traced back to ozone produced in the strongly industrialized Po basin as confirmed by various analyses. Backward trajectories clearly show advection from this region during foehn. NOy versus O3 correlation and comparison of O3 mixing ratios between Arosa and Mt. Cimone (Italy, 2165 m asl) suggest that ozone is the result of recent photochemical production (+5.6 ppb on average), either directly formed during the transport or via mixing of air processed in the Po basin boundary layer. The absence of a correlation between air parcel residence times over Europe and ozone mixing ratios at Arosa during foehn events is in contrast to a previous analysis, which suggested such correlation without reference to the origin of the air. In the case of south foehn, the continental scale influence of pollutants emission on ozone at Arosa appears to be far less important than the direct influence of the Po basin emissions. In contrast, winter time displays a different situation, with mean ozone reductions of about 4 ppb for air parcels passing the Po basin, probably caused by mixing with ozone-poor air from the Po basin boundary layer. r 2005 Elsevier Ltd. All rights reserved. Keywords: Mountains; Ozone; Case study; Trajectories; Trace gases
1. Introduction South foehn is a typical Alpine meteorological phenomenon characterized by south to north advection, which is caused by a strong pressure gradient across the Corresponding author. Tel.: +41 1 633 2748;
fax: +41 1 633 1058. E-mail address: [email protected] (J. Staehelin).
Alps leading to a descending air stream north of the Alps (Brinkmann, 1971; Hoinka, 1980; Seibert, 1990). South foehn may lead to high wind speed (10 m s1) at measurement stations located on exposed mountain sites. Piringer et al. (2001) described the influence of a strong foehn event on the meteorological structure of the boundary layer at a station located north of the Alps at low altitude and highlighted the typical changes in temperature, relative humidity, pressure and wind, which can be used to detect south foehn events. Seibert
1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.01.037
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(1990) showed that foehn air does not necessarily rise from low altitude at the southern edge of the Alps to the crest. Often, air reaching the ground at the northern edge of the Alps originates from about 2000 m altitude in the south. In the southern Alpine valleys close to the Alpine crest, cloudiness usually develops and may be accompanied by precipitation. However, the bulk of the air remains in the south and is usually not part of the foehn flow (Seibert, 1990). Precipitation on the windward side is not a necessary condition for foehn. Trace gas measurements at different sites show that foehn is often accompanied by a specific pattern in trace gas mixing ratios. Already, Go¨tz (1951) measured enhanced ozone mixing ratio in June during south foehn in Arosa. However, the overall chemical conditions at that time differed considerably from today: the mean surface ozone mixing ratio in Arosa was lower by more than a factor of two compared to that in the early 1990s (Staehelin et al., 1994) and photochemical activity in the Po Basin (Thielmann et al., 2002) was not as large as today. Baumann et al. (2001) investigated the ozone mixing ratios at stations located in the Rhine valley north of the Alps on single events during the Mesoscale Alpine Programme. Several foehn-induced ozone episodes were observed at several low altitude stations. Forrer et al. (2000) described the south foehn influence at the remote Alpine site Jungfraujoch (3580 m asl) showing a large variability in trace gas mixing ratios. In some foehn cases, the carbon monoxide (CO) mixing ratios were found to be twice as high as the monthly median value and nitrogen oxides (NOx) could exhibit even larger increases. Depending on the season, the ozone mixing ratio during south foehn at Jungfraujoch usually shows an increase in summer and a decrease in winter when compared to the median value. The climatology presented by Zellweger et al. (2003) confirmed that the station at Jungfraujoch is influenced by transport due to south foehn for about 2% of the time in winter and 6% in summer. Seibert et al. (2000) described one south foehn episode during spring showing ozone mixing ratio in the foehn area north of the Alps higher than at Mt. Cimone (2165 m asl), located south of the Po basin. The difference of 10–20 ppb was attributed to ozone production in the Po basin and in the southern Alpine foothills. In this paper, the influence of the south foehn on the measurements at the high alpine site Arosa is investigated. The potential influence of foehn events is not that obvious for Arosa, because the site is fairly well shielded against a direct inflow from the south due to the surrounding high mountains. This study includes ozone measurements performed at Mt. Cimone, which is located in the northern Apennines at approximately the same altitude as Arosa. During south foehn events Mt. Cimone is upwind of the Po basin. In addition, measurements of this study are
compared with a paper that described a positive correlation between surface ozone mixing ratios of air parcels arriving at Arosa and residence times of these air parcels over Europe (Pochanart et al., 2001). In the following, the term ‘‘foehn’’ designates always ‘‘south foehn’’, i.e. strong advection from south to north.
2. Methods and data 2.1. Trace gas measurements and sites The measurement station in Arosa (Lon. 09.68E, Lat. 46.78N) is located within the eastern part of the Swiss Alps at the end of the Plessur Valley at about 2030 m asl (Fig. 1). The station is located in a relatively wide basin at the upper end of the valley. The basin is surrounded by several mountain peaks reaching 2500–3000 m asl. The measurement site is located a few hundred meters above the village. The city of Chur (Lon. 09.32E, Lat 46.51N), at 500 m asl, with about 40 000 inhabitants and small industries is located at the entrance of the Plessur Valley leading to Arosa. We measured ozone (O3), nitrogen oxides (NOx), total reactive nitrogen (NOy), carbon monoxide (CO) and volatile organic compounds (VOC) at Arosa during 2 years (September 2000–August 2002). For the measurements standard instruments were used. The full description of the measurement instruments and the calibration procedures are contained in Campana (2003). We included ozone data from Mt. Cimone (Italy) obtained from CNR Research Station, Mt. Cimone, from Chur provided by the Cantonal Authority of Grisons and from Jungfraujoch measured within the network NABEL operated by EMPA. Mt. Cimone (Lon. 10.42E, Lat. 44.11N, 2165 m asl) is the highest peak of the northern Apennine, thus at an altitude comparable to Arosa. Similar to observations at the high mountain station Jungfraujoch (3578 m) (Zellweger et al., 2000) or Zugspitze (2960 m) (Lugauer and Winkler 2005), one might expect an influence of locally emitted and vertically transported pollutants during convection or frontal lifting events at Mt. Cimone. Nevertheless ozone measurements at this remote station represent under certain circumstances a good estimate of tropospheric ozone background in southern Europe at this altitude (Bonasoni et al., 2000a) similar to Jungfraujoch, which is often located in the undisturbed free troposphere (Zellweger et al., 2003). 2.2. Meteorological data for foehn identification Meteorological filters were defined in order to delimit periods characterized by strong south advection over the Alps and particularly in the area over Arosa (Table 1).
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Fig. 1. Measurement sites of chemical and meteorological data used in this study. The station at Mt. Cimone, located south west of the Po basin, is shown in Fig. 2.
Table 1 Criteria used to define periods with foehn at Arosa and in Chur
Station Foehn at Arosa Foehn in Chur
Pressure gradient
Wind direction
Wind speed I
Wind speed II
Relative humidity
Number of hours
Lugano—Zurich D42:1 hPa=100 km D42:1 hPa=100 km
Weissfluhjoch [1501–2501] [1501–2501]
Gu¨tsch 410 m/s 410 m/s
Arosa 44 m/s 44 m/s
Chur o65% o45%
— 882 h 486 h
The pressure gradient was calculated from the ANETZ stations Zurich and Lugano using pressure measurements reduced to sea level. The value of 2.1 hPa 100 km typical for foehn was cited in Forrer et al. (2000) and reported in a climatological study on foehn by Hoinka (1980) as a mean pressure gradient for foehn occurrence. Stable synoptic condition with southerly flows is ensured using the wind criterion at the high mountain station Weissfluhjoch. The typical high wind speed for foehn was checked according to Burri et al. (1993) at the well exposed station at Guetsch. The parameter relative humidity was considered, especially to detect strong foehn events reaching the valley floor.
These do not only include meteorological measurements performed at Arosa, but also consider measurements of other stations (Fig. 1). The criteria listed in Table 1 need to be fulfilled simultaneously. We identified 882 h of south foehn in the period between September 2000 and August 2002 (about 5% of the time), which were distributed over the 4 seasons as follows: spring 300 h, winter 210 h, fall 279 h, whereas a minimum was found in summer (103 h). Other than at high-altitude sites,
foehn occurrence in an inner valley is very often characterized by a striking drop in humidity (Piringer et al., 2001). To detect foehn events in Chur relative humidity in Table 1 was set to 45% in accordance with other observations made at locations in the Rhine Valley (Burri et al., 1993). The amount of hours selected dropped to 486 h, thus only 55% of the selected foehn events at Arosa reached simultaneously the valley floor in Chur.
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2.3. Trajectories The three-dimensional 10-days backwards trajectories used in this study were calculated by applying the model LAGRANTO (Wernli and Davies, 1997) to ECMWF wind fields with a horizontal resolution of 11 11 geographical grid covering the whole northern hemisphere and temporal resolution of 6 h. Trajectories were available for every day during the whole measurement campaign. During day time, the local influence due to polluted air masses is generally largest, especially when strong up slope winds develop. To reduce these possible influences trajectories were chosen to arrive at Arosa at 06 UTC at a model altitude of 800 hPa. Besides the trajectory starting at the position of the measurement site four additional trajectories were calculated with a displacement of 70.51 of latitude and longitude from the monitoring site. This procedure was applied in order to assess the reliability of the trajectories arriving at Arosa. For trajectories arriving at Arosa within a foehn period the dispersion of the five trajectories was low, i.e. the trajectories were showing a consistent and distinct flow in the last few days. The presented results of the trajectory analyses always refer to a mean trajectory calculated out of the bundle. Fig. 2 shows trajectories selected during foehn events (registered for the period 76 h relative to their arrival at Arosa), the majority of air parcels crossed the Po basin and none of the trajectories indicates a passage over the Adriatic area. According to the altitude of the trajectories air masses during foehn events did not necessarily originate from low altitude in the boundary layer South of the Alps (o900 hPa). Note that the real downward motion into the Arosa basin cannot be resolved by ECMWF trajectories because the complex topography at the receptor site is only coarsely described by this model. From these trajectories we calculated the residence time of air parcels over the ‘‘polluted’’ European continent that was defined as the latitude region from 351N to 601N and the longitude region from 101W to 301E. Within this region we considered only a passage over the land masses in order to better account for the distribution of emission sources. Thereafter, ozone mixing ratios at Arosa during foehn events were compared with air parcels residence time over Europe. Additionally, trajectories allowed a comparison between Arosa and Mt. Cimone. Ozone measurements at Mt. Cimone were available for the period from September 2000 to December 2001. We averaged ozone mixing ratios at both sites over 3 h and considered matches at the time of the closest passage of the trajectory, i.e. when the trajectory crossed a circular area with a radius of 2.71 around Mt. Cimone.
FOEHN DAYS
49.2N
latitude (deg)
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Mt. Cimone
32.9N 5.7 W
24.2E longitude (deg) FOEHN DAYS pressure
500 600 700 800 900 1000 -50
-40
-30
-20 hours
-10
0
Fig. 2. 10-d backward trajectories arriving at Arosa during foehn events (see Table 1). For clarity only 53 trajectories are depicted, corresponding to about half of the trajectories available within the foehn periods. Left panel: geographical distribution. Right panel: altitude distribution. Shown are only the last 48 hours before arriving at Arosa and only one trajectory per event is depicted. The Mt. Cimone is located in the Apennines (triangle).
3. Results and discussion 3.1. Ozone seasonal variation at Arosa and at Mt. Cimone Fig. 3 shows the seasonal variation of ozone mixing ratios at Arosa and Mt. Cimone. The typical ozone variation at remote sites in central Europe (Scheel et al., 1997) is characterized by a spring maximum, which is still recognizable at Arosa (maximum mean values in April–May). At Mt. Cimone the high spring ozone concentrations are followed by likewise high concentrations in summer. Mean ozone concentrations are clearly higher at Mt. Cimone than at Arosa during the warm season. The location of Mt. Cimone in the south and its close proximity to the Po basin are most probably responsible for the difference to Arosa. Bonasoni et al.
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cold season
cold season
O3 [ppb]
80 60 40 20 0
warm season
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean Mt. Cimone Min Mt. Cimone Max Mt. Cimone
Mean in Arosa Min Arosa Max Arosa
Fig. 3. Seasonal variations in ozone concentrations at Arosa (2035 m, gray symbols) and at Monte Cimone (2165 m, black symbols), from hourly averages of the 2001 data.
(2000a) explained high O3 mixing ratios at Mt. Cimone by the formation of mixing heights over the Po basin that can reach very high altitudes. In the cold season (from October to March) ozone concentrations at both sites become much more similar. Ozone mixing ratios at Mt. Cimone are slightly higher than at Arosa. Bonasoni et al. (2000b) similarly found at Zugspitze, in the Alps (2962 m asl), lower ozone concentrations than at Mt. Cimone in winter and explained the result by events of pollution advection from the northerly sector, which represent for the Alpine region an additional pollution source. During winter an inversion layer frequently forms over the Po basin (Baumann et al., 2001; Seibert et al., 1998) and the Mt. Cimone can be considered as decoupled from the polluted low boundary layer over the Po basin.
3.2. Case study of July 2002 A period with three strong South foehn events occurred in July 2002 as depicted in Fig. 4. These events were interrupted by humid and cold northerly advection. Abrupt changes in the meteorological parameters are typical for the onset and the breakdown of foehn. Ozone mixing ratios at Arosa during this extended foehn event strongly increased at each onset of the foehn. The maxima in ozone mixing ratios during these three events (gray shading) were similar to each other (76, 68 and 70 ppb, respectively) and much higher than the median in July 2002 (50 ppb). The NO and NOx peaks, which are regularly observed in the morning hours during days with weak synoptic flow, are caused by advection of local emissions by
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upslope winds (Campana, 2003). They disappeared during the periods with strong southerly advection (most pronounced for NO in Fig. 4). Conversely, the mixing ratios of CO and NOy did not show a similarly regular pattern as ozone. Nevertheless a clear increase in their mixing ratios was found at the onsets of the first two events. The ratios of NOx/NOy were similar during all the three events, 0.22, 0.18 and 0.25. The ratios of NOy/CO were 0.023, 0.022 and 0.013. This clearly indicates that photochemically well-aged air masses reached Arosa on these particular periods (Olszyna et al., 1994). Zellweger et al. (2003) reports for summer south foehn events at the Jungfraujoch a climatological mean NOx/NOy of 0.25 that agrees well with the ratio calculated for Arosa. The averaged NOy/CO is instead much lower (0.005 at Jungfraujoch) due to the generally lower NOy concentration at this high Alpine site. This result is further related to the much stronger influence of air with free tropospheric origin at Jungfraujoch than at Arosa and the shorter life time of NOy compared to CO. 3.3. Ozone mixing ratio at Arosa during foehn The difference DO3 between ozone mixing ratios during foehn and median values (OFoehn –OMedian ) for 3 3 the entire period September 2000–August 2002 was generally less pronounced than in the strong events in July 2002. The result shows high variability (Fig. 5). In each season, foehn can induce higher as well as lower ozone concentrations than the ozone median value. However, in most cases during summer and spring and to a lesser extent during fall, foehn causes enhanced ozone mixing ratios. Fig. 6 shows the difference DO3 versus the difference DNOy (NOyFoehn–NOyMedian) during the foehn events. In the case of precipitation south of the Alps (during about 332 h out of the 882 h with foehn occurrence), the occurrence of cases with a positive NOy difference strongly decreases. This is most pronounced in spring and summer. This observation reflects, probably, the high solubility of HNO3, i.e. the high efficiency of wet deposition (Munger et al., 1996). This result agrees with previous observation at Sonnblick showing that during advection from the Po basin high concentration of sulfur dioxide are often limited to ‘‘dry events’’ (Seibert et al., 1998). For foehn cases without precipitation south of the Alps in summer the scatter plot in Fig. 6 shows a positive correlation. This observation suggests a recent photochemical production of a part of the ozone transported to Arosa during foehn in spring and summer. This result combined with the trajectories in Fig. 2 provides evidence that photochemical air pollution of the Po basin is a significant source for ozone observed at Arosa during foehn in summer. In spring the
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wind speed Guetsch 270 wind direction Weissfluhjoch
8
180
4
90
0
0
200
8 6
150 100 50
CO NOx
4 2 0
0
O3 [ppb]
NOx [ppb]
360
12
80 70 60 50 40 30 20
O3 Arosa NOy NO
6 5 4 3 2 1 0
NO, NO y [ppb]
-1
wind speed [ms ] CO [ppb]
16
wind direction [˚]
M. Campana et al. / Atmospheric Environment 39 (2005) 2945–2955
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RH [%]
100 75 50
rel. humidity Arosa rel. humidity Chur
360
10 8 6 4 2 0 23.06.02
wind speed Arosa wind direction Arosa
270 180 90 0
27.06.02
01.07.02
05.07.02
09.07.02
13.07.02
wind direction [˚]
wind speed [ms-1]
25
17.07.02
Fig. 4. Time series of 1 h averages of chemical and meteorological measurements at Arosa during three foehn events (gray shading). Relative humidity in Chur and wind at Guetsch and Weissfluhjoch are also shown. Labels on the time axis refer to 00 CET.
30
Chur
Arosa
∆O3 [ppb]
20
10
0
-10 spring
summer
fall
winter
spring
summer
fall
winter
Fig. 5. Boxplot of the difference between ozone mixing ratios during foehn events and the corresponding monthly median at Arosa (left panel) and in Chur (right panel). Boxes indicate the median (central horizontal line) and the 75–25% percentiles, whiskers show the 95–5% range. Symbols: & mean value, ; 99% or (1%) value, — maximum or (minimum) value.
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chemically more active period of the year ozone mixing ratios south of the Po basin were on average lower than at Arosa (Table 2). Out of the selected 23 cases only three showed ozone values higher at Mt. Cimone than in Arosa. The ozone excess in Arosa was on average 5.6 ppb. Considering that during the warm season, ozone mixing ratios are usually higher at Mt. Cimone (Fig. 3), i.e. in strong contrast to the foehn cases, this result becomes even more convincing. During the foehn cases selected air parcels originated mostly from the Tyrrhenian Sea region and had not been in recent
correlation is slightly lower and in winter no correlation is found. This behavior of the correlation coefficients can be linked to the seasonality of photochemical activity in Europe which is much lower in the cold season. In the next step, the ozone values measured at Mt. Cimone during foehn were subtracted from those measured in Arosa in cases when trajectory calculations indicate that both stations were related to each other. This difference can be regarded, in a simple approach, as variation in ozone concentration in the air parcel along the trajectory. During foehn occurrence in the photo30
∆O3 [ppb]
20
10
0
-10
-2
0
Spring
R2=0.24
Summer
R2=0.43
Fall
R2=0.26
Winter
R2=0.00
2 ∆ NOy [ppb]
4
Spring Summer Fall Winter
6
-2
0
2 ∆ NOy [ppb]
4
6
Fig. 6. Variation of O3 and NOy concentrations at Arosa during foehn from the monthly median. Left: only cases without precipitation in Magadino and in Lugano (r ¼ 0 mm). Right: only cases with precipitation in Magadino or in Lugano (r40 mm). The small availability of NOy measurement in fall explains the small number of hours with foehn displayed in this plot. To discriminate events with and without precipitations data at the ANETZ stations Lugano and Magadino hourly averages were used and a time shift of 3 h with respect to the ozone measurements at Arosa was taken into account as a simple estimate of the traveling time between the southern Alpine foothills and Arosa.
Table 2 Averaged O3 mixing ratios and standard deviations south and north of the Po basin (i.e. at Arosa and at Mt. Cimone, respectively) during south foehn (data from September 2000 to December 2001) Season (and residence times t)
O3 Arosa (ppb)
O3 Mt. Cimone (ppb)
Difference in O3 (ppb)
No. foehn cases
Apr–Sep Oct–Mar Apr–Sep Dto12 h Apr–Sep Dt412 h
61.476.7 40.275.5 54.7714.3 65.075.6
55.875.1 44.274.2 50.9713.5 57.574.7
+5.674.7 4.075.1 3.775.4 7.572.8
23 37 12 11
The data are subdivided into a warm season (from April to September) and into a cold season (from October to March). Residence times over the Po basin have been estimated from trajectories between Arosa and Mt. Cimone.
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contact with a plume of a big agglomeration before passing over the Po basin. After having passed the mountainous Apennine region, air parcels were transported over the Po basin towards the Swiss Alpine region. The passage over the Po basin took place rather fast, according to trajectory the mean duration amounted to less than 1 day (17 h713 h). This increase in ozone between both stations can be interpreted either as photochemical production, occurring during the transport, or by mixing with ozone-rich air present over the Po basin. Due to strong solar insolation mixing heights over the Po basin can be rather high. The foehn flow can most probably incorporate pollutants even though the air parcels do not pass close to the surface of the Po basin. Bonasoni et al. (2000b), comparing ozone concentrations at Mt. Cimone to those at Zugspitze, found similar results for air parcels moving southwards. In the period from March to September the enhancement in ozone concentration ranged between 4.6 and 8.0 ppb. A more detailed analysis of these summer foehn events showed that for those 12 cases, in which trajectories were traveling rapidly over the Po basin (residence time between Mt. Cimone and Arosa Dto12 h), the increase was on average lower than in the 12 cases in which air parcels were residing longer over the Po basin (Table 2). This result suggests that the residence time over the Po basin could be an estimate of the exposure time to pollutant emissions in this region. 3.4. Comparison with air parcels residence times over Europe In the last part of Section 2.3, we discussed the possible role played by the residence time of an air parcel over the Po basin. Here we investigate the importance of the longer term history of an air parcel for the ozone mixing ratios. It turned out that this is not an important factor during foehn. Limiting the dataset to foehn events the ozone mixing ratio at Arosa is indeed independent of the total residence time over Europe, i.e. the history including also the time before the crossing of the Po basin. This specific behavior found for foehn events is in contrast with the results of the study presented by Pochanart et al. (2001). From the positive dependence between ozone mixing ratios at Arosa and residence times of air parcels over Europe, Pochanart et al. (2001) postulated an accumulation of regionally produced ozone taking place by increasing residence time. They assumed a ‘‘linear dependence’’ to represent the mean production of ozone due to emissions on the regional to continental scale. The result of the present analysis shows that in case of foehn events the passage over the strongly polluted Po basin is more important for the ozone mixing ratio than the long-term history.
3.5. Influence of foehn events on air quality in Chur Information contained in Section 3.4 and in Table 2 describes the source-receptor relationship on a regional and continental basis and identifies the Po basin as an important contributor to enhanced ozone concentrations at Arosa during foehn events. This may suggest that part of the enhanced ozone concentrations during foehn observed in Chur (Fig. 5, right panel), could also be caused by recent photochemical activity over the Po basin. However, for the understanding of these increases in ozone concentration one should also consider the development of strong temperature inversions, especially during winter and fall, which cause low median ozone mixing ratios at lower altitude. Low concentrations are due to dry deposition and titration with freshly emitted NO. Therefore, to a large extent the increase in ozone mixing ratio during foehn is caused by the strong winds which supply ozone-rich air from above and reduce the effectiveness of deposition and titration on the valley floor. 3.6. Influence of foehn on air pollutant concentrations in winter In comparison to Chur the ozone concentration increase at Arosa during south foehn in winter is much smaller (Fig. 5 left panel). The right panel of Fig. 5 depicts Arosa, Chur and Mt. Cimone in an averaged foehn situation in winter. During periods of persistent temperature inversion in winter Arosa is located in relatively ozone-rich air, well above the pool of cold and ozone-poor air which is building up in the lowest boundary layer (Campana, 2003). The lack of large regional photochemical ozone production in the cold season suggests that during advection of highly polluted air masses, as is the case for air advected from the boundary layer of the Po basin, the ozone concentration drops. At the high Alpine station Jungfraujoch Forrer et al. (2000) found in several episodes during foehn events in winter (1996 and 1997) a relatively strong and negative correlation between DCO and DO3 (D being defined as difference between actual measurement and median value) together with ozone concentration lower than 30 ppb. This was never the case for Arosa measurements (in the two years 2000–2002); low values (o30 ppb) during foehn were reached only once. This may suggest that Jungfraujoch is more sensitive and exposed to polluted air transport from the lower boundary layer over the Po basin than Arosa. However, the interpretation may be more difficult. The Jungfraujoch is in winter most of the time exposed to free tropospheric air (Zellweger et al., 2003) while Arosa is more exposed to transport of ozone poor air from the boundary layer North of the Alps and to local pollution. This fact results in lower ozone mean values at Arosa
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than at Jungfraujoch (Table 3), in other words the changes measured in DO3 are more discernible at Jungfraujoch than at Arosa. A comparison of absolute values of ozone concentrations during foehn did not show a significant difference between Arosa and Jungfraujoch (Table 3). Also minimal ozone mixing ratio measured during foehn did not differ much at these two Alpine sites (in the two years 2000–2002), which does not support the previous idea of the Jungfraujoch being more exposed to the Po basin during foehn than Arosa. The comparison between Arosa and Mt. Cimone for foehn events during the cold season shows that ozone mixing ratios at Arosa during foehn were on average 4.0 ppb lower than at Mt. Cimone, with only five cases out of 37 showing a higher concentration downstream (Table 2). Since both stations are located at the same altitude this difference in ozone can be understood as averaged ozone destruction due to primary pollutants emitted in the Po basin. The chemical signature in the air parcel depends on the mixing between higher altitude air and polluted boundary layer air. In a foehn case monitored in October 1999 Baumann et al. (2001) provided evidence that the foehn flow was crossing the Po basin above a capping inversion located at about 1000 m asl. Below this inversion layer the ozone content was less than 25 ppb. Ozone concentration measured in the foehn flow air above this inversion corresponded in this episode to values measured in the foehn air at Jungfraujoch and north of the Alps. In this case no important interaction took place between foehn flow and the Po basin boundary layer air. However, there are cases when such an interaction takes place. Such a foehn case occurred at the beginning of February 2001 when ozone concentrations at Jungfraujoch abruptly dropped to values of around 15 ppb lower than the median (30 ppb). CO and NOy increased from 100–120 ppb to 300–320 ppb and from 0.1–0.2 ppb to more than 10 ppb, respectively. During this event we measured similar
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ozone and NOy mixing ratios at Arosa as well (35 ppb, 7.5 ppb, respectively).
4. Conclusions Enhanced ozone concentrations during spring and summer foehn events showed that foehn can descend at Arosa when passing the Alps (about 5% of the time). The foehn flow can obviously penetrate into an inner Alpine area such as the surroundings of Arosa, which are normally not well exposed to the synoptic circulation. This is the case for strong advection events. Trajectories based on ECMWF analyses describe the motion from the south during foehn in reasonable accordance with the meteorological parameter used to detect foehn. The measurements in summer and spring together with the specific comparison of ozone mixing ratios upstream and downstream of the Po basin strongly point to transport of recently produced ozone towards and across the Alps. Compared to median values in spring and summer ozone mixing ratios were on average 5–10 ppb higher and the ozone increase in air crossing the Po basin amounted on average 5.6 ppb when using measurements at Arosa and at Mt. Cimone. The comparison between residence time over Europe and ozone mixing ratios at Arosa for south foehn events showed that these two parameters are uncorrelated, contradicting for these special periods the previous general analysis by Pochanart et al. (2001). This is interpreted as a stronger importance of pollutant emissions in the Po basin during foehn as compared to the large scale European influence. In contrast, the residence time over the Po basin was found to be more relevant for ozone mixing ratio at Arosa during summer. Foehn events showing high residence times over the Po basin (Dt412 h) were related on average to the highest ozone mixing ratios at Arosa.
Table 3 Mean ozone mixing ratios at Arosa and at Jungfraujoch in the cold season O3 [ppb]
Foehn at Arosaa Mean at Arosab Min during Foehn Arosac
Foehn at JFJa
Mean at JFJb
Min during Foehn JFJc
January February March October November December
42.4 42.8 49.7 42.0 39.1 39.9
41.8 44.2 51.1 45.8 41.0 42.8
46.6 46.5 51.9 47.6 42.7 45.0
33.1 35.4 34.9 33.0 31.6 35.5
42.2 42.1 48.1 40.0 37.3 39.1
35.0 35.3 33.1 33.5 31.0 34.5
Foehn period at Jungfraujoch was equated to foehn period at Arosa (data from September 2000 to August 2002). a During foehn. b Monthly average. c Minimum ozone mixing ratio during foehn period.
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In the cold season the picture is more complex than during the summer. In winter, although meteorological measurements reveal foehn at Arosa, the ozone mixing ratio does not indicate a clear pattern when comparing to all other situations. This observation is in contrast to results found at Jungfraujoch and is due to the comparably stronger exposition of Arosa to the boundary layer north of the Alps. In winter, comparison between ozone mixing ratios at Arosa and at Mt. Cimone allows to estimate an averaged ozone reduction of 4 ppb during the passage over the Po basin. The formation of a persistent inversion layer over the Po basin suggests that the interaction between foehn flow and the boundary layer over the Po basin, i.e. the stability of the atmosphere in the Po basin, are relevant for understanding of the trace gas behavior at Alpine sites during foehn events.
Acknowledgments This work was supported by research grants from ETHZ and from the Swiss National Science Foundation. We particularly thank MeteoSwiss for the access to the infrastructure at Arosa (Tschuggen) and for providing meteorological data. We also thank the staff at the CNR research station at Mt. Cimone, the Cantonal Authority of Grison in Chur, as well as the Swiss Agency for Environment in Bern for providing trace gas measurement data used in this study.
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