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Tropospheric ozone in the pre-alpine and alpine regions
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The Science of the Total Environment 156 (1994) 169-182
Tropospheric ozone in the pre-alpine and alpine regions S. S a n d r o n i *a, P. B a c c i b, G. B o f f a c, U . P e l l e g r i n i d, A. V e n t u r a d aEn~ironment Institute, Joint Research Centre, 21020 Ispra, Varese, Italy bENEL C.R.T.N., lrza Rubattino 54, 20134 Milano, Italy CDipartimento del Territorio del Canton Ticino, 6501 Bellinzona, Switzerland dECO. IrE. MA., Via Torino, 28041 Arona, Italy Received 11 February 1994; accepted 5 April 1994
Abstract
Surface ozone measurements taken at different altitudes in the south-western and eastern alpine regions from 1987 to 1991 are compared in terms of seasonal and daily fluctuations. The annual mean levels increase with altitude, ranging from about 20 parts-per-billion by volume, i.e. 10 -9 v / v (ppbv), on the plain to 50 ppbv at 3500 m. The transport of ozone a n d / o r precursors from the plain, the photochemical processes and the exchanges with the free troposphere are the main processes influencing its distribution in these regions. In the warm season, sites at intermediate altitudes up to about 1800 m may suffer from higher ozone exposure than sites at high altitudes. For instance, in July 1991, the monthly mean levels were 65 ppbv at 490 m (Brione), 70 ppbv at 920 m (Mottarone) and 67 ppbv at 1650 m (Cimetta), all higher than 50 ppbv at 3580 m (Jungfraujoch). The irregular distribution is due to the advection of an ozone front during the day and the persistent high levels during the night. The highest hourly levels (up to 185 ppbv) observed in the years 1989-91 were associated to subsidences of upper dry layers on a regional scale.
Keywords: Tropospheric ozone; Air quality; Photochemical processes; Mountainous regions
1. Introduction
While scientific interest in acid deposition is slowly decreasing, tropospheric ozone continues to be of significant concern worldwide. Ozone plays a key role in biogeochemical cycles, air quality and global change. There are two reasons for the current interest. One is the oxidation strength of ozone, which can be dangerous to human health and deleterious to forest ecosys-
* Corresponding author.
tems and agricultural crops (Prinz, 1988; Lippmann, 1991; Schenone and Lorenzini, 1992). The second reason is its role in forming the hydroxyl radical (OH) which initiates most organic oxidations in the troposphere and thus is a controlling agent in atmospheric chemistry (Finlayson-Pitts and Pitts, 1993). In recent decades its concentration at ground level in rural areas has shown a positive trend (Feister and Warmbt, 1987; Penkett, 1988; Volz and Kley, 1988; Janach, 1989; Reiter, 1990; Low et al., 1992). This trend is confirmed by the observation that in central Europe the present level is twice as high as that of a century ago
0048-9697/94/$07.00 © 1994 Elsevier Science BV. All rights reserved. SSD1 0048-9697(94)04205-2
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S. Sandroni et al. / Sci. Total Enciron. 156 (1994) 169-182
(Bojkov, 1986; Anfossi et al., 1991; Anfossi and Sandroni, 1994). Furthermore, Anfossi et al. (1991) and Marenco et al. (1993) have emphasized that an increase of the same order of magnitude has also occurred in the free troposphere. Two mechanisms have been proposed to account for high ozone levels in remote areas, namely, photochemical activity and stratospheric intrusions. The photochemical processes involve the transport of ozone a n d / o r of its precursors, VOC and nitrogen oxides (NOx), followed by in-situ or in-transit photochemical ozone production. Natural emissions contribute in these processes. In the alpine regions ozone is the most sensitive chemical parameter. In these regions, characterized by complex topography, some environmental damage is assigned to high photochemical activity, but there is limited information on the level of ozone and photo oxidants (Reiter et al., 1987; BUWAL, 1989; Janach, 1989; Bacci et al., 1990; Reiter, 1991; Puxbaum et al., 1991; Staehelin et al., 1993). As a pollutant, ozone should be brought under effective control, but the variability of its natural background level in the boundary layer makes the definition of the concentration above which it becomes a pollutant difficult to identify. In recent years the alpine regions and particularly those facing south, with densely populated valleys and densely forested slopes, have suffered from episodes of high ozone levels, even in remote valleys. Meteorological conditions have fostered high concentrations of ozone, when slow-moving anticyclones lasted some weeks over Europe. Levels greatly exceeding the maximum allowed values were recorded; emergency situations were declared, high-risk people were advised to stay at home and traffic restrictions were applied. Factors potentially responsible for high ozone levels in these regions are: (a)
the increased emission of precursors, due to intense tourist traffic in the warm season along the N-S motorways, around the lakes and in the valleys; (b) the persistence of anticyclonic situations, associated with high solar radiation and a weak wind regime in the lower layers.
An ideal measurement of tropospheric ozone would be a vertical profile, but this can seldom be achieved. Surface ozone measurements, being continuous in time, make it possible to investigate diurnal variations, changes associated with synoptic scale weather events and longer term effects such as seasonal trend. Since 1987 an increasing number of monitoring stations have been put into operation at different altitudes in northern Italy, southern Switzerland and Austria to collect as much information as possible on ozone levels, including related chemical and meteorological parameters. The data available has been collected and analysed. The objectives of this study are (1) to survey the ozone distribution and its fluctuations at different altitudes and (2) to collect data on the main factors causing periods of high ozone levels and their extent.
2. Regional scale meteorology The air mass circulation on the southern slope of the Alps is largely influenced by the plain to the South. The Po valley, ~ 450 km long and 250 km wide, is industrialized and densely populated; it is surrounded by mountain chains (Alps and Apennines) and the Adriatic sea to the East. The western Alps with a mean altitude of 3500 m are a strong barrier to Atlantic winds. The climatology of the Po valley is therefore characterized by weak winds (1-3 m-s -1) and particularly by mountain and valley breezes (Giuliacci, 1985). In the cold season, the pre-Alpine region rich in lakes and valleys, is only marginally influenced by the radiative inversions covering the Po valley and trapping the primary pollutants released close to the ground. The mountains do not suffer from the fog and severe pollution episodes that occur in the plain. Generally, more than one inversion with different stability is detected by rawinsondes; while the ground-based inversion may be dissolved during the day, higher inversions are stable. Under these conditions, the diurnal vertical diffusion is hindered and only a short-distance horizontal transport in lower layers might occur (Ambrosetti and Boffa, 1991). By contrast in the warm season, when pho-
S. Sandroni et al. ,/Sci. Total Environ. 156 (1994) 169-182
tochemical activity, vertical mixing and temperature gradient between valley and mountain are at their highest values, pollutants released in the plain are lifted up and transported towards the mountain ridges at a speed of 2-3 m. s -~ at ground level. Gaglione et al. (1988) have shown, experimentally, that on a warm summer day a tracer released downwind of a mountain chain 1200 m high (Campo dei Fiori) can float to the top in a few hours. While anabatic flow prevails during the day, at night a weak katabatic flow takes place (Broder and Gygax, 1985). The intensity of the anabatic winds varies if the mountain slope is in sunshine or in shadow. This advection is particularly important for relatively large N-S oriented valleys, such as the lake Maggiore-Ticino basin bordering the Gotthard pass and the Adige-Isarco valley bordering the Brenner pass. Under a reinforced southern wind, air masses can follow the Leventina valley and even cross the Gotthard pass (2100 m), as shown by tracer experiments performed in the Eurotrac/Tract/Transalp project (Lamprecht, 1990). Periods of transboundary transport of air pollutants through the eastern Alps over the Brenner pass are more frequent, because the mountain chain is lower (Camuffo et al., 1991). Such transalpine flow of air masses have also been simulated by Clerici et al. (1991).
plain, in the centre of the city (about 1.5 million inhabitants) ~ 30 m above ground; Ispra (209 m), sited in a semi-rural area near lake Maggiore, about 50 km N of Milan; Swiss urban stations: Chiasso (230 m), a border town with an annual transit of about 6 million cars; Lugano (290 m), a residential and commercial town; Mendrisio (300 m), a village ~ 10 km NW of Chiasso.
(b) stations at intermediate altitudes: --
--
(a)
stations in the plain: - - Milan Brera (138 m) sited in the Po
on the western side of Lake Maggiore: Aurigeno (340 m), a village in a side valley; Brione (490 m), a village ~ 300 m above Lake Maggiore; Mottarone (920 m), on a south facing woody slope; Cimetta (1650 m) on a ridge above Brione. on the eastern side of Lake Maggiore: Campo dei Fiori (1226 m), in a woody area; which came into operation in summer 1988 and 1989.
(c) stations at high altitudes: --
3. Experimental procedure 3.1. Measurement stations and instrumentation The measurement stations aod their characteristics are listed in Table 1 and their location is shown in Fig. 1. The different stations are managed by different organisations: the Italian Electricity Board (ENEL); Joint Research Centre (JRC), (Leyendecker et al., 1988-1992); Department of the Environment, Ticino, Switzerland (DOE-CH); Amt der Tiroler Landesregierung, Austria (ATL-A). Their location was chosen according to the following criteria; air quality, transboundary transport of pollution or forest damage. The stations may be grouped as follows:
171
(d)
in the eastem Alps: Brennero (1870 m), on the southern slope; Seefeld (1730 m), Mayrhofen (1910 m) and Seegrube (1960 m) on the northern slope. The Brennero station is sited in the Isarco valley, near the pass; Seegrube is above Innsbruck, in the main Inn valley; Seefeld and Mayrhofen are in side valleys.
alpine stations: - - Sestriere (2555 m), Plateau Rosa (3500 m) and Jungfraujoch (3580 m), which is a station of the Swiss NABEL network.
Because of the different affiliations of the stations (Table 1), the instuments used can differ considerably; most of them were equipped with an NO x monitor and meteorological instrumentation. VOC were only measured at a few sites. All ozone monitors were commercial instruments
172
S. Sandroni et al. /Sci. Total Environ. 156 (1994) 169-182
Table 1 Alpine and pre-alpine ozone measurement stations Ref. Fig. 1
Stations
Country
Type
Altitude (m asl)
Years
Institution
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Milano Brera Ispra Chiasso Lugano Mendrisio Aurigeno Brione Mottarone Campo dei Fiori Cimetta Seefeld Brennero Mayrhofen Seegrube Sestriere Plateau Rosa Jungfraujoch
I I CH CH CH CH CH I I CH A I A A I I CH
Urban Semi-rural Urban Urban Suburban Rural Rural, slope Woody Woody Mountain Mountain Mountain Mountain Mountain Alpine Alpine Alpine
138 209 230 290 300 340 490 920 1226 1650 1730 1870 1910 1960 2555 3500 3580
1987-1991 1987-1991 1988-1991 1990-1991 1990-1991 (p) 1989-1990 (p) 1989-1991 1987-1991 (p) 1988 and 1989 (p) 1991 (p) 1987-1991 1987-1991 (p) 1987-1991 1990-1991 1989-1990 (p) 1989-1990 (p) 1987-1991
ENEL-CRTN JRC, CEC DOE-CH DOE-CH DOE-CH DOE-CH DOE-CH ENEL-CRTN JRC, CEC DOE-CH ATL-A ENEL-CRTN ATL-A ATL-A ENEL-CRTN ENEL-CRTN NABEL
(p), incomplete data series.
acid-buffered KI) or gas-phase titration of NO/NO2; this data quality procedure was valid for all monitors. At low elevations, instruments were checked 2-3 times a month, however, in the Alps the severe meteorological conditions and the limited accessibility during the cold season drastically reduced the frequency of controls (2-3 times
(Dasibi, Environment or Thermoelectron) based on UV absorption. These monitors have a detection limit of 2 ppbv and an accuracy of + 20% for ozone values > 20 ppbv; their precision is + 20% at 30 ppbv. They were calibrated routinely by an ozone generator; manual tests were performed using a wet chemistry method (boric
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Fig. 1. Location of the alpine and pre-alpine ozone measurement stations.
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S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
a year) and, as a consequence, the number of validated measurements. Comparisons between JRC and ENEL monitors were made 1-2 times a year, twice between DOE-CH and JRC while only one comparison was made between ATL-A and ENEL. An agreement with a reference monitor within 5 ppbv was considered as acceptable. The distinct.ion between urban, suburban, semi-rural and rural stations is arbitrary: a 'rural' station should be far from emission sources. In reality all stations are more or less influenced by anthropogenic emissions; an alpine station may also be contaminated, to a lesser extent, for short periods of time.
3.2. Data base The original data format was different for each network; the concentrations were given in ppbv or in /zg.m -3, the unit required by national legislation; the averaging time was 30 min to 1 h. All data was organized in a data base. The chosen measurement unit was the ppbv, since this unit allows a direct comparison of data gathered at different altitudes without any correction factor. The chosen time unit was the hour. Daily and monthly means were computed from half-hourly or hourly mean values. The data base was organized in hourly, daily and monthly mean values. Considerable work was needed to validate the data recorded at the individual stations, where the periodical instrumental checks were used as a reference. The data base is incomplete, particularly for some stations as given in Table 1. Each station was put into operation or disconnected at a different date and there are gaps in the data series. The data series of Sestriere and Plateau Rosa are very incomplete; furthermore, only daily means and maxima were given for the Austrian stations (Seefeld, Seegrube and Mayrhofen) from the Amt der Tiroler Landesregierung (1987-1991) and monthly means for Jungfraujoch from BUWAL. Data series for stations in the plain and at intermediate altitude have few interruptions. Meteorological information was locally available at most stations. WMO charts and Meteosat images were available from the Meteorological Service of the Italian Air Force at Milan-Linate and Udine,
173
central and eastern Po valley, where routine rawinsondes were launched at 6- and 12-h intervals. 4. Results and discussion
4.1. Monthly and daily averages The monthly and yearly means for 1990 and 1991 for different stations are listed in Table 2. If validated daily data was partially available, the mean value is given in parentheses. The annual mean level increases with altitude, ranging from 20-25 ppbv in the plain to 45-50 ppbv at 3500 m. The monthly and daily mean values are similar for alpine stations above 2500 m. At high altitude a good agreement ( > 0.85) is observed for monthly means and daily maxima at the Austrian stations (Seegrube, Mayrhofen and Seefeld) only, independent of their different siting. Somewhat lower values compared with the Austrian stations have been observed at Brennero, since the site is partially influenced by traffic below (Camuffo et al., 1991). Unexpected levels are observed at intermediate altitudes, where in the warm season the monthly mean may be greater than those at high altitudes. Higher values than expected were observed at Brione (490 m), Mottarone (920 m) and Cimetta (1650 m). In July 1991 in particular, these sites suffered from ozone levels 10-20 ppbv higher than alpine stations. A low correlation ( > 0.70) is observed among these stations, even if they are located on the same mountain slope (Brione and Cimetta). Finally, a poor correlation is observed for stations in the plain, since they are influenced by local emissions and climate.
4.2. Seasonal and daily patterns The seasonal pattern is influenced by the local environment and altitude. In general, at the alpine stations the yearly maximum is observed in spring (April-May), the stratospheric contribution dominating, while at intermediate and at low altitudes it is observed in July-August. At intermediate altitudes and occasionally in semi-rural areas of the plain (e.g. in 1988) the seasonal pattern may show a second maximum in spring if many episodes of advection from the free troposphere have occurred. A pattern with a sharp maximum
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
174
in May was observed at Arosa (1840 m) in 1989-91 (Staehelin et al., 1993); a similar pattern occurred at Seefeld and Mayrhofen in 1989, but not in 1990 and 1991. At low and intermediate altitudes the photochemical activity is the dominant contributor. These patterns agree with the previous observations by Logan (1989) and Janach (1989). As regards urban, suburban and semi-rural stations in the plain, the pattern is related to the local emissions and climate. Generally, the daily pattern follows a bell shape distribution, with a minimum (10 ppbv) in the early morning and a maximum between 14:00 and 18:00 h CET. In the cold season (November to February) the Po valley has stable radiative inversions (frequently 50-200 m high) with dense fog and daily ozone levels
were lower than 10-15 ppbv. In the cold season, Lugano enjoys a milder climate and more sunny days than Milan; therefore, Lugano has a higher mean ozone level. In the warm season high values are reached downwind of an urban plume, e.g. Mendrisio. The bell-shape daily course is progressively smoothed as we move to higher altitudes, where the daily minimum approaches the maximum. At intermediate altitudes the daily and seasonal patterns are largely influenced by the evolution of the mixing layer. A comparison of the seasonal patterns through the year at sites at different altitudes are given in Fig. 2. In the cold season Mottarone (920 m) and Brennero (1870 m) are above the mixing height and their ozone
Table 2 Monthly and yearly mean ozone concentrations (in ppbv) for the years 1990/1991. In case of a limited number (40 < x < 90%) of validated daily data, the monthly mean is given in parentheses m (asl)
J
F
M
138 209 230 290 300 340 490 920 1730 1870 1910 1960 2555 3500
(0) 2 4 14 1 6 -7 . . . 3 11 17 23 (26) 37 43 44 (25) (39) 40 42 38 44 (37) (39) 31 --
12 23 11 13 . 25 36 (45) 48 44 44 49 (51) 43
3580 138 209 230 290 300 490 920 1650 1730 1870 1910 1960 3580
38 1 4 2 3 2 17 18 38 39 (34) 42 40 36
50 13 18 8 11 12 30 (34) -40 43 47 47 43
A
M
J
J
A
S
O
(50) 39 48 55 50 41 60 67 58 52 58 61
41 36 38 49 43 (39) 54 62 56 60 58 60 . .
21 22 19 31 24 -40 24 42 47 44 44
(8) 10 5 8 8 -19 15 37 -44 41
N
Year
D
1990
Milan Brera Ispra Chiasso Lugano Mendrisio Aurigeno Brione Mottarone Seefeld Brennero Mayrhofen Seegrube Sestriere Plateau Rosa
17 30 14 26
36 32 26 40
31 39 -53 46 50 50 -51
34 48 -55 (56) 57 59 (56) 55
32 31 27 38 40 30 46 -50 51 53 57 . 50
56 24 34 17 29 28 46 49 -52 54 58 51 50
63 30 43 27 38 38 48 53 -52 54 53 51 52
57 39 42 31 39 38 46 52 (53) 48 53 50 51 49
.
.
. .
.
. .
3 6 2 5 5
-19 18 34
--
3 6 2 4 4
--
(19) 24 17 25 (25)
(27)
19 15 34
35 (34) 46
(31)
(45)
42 38
39 37
48 48
--
--
(46)
--
--
(46)
34 1 8 2 5 6 16 29
(49) 21,5 25 19 25 24 36 (42)
(32)
(50)
38 35 38 35 43
45 45 49 44 45
1991
Jung~aujoch Milan Brera Ispra Chiasso Lugano Mendrisio Brione Mottarone Cimetta Seefeld Brennero Mayrhofen Seegrube Jungfraujoch
38 8 10 6 10 9 26 40 44 42 42 44 44 38
59 53 52 49 61 55 65 70 67 53 53 51 51 50
59 52 39 46 54 53 59 61 60 56 56 51 51 52
50 26 24 24 33 29 44 (42) (55) 47 49 45 45 43
-8 14 8 9 10 19 29 -37 38 34 35 43
35 3 8 4 4 5 14 24
-34 28 32 32 40
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
levels are nearly the same, but quite different to Ispra (209 m). By contrast, in the warm season, when there is a strong temperature gradient between valley and mountain, ozone-rich air masses in the plain are lifted up and driven as an 'ozone front' towards the mountain ridges. In the wannest hours of the day the convective motions balance the ozone level at Mottarone and Ispra (Fig. 2). Close to the mountains the polluted air masses coming from the plain mix with clean air when there is strong UV radiation; hence, ozone can also be locally formed. Near the mountains, the ozone daily course may show two peaks, the first (at 14:00-15:00 h CET) was caused by local photochemical processes, the second (at 17:00-19:00 h CET) was related to air masses transported from the plain. This front is responsible for an ozone accumulation at altitudes up to about 1800 m. Airborne measurements made in the Ticino valley (Paffrath et al., 1986; MetAir, 1992) and in the Isarco valley near Brennero (Schlager et al., 1992), have confirmed the presence of such a diurnal front: the positive concentration gradient from the ground to about 1000 m observed in the early morning in the valleys decreases progressively with time and is dissolved before noon. At intermediate altitudes (Brione, 490 m, Mottarone, 920 m, and to lower extent Cimetta, 1870 m) ozone daily maxima may be higher than at 3000
m. These observations disagree with measurements taken by Wolff et al. (1987) on a mountain slope 550 m high; by contrast, Reiter et al. (1987) observed higher levels at Wank, 1780 m above sea level (asl), than at Zugspitze (2964 m asl). Along the advection trajectory from the Po valley to the Alps, a time-lag is observed for the daily ozone maximum. Fig. 3 compares the daily evolution in a summer's day at stations sited in the Lake Maggiore basin at different altitudes: the maximum was observed at 16:00 h CET at Ispra (209 m) and Mottarone (920 m) but at ~ 23:00 h CET at Campo dei Fiori (1226 m). This indicates, the valley breeze started at 10:00 h CET and reversed its direction at ~ 19:00 h CET. At Campo dei Fiori the ozone daily course is reversed, with a minimum around noon and a maximum during the night. This maximum may also originate from a subsidence of air masses. Occasionally at the bottom of a valley, converging flows descending from the opposite slopes during the night, can give rise to an ozone peak (Broder and Gygax, 1985). An interesting case study is Brione, a village at 490 m asl above Lake Maggiore, on a natural balcony protected from the cold northern winds. Brione suffers from high ozone exposure throughout the year: in 1991, for instance, the hourly mean level of 60 ppbv was exceeded 1251 times, while the 120 ppbv level was exceeded 23 times.
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Fig. 2. Seasonal evolution of ozone at different altitudes.
Fig. 3. Diurnal patterns at Ispra (209 m), Mottarone (920 m) and Campo dei Fiori (1226 m) under southern wind conditions (26 August, 1989).
176
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
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Fig. 4. Comparison of ozone patterns at Brione (490 m) vs. Ispra (209 m) and Brennero (1870 m) on (a) 28 January, (b) 13 July and (c) 15 October, 1991.
The local ozone course depends on the evolution of the radiative inversions and the mixing layer. Fig. 4 shows some daily courses at Brione, a site in the plain (Ispra, 209 m) and an alpine station (Brennero, 1870 m). On a sunny winter day the lower inversion at ~ 200 m rises slowly up to ~ 600 m at 12:00 h CET; the ozone course, typical of a mountain station during the night, changes to a valley station during the day; around 19:00 h CET a new inversion is formed and Brione behaves as a mountain station (Fig. 4a). In the warm season (Fig. 4b) the site is merged into the mixing layer during the day but it may be above it during the night; as a consequence, during the day the ozone pattern is similar to Ispra, being affected by ozone and precursors carried by the valley breeze, but at night it behaves like a mountain station; generally, these changes occur at about
10:00 h CET and 21:00 h CET. On a cloudy day (Fig. 4c) Brione is above the mixing layer and the ozone daily course is similar to that of a mountain station. By contrast, the ozone courses at Aurigeno (300 m asl), sited in an isolated side valley, are quite similar to a semi-rural site in the plain like Ispra. Particular daily courses occur during foehn episodes. In the pre-alpine region ozone concentrations comparable with photochemical processes may be measured in conjunction with intense and persistent downslope winds (northern foehn). These winds, particularly evident in the cold season (November to April), blow from N-NW with an average speed of 10 m. s-1 and with gusts up to 30 m. s-1. They may last from a few hours to 2-3 days and are characterized by a drop in relative humidity and pressure and an increase in temperature. These events are also mirrored by comparable ozone maxima in the plain and daily means in altitude. By this process ozone-rich air masses are transported from the free troposphere to the ground. An example of this situation is given in Fig. 5, which refers to February 8-9, 1988. A sharp increase of ozone concentration occurred simultaneously with the strong northern wind (foehn), which started during the night, at Ispra. During this event, the levels at Ispra and Mottarone are similar, supporting the hypothesis of a dynamic mixing of free and low troposphere. In the pre-alpine region foehn events occur 30-40 days a year: they are particularly strong in the cold season (Gandino et al., 1990); in summer they are weak and the advected ozone level may be lower than that formed photochemically. The contribution of these events to the monthly means is negligible, except in the case of several episodes in the same month, e.g. in February and March 1988 (7 and 6 events, respectively). Air masses from the lower stratosphere may be transported downward along isoentropic surfaces (jet streams), as described elsewhere (Reiter et al., 1987). The stratospheric intrusions can be seen on Meteosat images and charts at 500 hPa; they last for 1-2 days and are characterised by increased levels of cosmogenic radionuclides, especially 7Be; at 3000 m the 7Be concentration may increase from 90 up to 200-300 fCi. m-3. In
S. Sandroni et al. / Sci. Total Enciron. 156 (1994) 169-182
700 t/
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80
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20
4
8
12
16
20
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8
12
16
20
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Fig. 5. Ozone course during a foehn event during the night from 8 to 9 February, 1988 at Ispra (209 m) and Mottarone (920 m). Because of the event the wind speed increased from 1 to 16.6 m . s -1 while the pressure drop was 10 hPa.
1988 only one episode with a 7Be increase from 80 to 162 fCi.m -3 was recorded at Mottarone (920 m) on February 13. At lower altitudes no intrusion has been recorded; a foehn event may occur simultaneously. According to Reiter (1990), the lowest elevation to which a stratospheric intrusion can penetrate in the alpine region is about 1400-1600 m; at 3000 m their contribution to the monthly mean is of the order of 1-2 ppbv ozone. 4.3. Ozone levels at intermediate altitudes
As previously mentioned, in the warm season higher ozone levels may be observed at intermediate than at high altitudes. A statistical evaluation of ozone distribution as 8-h average concentration for the time intervals (00:00-08:00 h CET)
177
and (09:00-17:00 h CET) at Ispra (209 m), Brione (490 m) and Mottarone (920 m) in the coldest and the warmest months of 1991 is given in Tables 3 and 4. In the cold season the ozone distribution at Brione is similar to that at Mottarone; furthermore, while the day to night gradient is positive at Ispra and nearly absent at Mottarone, it is negative at Brione because of the advection of ozonepoor air from the plain (Table 3). In the warm season (Table 4) the day-to-night increase is quite sharp at Ispra and Mottarone but lower at Brione. A reference parameter for high ozone levels is the 'excess ozone' (Simpson, 1993). The term excess ozone means the sum over all hours in a given time period (e.g. 1 month) of all ozone concentrations in excess of 75 ppbv; it is expressed as ppbv. h. month-1. The 75 ppbv corresponds to the 1 h critical level of ozone, above which vegetation damage is observed, as adopted by the UN-ECE. The computed values of excess ozone for some representative stations for AprilSeptember 1991 are given in Table 5. At low and intermediate altitudes these values are quite high, then they decrease with altitude from about 1800 m. The highest values have been observed at Brione (490 m) in April and July. Another reference parameter for high levels is the frequency of ozone levels exceeding health and vegetation protection thresholds (Angle and Sandhu, 1989; Beck and Grennfelt, 1993). At present, Switzerland, Austria and Italy have different legislation; the EEC threshold value for vegetation protection (100 ppbv as 1 h average; Directive 92/72/EEC) can be considered a good com-
Table 3 Distribution of ozone concentration as 8-h average in January 1991 Concentration range (ppbv)
from 00:00 to 08:00 h CET Ispra
Brione
Mottarone
Ispra
Brione
Mottarone
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80
30 1 0 0 0 0 0 0
8 7 11 5 0 0 0 0
5 13 12 1 0 0 0 0
23 8 0 0 0 0 0 0
8 17 6 0 0 0 0 0
3 13 12 3 0 0 0 0
from 09:00 to 17:00 h CET
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
178
Table 4 Distribution of ozone concentration as 8-h average in July 1991 Concentration range (ppbv)
from 00:00 to 08:00 h CET Ispra
Brione
Mottarone
Ispra
Brione
Mottarone
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 100-120
7 11 6 5 2 0 0 0 0 0 0
0 0 1 6 8 4 5 6 1 0 0
0 0 0 2 8 6 9 3 3 0 0
0 0 0 0 5 3 4 4 6 3 6
0 0 1 2 8 3 5 8 2 2 0
0 0 0 0 3 3 5 8 6 6 0
from 09:00 to 17:00 h CET
24,1H
promise. The computed monthly values for the period April-September 1991 are listed in Table 6. The occurrence of ozone concentrations exceeding acceptable levels have been limited up to 1000 m, high values have been observed at Brione in July and at Mottarone in August.
4.4. Photochemical pollution episodes The selection of criteria for definition of an ozone episode is somewhat arbitrary, particularly in terms of ozone concentration (Logan, 1989). For instance, an episode may be defined as the simultaneous occurrence of an average value exceeding 70 ppbv from 10:00 to 16:00 h LT at four or more sites for 2 or more consecutive days; an alternative definition is the simultaneous occurrence of the daily maximum value exceeding 80 ppbv at four or more sites for 2 or more consecutive days. Here we have focused our attention on
the highest levels which occurred in the years under consideration. During the warm season in the alpine regions hourly ozone means frequently exceeded the limit values allowed by the national legislation, i.e. 60 ppbv in Switzerland and Austria and 100 ppbv in Italy; these values should not be exceeded more than once a year. In 1991 this value was exceeded 758 times at Mendrisio and 1251 times at Brione. At Ispra the maximum hourly means were 142 ppbv in 1987 (July 26), 135 ppbv in 1988 (July 20), 135 ppbv in 1989 (July 19), 128 ppbv in 1990 (August 3) and 173 ppbv in 1991 (July 14). The maximum hourly mean in the region (185 ppbv) was recorded at Chiasso on July 23, 1990. Generally, episodes lasted 4-5 days and were limited to one basin; sometimes they extended over the western and eastern alpine regions and in a few cases levels remained high throughout the night.
Table 5 Monthly values of excess ozone (units: 100 ppbv. h. m o n t h - 1 ) for April-September 1991 at some representative stations
Ispra Chiasso Lugano Mendrisio Brione Mottarone Brennero
m (asl)
A
M
J
J
A
S
209 230 290 300 490 920 1870
0.2 0 0 0.1 2.3 0.8 0
9.0 3.2 6.8 7.3 6.5 5.5 0
16.3 8.2 9.0 7.5 11.5 8.7 2.0
54.3 41.5 45.3 47.4 57.3 45.9 1.1
16.8 28.4 17.8 39.8 21.1 7.3 --
5.5 8.7 6.1 9.9 11.7 -0.7
S. Sandroni et al. /ScL Total Environ. 156 (1994) 169-182
179
Table 6 Frequency (in %) of hourly values exceeding the EEC vegetation protection threshold (100 ppbv as 1 h average) for April September 1991
Ispra Brione Mottarone Brennero Seegrube
m (asl)
A
M
J
J
A
S
209 490 920 1870 1960
0 0 0 0 0
0.5 0.3 0 0 0
2.6 1.5 0.3 0 0
13.0 12.4 1.9 0 0
2.1 2.4 9.8 0 0
0.3 0.1 0 0 0
Following a request from regional authorities, an investigation of the possible relationship between these episodes and meteorological conditions (Davies et al., 1992) and the emission of precursors. In particular, in the Italian-Swiss lake region, meteorological data (and data for a few chemical parameters, mostly NO x) have been routinely available at different altitudes and significant mechanisms could be identified.
4. 5. Meteorological conditions. All the episodes we observed in the years 1987-91 were associated with anticyclonic conditions lasting some days, intense solar radiation and weak winds (1-2 m.s -1) from the south. In July 1989 a slow-moving anticyclone dominated over central Europe up to the 19th, when its centre moved to England allowing the transit of a weak front. A high pressure in the upper layers had a stabilizing effect on the atmosphere. The mixing height (calculated according to Holzworth, 1974) had an initial level of 1600-1700 m, then it rose to 2600 m on the 17th. A dry and warm layer, typical precursor of a subsidence, extended up to 3600 m. During the days from the 16th to the 19th the subsidence moved progressively down to the ground carrying clear air masses from the upper layer to mix with polluted air masses in the lower layers. During the days of the 16th-19th, ozone daily maxima increased progressively and reached 135 ppbv at Ispra and similar levels at Aurigeno and Brione (Fig. 6). The maximum was observed at around 16:00 h CET, about 4 h after the maximum temperature (32°C) and solar radiation (918 W/cm2); wind from the south at 1.4 m . s -1.
-
-
A series of episodes occurred in July and August 1990. From the beginning of July to August 6 a persistent anticyclone was observed at 500 hPa centred over North Africa and extending over the alpine region; during the same time period a large high-pressure field ( P > 1013 hPa) occurred at 850 hPa over western Europe. The mixing height rose up to 1500-2000 m, while its minima occurred together with a subsidence (July 23) and a front transit (August 6). Fig. 7 shows the progressive ozone increase from 18 to 23 July, together with an increase in temperature and solar radiation, at three stations of the Lake Maggiore basin up to the level of 185 ppbv at Chiasso on the 23rd. With a time lag of 1-3 days high levels were observed both in the western and the eastern alpine regions, e.g. during the last days of July (Ispra, 121 ppbv and Mottarone, 115 ppbv; Seefeld, 97 ppbv; Seegrube, 98 ppbv and
7 Brion¢
140
........... •. - ~ .
2
•
[spra
6 AmH|eno
120
100
SO
4O
",~,
:
i
¢"
ii
CET
Fig. 6. Ozone course during a photochemical smog episode on 16-18 July, 1989 at Ispra (209 m), Aurigeno (340 m) and Brione (490 m).
180
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182 2OO
/~,.,
Mayrhofen, 116 ppbv). A persistent anticyclone with a few interruptions also occurred in summer 1991. The thermal mixing height was almost stabilized at about 1800-1900 m, reaching 2400 m. During that period a dry layer with irregular altitudes was observed. Ozone maxima coincided with subsidences that occurred on July 14 (173 ppbv ozone and 17 ppbv PAN at Ispra), July 23 and on August 3. The vertical dynamics of the troposphere is likely to be one of the main parameters controlling the occurrence of high ozone levels in the alpine regions. In particular, the altitude, the thickness and the vertical motion of the dry (and relatively warm) upper air layer determine the evolution of the subsidence. These descending motions are an ideal carrier of air masses from the lower free troposphere to ground level. A direct relationship between the increase in ozone and the mean thermal gradients has not yet been established.
troposphere. At low and intermediate altitudes monthly maxima occur in summer, together with intense photochemical activity; high levels in spring are associated with advections from free troposphere and stratosphere. Particular attention has been paid to intermediate altitudes, i.e. from 400 to about 1800 m, where in the warm season monthly mean levels are comparable, or even 10-20 ppbv larger than at higher altitude levels. Also the levels of excess ozone and the frequency with which they exceed health and vegetation protection thresholds are higher than expected for that altitude. The reason for the higher levels is twofold, (1) the advection of ozone a n d / o r precursors from the plain (ozone front) towards the areas of high ground during the day and (2) the persistent high levels during the night. Sites at intermediate altitudes behave as mountain sites during the night and plain stations during the day. Since the photochemically produced ozone levels in the plain are higher than at 3000 m, the mean daily level at intermediate altitudes may exceed the corresponding one at high altitudes. Physical processes play an important role in redistributing ozone levels over such a complex terrain. The main mechanisms are (a) the horizontal transport on the regional scale, (b) the convective motions of thermal origin or associated with a dynamical instability. Stratospheric intrusions (c) have a negligible incidence below 1800 m, but downslope transport from free troposphere (foehn events) is common in the cold season. Above ~ 1800 m, the ozone course is weakly influenced by photochemical activity at the bottom of the valley. A reverse ozone daily course may be observed > 1000 m, in comparison with the valley. Finally, high-levels seem to be well correlated with a subsidence of dry upper layer in anticyclonic conditions.
5. Conclusions
Acknowledgements
In the alpine regions ozone increases with altitude, the yearly mean ranging about from 20 ppbv in the plain to 50 ppbv at 3000 m. The seasonal course is largely influenced by synoptic meteorological conditions and by exchanges with free
This study has been supported by the CNRENEL Project - - Interaction of energy systems with human health and the environment - - Rome, Italy. We would also like to thank W. Leyendecker, JRC Ispra, the ENEL-C.R.T.N., Milan,
ISO . . . . . . . . . . . . . . .
Brlone lspra
. . . . Ch,~,o
~
/ a
~
/
'~
r
8o 6o
io 2o
CET
Fig. 7. Photochemical smog episode on 18-23 July, 1990 at a semi-rural (Ispra, 209 m), an urban (Chiasso, 230 m) and a rural site (Brione, 490 m).
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
the Dipartimento del Territorio del Canton Ticino, BeUinzona, and the Amt der Tiroler Landesregierung, Innsbruck, for making data available. The authors are also indebted to G. Queirazza and G. Bonforte, ENEL C.R.T.N., for radionuclide measurements and to the Italian Meteorological Service of the Air Force, Milan for meteorological charts. References Ambrosetti P. and G. Boffa, 1991. Inquinamento atmosferico invernale nel Ticino: effetti di trasporto orizzontale. Boll. Soc. Tic. Sci. Natur., 79: 13-22. Amt der Tiroler Landesregiernng, Abt. Umweltschutz, 1987-1991. Montasberichte. Anfossi D., S. Sandroni and S. Viarengo, 1991. Tropospheric ozone in the nineteenth century: the Moncalieri series. J. Geophys. Res., 96: 17, 349-317, 352. Anfossi D. and S. Sandroni, 1994. Surface ozone at mid latitudes in the past century. Nuovo Cimento C, 17: 199-205. Angle R.P. and H.S. Sandhu, 1989. Urban and rural ozone concentrations in Alberta, Canada. Atmos. Environ., 23: 215-221. Bacci P., S. Sandroni and A. Ventura, 1990. Patterns of tropospheric ozone in the pre-alpine region. Sci. Total Environm., 96: 297-312. Beck J. and P. Grennfelt, 1993. Distribution of ozone over Europe. A contribution to subproject TOR. In: P.M. Borrell, P. Borrell, T. Cvita~ and W. Seiler (Eds), Proceed. Eurotrac Symposium '92, SPB Academic Publ., The Hague, The Netherlands, pp. 43-58. Bojkov R.D., 1986. Surface ozone during the second half of the nineteenth century. J. Climat. Appl. Met., 25: 343-352. Broder B. and H. Gygax, 1985. The influence of locally induced wind systems on the effectiveness of nocturnal dry deposition of ozone. Atmos. Environ., 19: 1627-1637. BUWAL (1989) Ozon in der Schweiz. Schriftenrehie Umweltschutz 101. Bundesanstalt tur Umwelt, Wald und Landschaft, Bern. Camuffo D., A. Bernardi and P. Bacci, 1991. Transboundary transport of atmospheric pollutants through the eastern Alps. Atmos. Environm., 25A: 2863-2871. Clerici G.C., R. Salerno and S. Sandroni, 1991. Time evolution of breeze circulation in south alpine valleys In: H. van Dop and D.G. Steyn (Eds.), Air Pollution Modelling and its Applications, VIII, Plenum Press, NY, pp. 349-360. Davies T.D., P.M. Kelly, P.S. Low and C.E. Low, 1992. Surface ozone concentrations in Europe: links with the regionalscale atmospheric circulation. J. Geophys. Res., 97: 9819-9832. Feister U. and W. Warmbt, 1987. Long-term measurements of surface ozone in the German Democratic Republic. J. Atmos. Chem., 5: 1-21.
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Finlayson-Pitts B.J. and J.N. Pitts Jr., 1993. Atmospheric chemistry of tropospheric ozone formation: scientific and regulatory implications. J. Air Waste Manage. Assoc., 43: 1091-1100. Gaglione P., G. Graziani and S.E. Gryning, 1988. Perfluorocarbon tracer experiments in a lake-mountain area (Campo dei Fiori experiment), paper presented at the EURASAP Conference, 25-27 October, Rise National Laboratories, Denmark. Gandino C., W. Leyendecker and S. Sandroni, 1990. The northern foehn as ozone carrier. Nuovo Cimento C., 13: 669-676. Giuliacci M., 1985. Climatologia Statica e Dinamica della Valpadana. CNR Report A Q / 3 / 1 8 , Milano. Hoizworth G.C., 1974. Climatological Aspects of the Composition and Pollution of the Atmosphere, WMO Techn. Note, 139. Janach W.E., 1989. Surface ozone: trend details, seasonal variations, and interpretation. J. Geophys. Res., 94: 18, 289-18, 295. Lamprecht R., 1990. First result of tracer experiments over complex alpine terrain: TRANSALP, paper presented at the Int. Conf. on Alpine Meteorology, Engelberg, Switzerland, September. Leyendecker W., C. Brnn, H. Geiss-Horsch and G. SerriniLanza, 1988-1992. Activity of JRC Emep station: Annual Reports. Lippmann M., 1991. Health effects of tropospheric ozone. Environ. Sci. Technol., 25: 1954-1962. Logan J.A., 1989. Ozone in rural areas of the United States. J. Geophys. Res., 94: 8511-8532. Low P.S., P.M. Kelly and T.D. Davies, 1992. Variations in surface ozone trends over Europe. Geophys. Res. Lett., 19: 1117-1120. Marenco A., H. Gouget, P. Nedelec, J.P. Pag6e and F. Karcher, 1993. Evidence of long-term evolution of ozone at mid-latitudes of the northern hemisphere from the series of Pic du Midi observatory: consequences for radiative forcing, J. Goephys. Res. (in prep). MetAir (1992) Sommersmog-Messfliige im Tessin und in der Westschweiz 1991, MetAir Bericht, Illnau, Switzerland. Paffrath D., W. Peters and H. Stangl, 1986. Airborne measurements of ozone and other pollutants north and south of the Alps. Paper presented at the 7th Clean Air Congress, Sydney, Australia, 3: 194-203. Penkett S.A., 1988. Indications and causes of ozone increase in the troposphere. In: F.S. Rowland and I.S.A. Isaksen (Eds), The changing atmosphere, Wiley, NY, pp. 91-103. Prinz B., 1988. Ozone effects on vegetation. In: I.S.A. Isaksen (Ed.), Tropospheric Ozone, Reidel Publ. Co. Dordrecht, pp. 161-184. Puxbaum H., K. Gabler, S. Smidt and F. Glattes, 1991. A one-year record of ozone profiles in an alpine valley (Zillertal, Tyrol, Austria, 600-2000 m asl). Atmos. Environ., 25A: 1759-1765.
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Reiter R., R. Sladkovic and H.J. Kanter, i987. Concentration of trace gases in the lower troposphere, simultaneously recorded at neighboring mountain stations. Part II: Ozone. Met. Atmos. Phys., 37: 27-47. Reiter R., 1990. The ozone trend in the layer of 2 to 3 km asl since 1978 and the typical time variations of the ozone profile between ground and 3 km asl. Met. Atmos. Phys., 42: 91-104. Reiter R., 1991. On the mean daily and seasonal variations of the vertical ozone profiles in the lower troposphere. Atmos. Environ., 25A: 1751-1757. Schenone G. and G. Lorenzini, 1992. Effects of regional air pollution on crops in Italy. Agric., Ecosyst. Environ., 38: 51-59. Schlager H., J. Graf, M. Krautstrunk and M. Briinner, 1992. Messung und Modellierung des Schadstoffverhaltens im
Alpenbereich (MEMOSA Projekt). DLR Report, Oberpfaffenhofen (Germany). Simpson D., 1993. Photochemical model calculations over Europe for two extended summer periods: 1985 and 1989. Model results and comparison with observations. Atmos. Environ., 27A: 921-943. Staehelin J., J. Thudium, R. Buehler, A. Volz-Thomas and W. Graber, 1993. Trends in surface ozone concentrations at Arosa. Atmos. Environ., 28: 75-87. Volz A. and D. Kley, 1988. Evaluation of the Montsouris series of ozone measurements made in the nineteenth century. Nature, 332: 240-242. Wolff G.T., P.J. Lioy and R.S. Taylor, 1987. The diurnal variation of ozone at different altitudes on a rural mountain in the eastern United States. J. Am. Phys. Chem. Assoc., 37: 45-48.
ELSEVIER
~
tw~m~k
Itl*w~
The Science of the Total Environment 156 (1994) 169-182
Tropospheric ozone in the pre-alpine and alpine regions S. S a n d r o n i *a, P. B a c c i b, G. B o f f a c, U . P e l l e g r i n i d, A. V e n t u r a d aEn~ironment Institute, Joint Research Centre, 21020 Ispra, Varese, Italy bENEL C.R.T.N., lrza Rubattino 54, 20134 Milano, Italy CDipartimento del Territorio del Canton Ticino, 6501 Bellinzona, Switzerland dECO. IrE. MA., Via Torino, 28041 Arona, Italy Received 11 February 1994; accepted 5 April 1994
Abstract
Surface ozone measurements taken at different altitudes in the south-western and eastern alpine regions from 1987 to 1991 are compared in terms of seasonal and daily fluctuations. The annual mean levels increase with altitude, ranging from about 20 parts-per-billion by volume, i.e. 10 -9 v / v (ppbv), on the plain to 50 ppbv at 3500 m. The transport of ozone a n d / o r precursors from the plain, the photochemical processes and the exchanges with the free troposphere are the main processes influencing its distribution in these regions. In the warm season, sites at intermediate altitudes up to about 1800 m may suffer from higher ozone exposure than sites at high altitudes. For instance, in July 1991, the monthly mean levels were 65 ppbv at 490 m (Brione), 70 ppbv at 920 m (Mottarone) and 67 ppbv at 1650 m (Cimetta), all higher than 50 ppbv at 3580 m (Jungfraujoch). The irregular distribution is due to the advection of an ozone front during the day and the persistent high levels during the night. The highest hourly levels (up to 185 ppbv) observed in the years 1989-91 were associated to subsidences of upper dry layers on a regional scale.
Keywords: Tropospheric ozone; Air quality; Photochemical processes; Mountainous regions
1. Introduction
While scientific interest in acid deposition is slowly decreasing, tropospheric ozone continues to be of significant concern worldwide. Ozone plays a key role in biogeochemical cycles, air quality and global change. There are two reasons for the current interest. One is the oxidation strength of ozone, which can be dangerous to human health and deleterious to forest ecosys-
* Corresponding author.
tems and agricultural crops (Prinz, 1988; Lippmann, 1991; Schenone and Lorenzini, 1992). The second reason is its role in forming the hydroxyl radical (OH) which initiates most organic oxidations in the troposphere and thus is a controlling agent in atmospheric chemistry (Finlayson-Pitts and Pitts, 1993). In recent decades its concentration at ground level in rural areas has shown a positive trend (Feister and Warmbt, 1987; Penkett, 1988; Volz and Kley, 1988; Janach, 1989; Reiter, 1990; Low et al., 1992). This trend is confirmed by the observation that in central Europe the present level is twice as high as that of a century ago
0048-9697/94/$07.00 © 1994 Elsevier Science BV. All rights reserved. SSD1 0048-9697(94)04205-2
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S. Sandroni et al. / Sci. Total Enciron. 156 (1994) 169-182
(Bojkov, 1986; Anfossi et al., 1991; Anfossi and Sandroni, 1994). Furthermore, Anfossi et al. (1991) and Marenco et al. (1993) have emphasized that an increase of the same order of magnitude has also occurred in the free troposphere. Two mechanisms have been proposed to account for high ozone levels in remote areas, namely, photochemical activity and stratospheric intrusions. The photochemical processes involve the transport of ozone a n d / o r of its precursors, VOC and nitrogen oxides (NOx), followed by in-situ or in-transit photochemical ozone production. Natural emissions contribute in these processes. In the alpine regions ozone is the most sensitive chemical parameter. In these regions, characterized by complex topography, some environmental damage is assigned to high photochemical activity, but there is limited information on the level of ozone and photo oxidants (Reiter et al., 1987; BUWAL, 1989; Janach, 1989; Bacci et al., 1990; Reiter, 1991; Puxbaum et al., 1991; Staehelin et al., 1993). As a pollutant, ozone should be brought under effective control, but the variability of its natural background level in the boundary layer makes the definition of the concentration above which it becomes a pollutant difficult to identify. In recent years the alpine regions and particularly those facing south, with densely populated valleys and densely forested slopes, have suffered from episodes of high ozone levels, even in remote valleys. Meteorological conditions have fostered high concentrations of ozone, when slow-moving anticyclones lasted some weeks over Europe. Levels greatly exceeding the maximum allowed values were recorded; emergency situations were declared, high-risk people were advised to stay at home and traffic restrictions were applied. Factors potentially responsible for high ozone levels in these regions are: (a)
the increased emission of precursors, due to intense tourist traffic in the warm season along the N-S motorways, around the lakes and in the valleys; (b) the persistence of anticyclonic situations, associated with high solar radiation and a weak wind regime in the lower layers.
An ideal measurement of tropospheric ozone would be a vertical profile, but this can seldom be achieved. Surface ozone measurements, being continuous in time, make it possible to investigate diurnal variations, changes associated with synoptic scale weather events and longer term effects such as seasonal trend. Since 1987 an increasing number of monitoring stations have been put into operation at different altitudes in northern Italy, southern Switzerland and Austria to collect as much information as possible on ozone levels, including related chemical and meteorological parameters. The data available has been collected and analysed. The objectives of this study are (1) to survey the ozone distribution and its fluctuations at different altitudes and (2) to collect data on the main factors causing periods of high ozone levels and their extent.
2. Regional scale meteorology The air mass circulation on the southern slope of the Alps is largely influenced by the plain to the South. The Po valley, ~ 450 km long and 250 km wide, is industrialized and densely populated; it is surrounded by mountain chains (Alps and Apennines) and the Adriatic sea to the East. The western Alps with a mean altitude of 3500 m are a strong barrier to Atlantic winds. The climatology of the Po valley is therefore characterized by weak winds (1-3 m-s -1) and particularly by mountain and valley breezes (Giuliacci, 1985). In the cold season, the pre-Alpine region rich in lakes and valleys, is only marginally influenced by the radiative inversions covering the Po valley and trapping the primary pollutants released close to the ground. The mountains do not suffer from the fog and severe pollution episodes that occur in the plain. Generally, more than one inversion with different stability is detected by rawinsondes; while the ground-based inversion may be dissolved during the day, higher inversions are stable. Under these conditions, the diurnal vertical diffusion is hindered and only a short-distance horizontal transport in lower layers might occur (Ambrosetti and Boffa, 1991). By contrast in the warm season, when pho-
S. Sandroni et al. ,/Sci. Total Environ. 156 (1994) 169-182
tochemical activity, vertical mixing and temperature gradient between valley and mountain are at their highest values, pollutants released in the plain are lifted up and transported towards the mountain ridges at a speed of 2-3 m. s -~ at ground level. Gaglione et al. (1988) have shown, experimentally, that on a warm summer day a tracer released downwind of a mountain chain 1200 m high (Campo dei Fiori) can float to the top in a few hours. While anabatic flow prevails during the day, at night a weak katabatic flow takes place (Broder and Gygax, 1985). The intensity of the anabatic winds varies if the mountain slope is in sunshine or in shadow. This advection is particularly important for relatively large N-S oriented valleys, such as the lake Maggiore-Ticino basin bordering the Gotthard pass and the Adige-Isarco valley bordering the Brenner pass. Under a reinforced southern wind, air masses can follow the Leventina valley and even cross the Gotthard pass (2100 m), as shown by tracer experiments performed in the Eurotrac/Tract/Transalp project (Lamprecht, 1990). Periods of transboundary transport of air pollutants through the eastern Alps over the Brenner pass are more frequent, because the mountain chain is lower (Camuffo et al., 1991). Such transalpine flow of air masses have also been simulated by Clerici et al. (1991).
plain, in the centre of the city (about 1.5 million inhabitants) ~ 30 m above ground; Ispra (209 m), sited in a semi-rural area near lake Maggiore, about 50 km N of Milan; Swiss urban stations: Chiasso (230 m), a border town with an annual transit of about 6 million cars; Lugano (290 m), a residential and commercial town; Mendrisio (300 m), a village ~ 10 km NW of Chiasso.
(b) stations at intermediate altitudes: --
--
(a)
stations in the plain: - - Milan Brera (138 m) sited in the Po
on the western side of Lake Maggiore: Aurigeno (340 m), a village in a side valley; Brione (490 m), a village ~ 300 m above Lake Maggiore; Mottarone (920 m), on a south facing woody slope; Cimetta (1650 m) on a ridge above Brione. on the eastern side of Lake Maggiore: Campo dei Fiori (1226 m), in a woody area; which came into operation in summer 1988 and 1989.
(c) stations at high altitudes: --
3. Experimental procedure 3.1. Measurement stations and instrumentation The measurement stations aod their characteristics are listed in Table 1 and their location is shown in Fig. 1. The different stations are managed by different organisations: the Italian Electricity Board (ENEL); Joint Research Centre (JRC), (Leyendecker et al., 1988-1992); Department of the Environment, Ticino, Switzerland (DOE-CH); Amt der Tiroler Landesregierung, Austria (ATL-A). Their location was chosen according to the following criteria; air quality, transboundary transport of pollution or forest damage. The stations may be grouped as follows:
171
(d)
in the eastem Alps: Brennero (1870 m), on the southern slope; Seefeld (1730 m), Mayrhofen (1910 m) and Seegrube (1960 m) on the northern slope. The Brennero station is sited in the Isarco valley, near the pass; Seegrube is above Innsbruck, in the main Inn valley; Seefeld and Mayrhofen are in side valleys.
alpine stations: - - Sestriere (2555 m), Plateau Rosa (3500 m) and Jungfraujoch (3580 m), which is a station of the Swiss NABEL network.
Because of the different affiliations of the stations (Table 1), the instuments used can differ considerably; most of them were equipped with an NO x monitor and meteorological instrumentation. VOC were only measured at a few sites. All ozone monitors were commercial instruments
172
S. Sandroni et al. /Sci. Total Environ. 156 (1994) 169-182
Table 1 Alpine and pre-alpine ozone measurement stations Ref. Fig. 1
Stations
Country
Type
Altitude (m asl)
Years
Institution
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Milano Brera Ispra Chiasso Lugano Mendrisio Aurigeno Brione Mottarone Campo dei Fiori Cimetta Seefeld Brennero Mayrhofen Seegrube Sestriere Plateau Rosa Jungfraujoch
I I CH CH CH CH CH I I CH A I A A I I CH
Urban Semi-rural Urban Urban Suburban Rural Rural, slope Woody Woody Mountain Mountain Mountain Mountain Mountain Alpine Alpine Alpine
138 209 230 290 300 340 490 920 1226 1650 1730 1870 1910 1960 2555 3500 3580
1987-1991 1987-1991 1988-1991 1990-1991 1990-1991 (p) 1989-1990 (p) 1989-1991 1987-1991 (p) 1988 and 1989 (p) 1991 (p) 1987-1991 1987-1991 (p) 1987-1991 1990-1991 1989-1990 (p) 1989-1990 (p) 1987-1991
ENEL-CRTN JRC, CEC DOE-CH DOE-CH DOE-CH DOE-CH DOE-CH ENEL-CRTN JRC, CEC DOE-CH ATL-A ENEL-CRTN ATL-A ATL-A ENEL-CRTN ENEL-CRTN NABEL
(p), incomplete data series.
acid-buffered KI) or gas-phase titration of NO/NO2; this data quality procedure was valid for all monitors. At low elevations, instruments were checked 2-3 times a month, however, in the Alps the severe meteorological conditions and the limited accessibility during the cold season drastically reduced the frequency of controls (2-3 times
(Dasibi, Environment or Thermoelectron) based on UV absorption. These monitors have a detection limit of 2 ppbv and an accuracy of + 20% for ozone values > 20 ppbv; their precision is + 20% at 30 ppbv. They were calibrated routinely by an ozone generator; manual tests were performed using a wet chemistry method (boric
°
•
6
. 4
~
f
%
'o
I
~.J.
~
"--
.
--
/ f ~ --
--' ~ w I
" -~
~) ('~'~" _. - I _ * ]b
~'~
T .L
f'~ ......
..// ~') --
~ ~"
~
f •J
~
~
~
~ ~
~'~
•
"
1
lMilan*ere,~
~ il~ Pr ~ IL~ ~
I I I I i J
tugano § IVendr Jsio 6 Aur i geno 7 Br bone 8 F/or tarone 9 Campo del Flora 10 Clmet t a
~
I
II Seefeld
~'[ ~L ~
] [ l t [
12 Brennero 13 Mlyrhofen
_...d,"
~ e'l " ~ .
I
4
14 15 16 l?
Seegr ube Sest riere Plateau Rosa Jungfraujoch
Fig. 1. Location of the alpine and pre-alpine ozone measurement stations.
/
, /
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
a year) and, as a consequence, the number of validated measurements. Comparisons between JRC and ENEL monitors were made 1-2 times a year, twice between DOE-CH and JRC while only one comparison was made between ATL-A and ENEL. An agreement with a reference monitor within 5 ppbv was considered as acceptable. The distinct.ion between urban, suburban, semi-rural and rural stations is arbitrary: a 'rural' station should be far from emission sources. In reality all stations are more or less influenced by anthropogenic emissions; an alpine station may also be contaminated, to a lesser extent, for short periods of time.
3.2. Data base The original data format was different for each network; the concentrations were given in ppbv or in /zg.m -3, the unit required by national legislation; the averaging time was 30 min to 1 h. All data was organized in a data base. The chosen measurement unit was the ppbv, since this unit allows a direct comparison of data gathered at different altitudes without any correction factor. The chosen time unit was the hour. Daily and monthly means were computed from half-hourly or hourly mean values. The data base was organized in hourly, daily and monthly mean values. Considerable work was needed to validate the data recorded at the individual stations, where the periodical instrumental checks were used as a reference. The data base is incomplete, particularly for some stations as given in Table 1. Each station was put into operation or disconnected at a different date and there are gaps in the data series. The data series of Sestriere and Plateau Rosa are very incomplete; furthermore, only daily means and maxima were given for the Austrian stations (Seefeld, Seegrube and Mayrhofen) from the Amt der Tiroler Landesregierung (1987-1991) and monthly means for Jungfraujoch from BUWAL. Data series for stations in the plain and at intermediate altitude have few interruptions. Meteorological information was locally available at most stations. WMO charts and Meteosat images were available from the Meteorological Service of the Italian Air Force at Milan-Linate and Udine,
173
central and eastern Po valley, where routine rawinsondes were launched at 6- and 12-h intervals. 4. Results and discussion
4.1. Monthly and daily averages The monthly and yearly means for 1990 and 1991 for different stations are listed in Table 2. If validated daily data was partially available, the mean value is given in parentheses. The annual mean level increases with altitude, ranging from 20-25 ppbv in the plain to 45-50 ppbv at 3500 m. The monthly and daily mean values are similar for alpine stations above 2500 m. At high altitude a good agreement ( > 0.85) is observed for monthly means and daily maxima at the Austrian stations (Seegrube, Mayrhofen and Seefeld) only, independent of their different siting. Somewhat lower values compared with the Austrian stations have been observed at Brennero, since the site is partially influenced by traffic below (Camuffo et al., 1991). Unexpected levels are observed at intermediate altitudes, where in the warm season the monthly mean may be greater than those at high altitudes. Higher values than expected were observed at Brione (490 m), Mottarone (920 m) and Cimetta (1650 m). In July 1991 in particular, these sites suffered from ozone levels 10-20 ppbv higher than alpine stations. A low correlation ( > 0.70) is observed among these stations, even if they are located on the same mountain slope (Brione and Cimetta). Finally, a poor correlation is observed for stations in the plain, since they are influenced by local emissions and climate.
4.2. Seasonal and daily patterns The seasonal pattern is influenced by the local environment and altitude. In general, at the alpine stations the yearly maximum is observed in spring (April-May), the stratospheric contribution dominating, while at intermediate and at low altitudes it is observed in July-August. At intermediate altitudes and occasionally in semi-rural areas of the plain (e.g. in 1988) the seasonal pattern may show a second maximum in spring if many episodes of advection from the free troposphere have occurred. A pattern with a sharp maximum
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
174
in May was observed at Arosa (1840 m) in 1989-91 (Staehelin et al., 1993); a similar pattern occurred at Seefeld and Mayrhofen in 1989, but not in 1990 and 1991. At low and intermediate altitudes the photochemical activity is the dominant contributor. These patterns agree with the previous observations by Logan (1989) and Janach (1989). As regards urban, suburban and semi-rural stations in the plain, the pattern is related to the local emissions and climate. Generally, the daily pattern follows a bell shape distribution, with a minimum (10 ppbv) in the early morning and a maximum between 14:00 and 18:00 h CET. In the cold season (November to February) the Po valley has stable radiative inversions (frequently 50-200 m high) with dense fog and daily ozone levels
were lower than 10-15 ppbv. In the cold season, Lugano enjoys a milder climate and more sunny days than Milan; therefore, Lugano has a higher mean ozone level. In the warm season high values are reached downwind of an urban plume, e.g. Mendrisio. The bell-shape daily course is progressively smoothed as we move to higher altitudes, where the daily minimum approaches the maximum. At intermediate altitudes the daily and seasonal patterns are largely influenced by the evolution of the mixing layer. A comparison of the seasonal patterns through the year at sites at different altitudes are given in Fig. 2. In the cold season Mottarone (920 m) and Brennero (1870 m) are above the mixing height and their ozone
Table 2 Monthly and yearly mean ozone concentrations (in ppbv) for the years 1990/1991. In case of a limited number (40 < x < 90%) of validated daily data, the monthly mean is given in parentheses m (asl)
J
F
M
138 209 230 290 300 340 490 920 1730 1870 1910 1960 2555 3500
(0) 2 4 14 1 6 -7 . . . 3 11 17 23 (26) 37 43 44 (25) (39) 40 42 38 44 (37) (39) 31 --
12 23 11 13 . 25 36 (45) 48 44 44 49 (51) 43
3580 138 209 230 290 300 490 920 1650 1730 1870 1910 1960 3580
38 1 4 2 3 2 17 18 38 39 (34) 42 40 36
50 13 18 8 11 12 30 (34) -40 43 47 47 43
A
M
J
J
A
S
O
(50) 39 48 55 50 41 60 67 58 52 58 61
41 36 38 49 43 (39) 54 62 56 60 58 60 . .
21 22 19 31 24 -40 24 42 47 44 44
(8) 10 5 8 8 -19 15 37 -44 41
N
Year
D
1990
Milan Brera Ispra Chiasso Lugano Mendrisio Aurigeno Brione Mottarone Seefeld Brennero Mayrhofen Seegrube Sestriere Plateau Rosa
17 30 14 26
36 32 26 40
31 39 -53 46 50 50 -51
34 48 -55 (56) 57 59 (56) 55
32 31 27 38 40 30 46 -50 51 53 57 . 50
56 24 34 17 29 28 46 49 -52 54 58 51 50
63 30 43 27 38 38 48 53 -52 54 53 51 52
57 39 42 31 39 38 46 52 (53) 48 53 50 51 49
.
.
. .
.
. .
3 6 2 5 5
-19 18 34
--
3 6 2 4 4
--
(19) 24 17 25 (25)
(27)
19 15 34
35 (34) 46
(31)
(45)
42 38
39 37
48 48
--
--
(46)
--
--
(46)
34 1 8 2 5 6 16 29
(49) 21,5 25 19 25 24 36 (42)
(32)
(50)
38 35 38 35 43
45 45 49 44 45
1991
Jung~aujoch Milan Brera Ispra Chiasso Lugano Mendrisio Brione Mottarone Cimetta Seefeld Brennero Mayrhofen Seegrube Jungfraujoch
38 8 10 6 10 9 26 40 44 42 42 44 44 38
59 53 52 49 61 55 65 70 67 53 53 51 51 50
59 52 39 46 54 53 59 61 60 56 56 51 51 52
50 26 24 24 33 29 44 (42) (55) 47 49 45 45 43
-8 14 8 9 10 19 29 -37 38 34 35 43
35 3 8 4 4 5 14 24
-34 28 32 32 40
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
levels are nearly the same, but quite different to Ispra (209 m). By contrast, in the warm season, when there is a strong temperature gradient between valley and mountain, ozone-rich air masses in the plain are lifted up and driven as an 'ozone front' towards the mountain ridges. In the wannest hours of the day the convective motions balance the ozone level at Mottarone and Ispra (Fig. 2). Close to the mountains the polluted air masses coming from the plain mix with clean air when there is strong UV radiation; hence, ozone can also be locally formed. Near the mountains, the ozone daily course may show two peaks, the first (at 14:00-15:00 h CET) was caused by local photochemical processes, the second (at 17:00-19:00 h CET) was related to air masses transported from the plain. This front is responsible for an ozone accumulation at altitudes up to about 1800 m. Airborne measurements made in the Ticino valley (Paffrath et al., 1986; MetAir, 1992) and in the Isarco valley near Brennero (Schlager et al., 1992), have confirmed the presence of such a diurnal front: the positive concentration gradient from the ground to about 1000 m observed in the early morning in the valleys decreases progressively with time and is dissolved before noon. At intermediate altitudes (Brione, 490 m, Mottarone, 920 m, and to lower extent Cimetta, 1870 m) ozone daily maxima may be higher than at 3000
m. These observations disagree with measurements taken by Wolff et al. (1987) on a mountain slope 550 m high; by contrast, Reiter et al. (1987) observed higher levels at Wank, 1780 m above sea level (asl), than at Zugspitze (2964 m asl). Along the advection trajectory from the Po valley to the Alps, a time-lag is observed for the daily ozone maximum. Fig. 3 compares the daily evolution in a summer's day at stations sited in the Lake Maggiore basin at different altitudes: the maximum was observed at 16:00 h CET at Ispra (209 m) and Mottarone (920 m) but at ~ 23:00 h CET at Campo dei Fiori (1226 m). This indicates, the valley breeze started at 10:00 h CET and reversed its direction at ~ 19:00 h CET. At Campo dei Fiori the ozone daily course is reversed, with a minimum around noon and a maximum during the night. This maximum may also originate from a subsidence of air masses. Occasionally at the bottom of a valley, converging flows descending from the opposite slopes during the night, can give rise to an ozone peak (Broder and Gygax, 1985). An interesting case study is Brione, a village at 490 m asl above Lake Maggiore, on a natural balcony protected from the cold northern winds. Brione suffers from high ozone exposure throughout the year: in 1991, for instance, the hourly mean level of 60 ppbv was exceeded 1251 times, while the 120 ppbv level was exceeded 23 times.
80
100 ¸ - -
8 Mottarone I J 9 CampodeiFior/ _ _ 2 Ispra
I~pra(2oom) 70.
~
175
Brione (480m)
60-
Mottarone (920m)
50"
Brennero (1870m) --x-Jungfrau (3475m)
40-
80
"~ 6O ~ ~
/., ,. ,.,
40
~ 30/' 2O
'\,...
/
20"
',..
/
...~
i
10-
2
4
6
8
10
12
14
16
18
20
22
24
CET J
F
M
A
M
J
J
A
S
O
N
D
CET
Fig. 2. Seasonal evolution of ozone at different altitudes.
Fig. 3. Diurnal patterns at Ispra (209 m), Mottarone (920 m) and Campo dei Fiori (1226 m) under southern wind conditions (26 August, 1989).
176
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
7 IIIrioa* - -
|
l|p~
,0-
cer
f~J
b
.
c_,,-r
C
'..
u
•
ro
t 2cL.r
~4
Te
~.
~
~
~
Fig. 4. Comparison of ozone patterns at Brione (490 m) vs. Ispra (209 m) and Brennero (1870 m) on (a) 28 January, (b) 13 July and (c) 15 October, 1991.
The local ozone course depends on the evolution of the radiative inversions and the mixing layer. Fig. 4 shows some daily courses at Brione, a site in the plain (Ispra, 209 m) and an alpine station (Brennero, 1870 m). On a sunny winter day the lower inversion at ~ 200 m rises slowly up to ~ 600 m at 12:00 h CET; the ozone course, typical of a mountain station during the night, changes to a valley station during the day; around 19:00 h CET a new inversion is formed and Brione behaves as a mountain station (Fig. 4a). In the warm season (Fig. 4b) the site is merged into the mixing layer during the day but it may be above it during the night; as a consequence, during the day the ozone pattern is similar to Ispra, being affected by ozone and precursors carried by the valley breeze, but at night it behaves like a mountain station; generally, these changes occur at about
10:00 h CET and 21:00 h CET. On a cloudy day (Fig. 4c) Brione is above the mixing layer and the ozone daily course is similar to that of a mountain station. By contrast, the ozone courses at Aurigeno (300 m asl), sited in an isolated side valley, are quite similar to a semi-rural site in the plain like Ispra. Particular daily courses occur during foehn episodes. In the pre-alpine region ozone concentrations comparable with photochemical processes may be measured in conjunction with intense and persistent downslope winds (northern foehn). These winds, particularly evident in the cold season (November to April), blow from N-NW with an average speed of 10 m. s-1 and with gusts up to 30 m. s-1. They may last from a few hours to 2-3 days and are characterized by a drop in relative humidity and pressure and an increase in temperature. These events are also mirrored by comparable ozone maxima in the plain and daily means in altitude. By this process ozone-rich air masses are transported from the free troposphere to the ground. An example of this situation is given in Fig. 5, which refers to February 8-9, 1988. A sharp increase of ozone concentration occurred simultaneously with the strong northern wind (foehn), which started during the night, at Ispra. During this event, the levels at Ispra and Mottarone are similar, supporting the hypothesis of a dynamic mixing of free and low troposphere. In the pre-alpine region foehn events occur 30-40 days a year: they are particularly strong in the cold season (Gandino et al., 1990); in summer they are weak and the advected ozone level may be lower than that formed photochemically. The contribution of these events to the monthly means is negligible, except in the case of several episodes in the same month, e.g. in February and March 1988 (7 and 6 events, respectively). Air masses from the lower stratosphere may be transported downward along isoentropic surfaces (jet streams), as described elsewhere (Reiter et al., 1987). The stratospheric intrusions can be seen on Meteosat images and charts at 500 hPa; they last for 1-2 days and are characterised by increased levels of cosmogenic radionuclides, especially 7Be; at 3000 m the 7Be concentration may increase from 90 up to 200-300 fCi. m-3. In
S. Sandroni et al. / Sci. Total Enciron. 156 (1994) 169-182
700 t/
t I I
8 Mmtar~me
......
2 Ispta
80
4o
20
4
8
12
16
20
24 CET
4
8
12
16
20
24
Fig. 5. Ozone course during a foehn event during the night from 8 to 9 February, 1988 at Ispra (209 m) and Mottarone (920 m). Because of the event the wind speed increased from 1 to 16.6 m . s -1 while the pressure drop was 10 hPa.
1988 only one episode with a 7Be increase from 80 to 162 fCi.m -3 was recorded at Mottarone (920 m) on February 13. At lower altitudes no intrusion has been recorded; a foehn event may occur simultaneously. According to Reiter (1990), the lowest elevation to which a stratospheric intrusion can penetrate in the alpine region is about 1400-1600 m; at 3000 m their contribution to the monthly mean is of the order of 1-2 ppbv ozone. 4.3. Ozone levels at intermediate altitudes
As previously mentioned, in the warm season higher ozone levels may be observed at intermediate than at high altitudes. A statistical evaluation of ozone distribution as 8-h average concentration for the time intervals (00:00-08:00 h CET)
177
and (09:00-17:00 h CET) at Ispra (209 m), Brione (490 m) and Mottarone (920 m) in the coldest and the warmest months of 1991 is given in Tables 3 and 4. In the cold season the ozone distribution at Brione is similar to that at Mottarone; furthermore, while the day to night gradient is positive at Ispra and nearly absent at Mottarone, it is negative at Brione because of the advection of ozonepoor air from the plain (Table 3). In the warm season (Table 4) the day-to-night increase is quite sharp at Ispra and Mottarone but lower at Brione. A reference parameter for high ozone levels is the 'excess ozone' (Simpson, 1993). The term excess ozone means the sum over all hours in a given time period (e.g. 1 month) of all ozone concentrations in excess of 75 ppbv; it is expressed as ppbv. h. month-1. The 75 ppbv corresponds to the 1 h critical level of ozone, above which vegetation damage is observed, as adopted by the UN-ECE. The computed values of excess ozone for some representative stations for AprilSeptember 1991 are given in Table 5. At low and intermediate altitudes these values are quite high, then they decrease with altitude from about 1800 m. The highest values have been observed at Brione (490 m) in April and July. Another reference parameter for high levels is the frequency of ozone levels exceeding health and vegetation protection thresholds (Angle and Sandhu, 1989; Beck and Grennfelt, 1993). At present, Switzerland, Austria and Italy have different legislation; the EEC threshold value for vegetation protection (100 ppbv as 1 h average; Directive 92/72/EEC) can be considered a good com-
Table 3 Distribution of ozone concentration as 8-h average in January 1991 Concentration range (ppbv)
from 00:00 to 08:00 h CET Ispra
Brione
Mottarone
Ispra
Brione
Mottarone
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80
30 1 0 0 0 0 0 0
8 7 11 5 0 0 0 0
5 13 12 1 0 0 0 0
23 8 0 0 0 0 0 0
8 17 6 0 0 0 0 0
3 13 12 3 0 0 0 0
from 09:00 to 17:00 h CET
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
178
Table 4 Distribution of ozone concentration as 8-h average in July 1991 Concentration range (ppbv)
from 00:00 to 08:00 h CET Ispra
Brione
Mottarone
Ispra
Brione
Mottarone
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 100-120
7 11 6 5 2 0 0 0 0 0 0
0 0 1 6 8 4 5 6 1 0 0
0 0 0 2 8 6 9 3 3 0 0
0 0 0 0 5 3 4 4 6 3 6
0 0 1 2 8 3 5 8 2 2 0
0 0 0 0 3 3 5 8 6 6 0
from 09:00 to 17:00 h CET
24,1H
promise. The computed monthly values for the period April-September 1991 are listed in Table 6. The occurrence of ozone concentrations exceeding acceptable levels have been limited up to 1000 m, high values have been observed at Brione in July and at Mottarone in August.
4.4. Photochemical pollution episodes The selection of criteria for definition of an ozone episode is somewhat arbitrary, particularly in terms of ozone concentration (Logan, 1989). For instance, an episode may be defined as the simultaneous occurrence of an average value exceeding 70 ppbv from 10:00 to 16:00 h LT at four or more sites for 2 or more consecutive days; an alternative definition is the simultaneous occurrence of the daily maximum value exceeding 80 ppbv at four or more sites for 2 or more consecutive days. Here we have focused our attention on
the highest levels which occurred in the years under consideration. During the warm season in the alpine regions hourly ozone means frequently exceeded the limit values allowed by the national legislation, i.e. 60 ppbv in Switzerland and Austria and 100 ppbv in Italy; these values should not be exceeded more than once a year. In 1991 this value was exceeded 758 times at Mendrisio and 1251 times at Brione. At Ispra the maximum hourly means were 142 ppbv in 1987 (July 26), 135 ppbv in 1988 (July 20), 135 ppbv in 1989 (July 19), 128 ppbv in 1990 (August 3) and 173 ppbv in 1991 (July 14). The maximum hourly mean in the region (185 ppbv) was recorded at Chiasso on July 23, 1990. Generally, episodes lasted 4-5 days and were limited to one basin; sometimes they extended over the western and eastern alpine regions and in a few cases levels remained high throughout the night.
Table 5 Monthly values of excess ozone (units: 100 ppbv. h. m o n t h - 1 ) for April-September 1991 at some representative stations
Ispra Chiasso Lugano Mendrisio Brione Mottarone Brennero
m (asl)
A
M
J
J
A
S
209 230 290 300 490 920 1870
0.2 0 0 0.1 2.3 0.8 0
9.0 3.2 6.8 7.3 6.5 5.5 0
16.3 8.2 9.0 7.5 11.5 8.7 2.0
54.3 41.5 45.3 47.4 57.3 45.9 1.1
16.8 28.4 17.8 39.8 21.1 7.3 --
5.5 8.7 6.1 9.9 11.7 -0.7
S. Sandroni et al. /ScL Total Environ. 156 (1994) 169-182
179
Table 6 Frequency (in %) of hourly values exceeding the EEC vegetation protection threshold (100 ppbv as 1 h average) for April September 1991
Ispra Brione Mottarone Brennero Seegrube
m (asl)
A
M
J
J
A
S
209 490 920 1870 1960
0 0 0 0 0
0.5 0.3 0 0 0
2.6 1.5 0.3 0 0
13.0 12.4 1.9 0 0
2.1 2.4 9.8 0 0
0.3 0.1 0 0 0
Following a request from regional authorities, an investigation of the possible relationship between these episodes and meteorological conditions (Davies et al., 1992) and the emission of precursors. In particular, in the Italian-Swiss lake region, meteorological data (and data for a few chemical parameters, mostly NO x) have been routinely available at different altitudes and significant mechanisms could be identified.
4. 5. Meteorological conditions. All the episodes we observed in the years 1987-91 were associated with anticyclonic conditions lasting some days, intense solar radiation and weak winds (1-2 m.s -1) from the south. In July 1989 a slow-moving anticyclone dominated over central Europe up to the 19th, when its centre moved to England allowing the transit of a weak front. A high pressure in the upper layers had a stabilizing effect on the atmosphere. The mixing height (calculated according to Holzworth, 1974) had an initial level of 1600-1700 m, then it rose to 2600 m on the 17th. A dry and warm layer, typical precursor of a subsidence, extended up to 3600 m. During the days from the 16th to the 19th the subsidence moved progressively down to the ground carrying clear air masses from the upper layer to mix with polluted air masses in the lower layers. During the days of the 16th-19th, ozone daily maxima increased progressively and reached 135 ppbv at Ispra and similar levels at Aurigeno and Brione (Fig. 6). The maximum was observed at around 16:00 h CET, about 4 h after the maximum temperature (32°C) and solar radiation (918 W/cm2); wind from the south at 1.4 m . s -1.
-
-
A series of episodes occurred in July and August 1990. From the beginning of July to August 6 a persistent anticyclone was observed at 500 hPa centred over North Africa and extending over the alpine region; during the same time period a large high-pressure field ( P > 1013 hPa) occurred at 850 hPa over western Europe. The mixing height rose up to 1500-2000 m, while its minima occurred together with a subsidence (July 23) and a front transit (August 6). Fig. 7 shows the progressive ozone increase from 18 to 23 July, together with an increase in temperature and solar radiation, at three stations of the Lake Maggiore basin up to the level of 185 ppbv at Chiasso on the 23rd. With a time lag of 1-3 days high levels were observed both in the western and the eastern alpine regions, e.g. during the last days of July (Ispra, 121 ppbv and Mottarone, 115 ppbv; Seefeld, 97 ppbv; Seegrube, 98 ppbv and
7 Brion¢
140
........... •. - ~ .
2
•
[spra
6 AmH|eno
120
100
SO
4O
",~,
:
i
¢"
ii
CET
Fig. 6. Ozone course during a photochemical smog episode on 16-18 July, 1989 at Ispra (209 m), Aurigeno (340 m) and Brione (490 m).
180
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182 2OO
/~,.,
Mayrhofen, 116 ppbv). A persistent anticyclone with a few interruptions also occurred in summer 1991. The thermal mixing height was almost stabilized at about 1800-1900 m, reaching 2400 m. During that period a dry layer with irregular altitudes was observed. Ozone maxima coincided with subsidences that occurred on July 14 (173 ppbv ozone and 17 ppbv PAN at Ispra), July 23 and on August 3. The vertical dynamics of the troposphere is likely to be one of the main parameters controlling the occurrence of high ozone levels in the alpine regions. In particular, the altitude, the thickness and the vertical motion of the dry (and relatively warm) upper air layer determine the evolution of the subsidence. These descending motions are an ideal carrier of air masses from the lower free troposphere to ground level. A direct relationship between the increase in ozone and the mean thermal gradients has not yet been established.
troposphere. At low and intermediate altitudes monthly maxima occur in summer, together with intense photochemical activity; high levels in spring are associated with advections from free troposphere and stratosphere. Particular attention has been paid to intermediate altitudes, i.e. from 400 to about 1800 m, where in the warm season monthly mean levels are comparable, or even 10-20 ppbv larger than at higher altitude levels. Also the levels of excess ozone and the frequency with which they exceed health and vegetation protection thresholds are higher than expected for that altitude. The reason for the higher levels is twofold, (1) the advection of ozone a n d / o r precursors from the plain (ozone front) towards the areas of high ground during the day and (2) the persistent high levels during the night. Sites at intermediate altitudes behave as mountain sites during the night and plain stations during the day. Since the photochemically produced ozone levels in the plain are higher than at 3000 m, the mean daily level at intermediate altitudes may exceed the corresponding one at high altitudes. Physical processes play an important role in redistributing ozone levels over such a complex terrain. The main mechanisms are (a) the horizontal transport on the regional scale, (b) the convective motions of thermal origin or associated with a dynamical instability. Stratospheric intrusions (c) have a negligible incidence below 1800 m, but downslope transport from free troposphere (foehn events) is common in the cold season. Above ~ 1800 m, the ozone course is weakly influenced by photochemical activity at the bottom of the valley. A reverse ozone daily course may be observed > 1000 m, in comparison with the valley. Finally, high-levels seem to be well correlated with a subsidence of dry upper layer in anticyclonic conditions.
5. Conclusions
Acknowledgements
In the alpine regions ozone increases with altitude, the yearly mean ranging about from 20 ppbv in the plain to 50 ppbv at 3000 m. The seasonal course is largely influenced by synoptic meteorological conditions and by exchanges with free
This study has been supported by the CNRENEL Project - - Interaction of energy systems with human health and the environment - - Rome, Italy. We would also like to thank W. Leyendecker, JRC Ispra, the ENEL-C.R.T.N., Milan,
ISO . . . . . . . . . . . . . . .
Brlone lspra
. . . . Ch,~,o
~
/ a
~
/
'~
r
8o 6o
io 2o
CET
Fig. 7. Photochemical smog episode on 18-23 July, 1990 at a semi-rural (Ispra, 209 m), an urban (Chiasso, 230 m) and a rural site (Brione, 490 m).
S. Sandroni et al. / Sci. Total Environ. 156 (1994) 169-182
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