Atmospheric Environment 34 (2000) 5191}5198
A climatology of regional background ozone at di!erent
elevations in Switzerland (1992}1998)
S. BroK nnimann *, E. Schuepbach , P. Zanis , B. Buchmann, H. Wanner
Institute of Geography, University of Berne, Hallerstrasse 12, CH-3012 Berne, Switzerland
Swiss Federal Laboratories for Materials Testing and Research EMPA, U$ berlandstr. 129, CH-8600 Du( bendorf, Switzerland
Received 13 December 1999; received in revised form 6 March 2000; accepted 24 March 2000
Abstract
The ozone records of several monitoring stations in Switzerland from 1992 to 1998 are investigated with respect to the
variability observed during regional background conditions, i.e. conditions with little detectable local or regional-scale
in#uences as evident by NO and CO concentrations. The sites cover di!erent altitudes between 490 and 3600 m asl.
V
They are characteristic of near-surface conditions, the top of the planetary boundary layer or residual layer, the complex
atmosphere in an alpine valley, and the free troposphere. The results reveal a distinctly di!erent ozone variability (diurnal
cycles, seasonal cycles, trends) during regional background conditions compared to all days. The estimated annual
average ozone concentration under these conditions is between 33 and 50 ppb, dependent on altitude, with a spring
maximum and an autumn/winter minimum. Di!erences in background ozone are found depending on the synoptic
weather type. For all sites a positive ozone trend is calculated for background conditions that is larger than for all data.
For the latter, the trends appear to be stronger positive for the last 7 years than for the last 11 years. 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: Background air; Ozone; NO ; Photo-oxidants; Carbon monoxide; Synoptic weather types
V
1. Introduction
The concentrations of lower tropospheric ozone in
Europe exhibit di!erent temporal and spatial scales of
variability. To evaluate the e!ect of emission control
strategies taken in one region it is important to know the
contribution of mechanisms on di!erent scales to the
observed temporal development of surface ozone (Jacob
et al., 1999). These mechanisms comprise hemispheric or
continental scale chemical and transport processes, in#ux
from the stratosphere, regional scale ozone formation,
mesoscale meteorological processes and regional wind
systems, urban plumes, local ozone titration, and depos-
* Corresponding author. Fax: #41-31-631-8511.
E-mail address: broenn@giub.unibe.ch (S. BroK nnimann).
Now at: Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Campus Box 149, 54006 Thessaloniki,
Greece.
ition. Several studies have shown that di!erent features of
temporal ozone variability emerge from these mechanisms, overlapping in a single ozone record. By splitting
the ozone time series of Mace Head, Ireland, by the
origin of air masses, Simmonds et al. (1997) found positive and negative trends in polluted and unpolluted air
masses, respectively. At the high-alpine site Jungfraujoch
(3580 m asl) in the Swiss Alps, di!erent frequency distributions, changes in the seasonal ozone cycles, and nonstationary trends were found when splitting the ozone
record by the two main wind directions, wind speed and
time windows (Schuepbach et al., 1999, 2000).
In this study, the focus is on lower tropospheric ozone
in Switzerland during relatively unpolluted conditions,
by which we understand air masses with little detectable
local to regional scale in#uences, often referred to as
`background conditionsa. The term background ozone is
loaded with some confusion. Following Kourtidis et al.
(1997), `natural backgrounda and `regional backgrounda can be distinguished. Natural background
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 1 9 3 - X
S. Bro( nnimann et al. / Atmospheric Environment 34 (2000) 5191}5198
5192
means ozone generated chemically in the troposphere
from biogenic or geogenic emissions plus ozone transported from the stratosphere. `Regional backgrounda
ozone is the ozone level over a large area
(1000;1000 km) produced by the mixing of air masses
of di!erent origin outside and inside the de"ned area
(Kourtidis et al., 1997). Ozone concentrations under the
so-called background conditions have been widely
studied at di!erent locations in Europe (Scheel et al.,
1997 and references therein). One motivation for these
studies is to obtain knowledge about ozone budgets
in relatively undisturbed conditions. Yet, even these
relatively undisturbed conditions have been anthropogenically perturbed for decades. Surface ozone concentrations have roughly doubled since pre-industrial times
(Volz and Kley, 1988). Since about 1950 ozone concentrations have increased in much of the lower and midtroposphere at northern midlatitudes by 1}2% yr\,
with a suggested levelling o! since about 1980 (Staehelin
et al., 1994; Logan et al., 1999).
Background conditions are often addressed by analysing ozone at remote or high altitude sites, which are far
from emission sources, or coastal sites with distinct clean
air wind sectors. The present study makes use of six time
series measured at di!erent elevations and distance from
emission sources in Switzerland to investigate the spatiotemporal variation in ozone concentrations that is related to regional background conditions. The study focuses on the altitude dependence, diurnal and seasonal
cycles, the dependence on synoptic weather types, and
possible trends.
which O , NO , and CO are used in this study. The
V
measurements are described in more detail by EMPA
(1994). The sites are displayed in Fig. 1, the most important features are given in Table 1. The locations cover
di!erent altitudes between 490 and 3600 m asl. Payerne,
a rural site on the Swiss Plateau, and LaK geren on a
forested slope are relatively close to emission sources.
Chaumont and Rigi, at around 1100 m asl, have little
local in#uence but can be a!ected by polluted air masses
from the Swiss Plateau. The Davos site is on a forested
slope in an alpine valley; it can be in#uenced by emissions
from the nearby road across the FluK ela pass, but is
generally only weakly polluted. Finally, Jungfraujoch
is a high-alpine site with negligible local emissions. Each
site is characteristic of either near-surface conditions, the
top of the planetary boundary layer or residual layer,
2. Data and methods
Time series from six sites in the Swiss Air Quality
Network NABEL are investigated. The data comprise
di!erent chemical and meteorological measurements of
Fig. 1. Map showing the location of the monitoring sites used in
this study. Shaded areas are located above 1000 m (light) and
2000 m asl (dark), respectively.
Table 1
Characteristics of the measurement sites used in this study. Ozone and NO are annual averages from 1992 to 1998. SP"Swiss Plateau
V
Site
Altitude (m asl)
O (ppb)
NO (ppb)
V
Description of the sites
Payerne
494
24.7
14.4
LaK geren
732
31.1
10.4
Rigi
Chaumont
Davos
1030
1140
1669
39.0
40.9
39.2
5.7
4.9
3.2
Jungfraujoch
3580
50.2
0.5
Rural, on the SP, 400 m from road (ca 7000
vehicles day\)
Forested slope, on a 45 m tower, 350 m above the SP,
4 km from highway on the SP (ca 80'000 vehicles day\),
S exposition
Rural, on a terrace, 600 m above the SP, NW exposition
Rural, on a ridge, 700 m above the SP, SE exposition
Forested slope, on a 35 m tower, 100 m above valley
#oor, 500 m from road (ca 3000 vehicles day\)
High alpine, on a saddle, NW/SE wind channelling
S. Bro( nnimann et al. / Atmospheric Environment 34 (2000) 5191}5198
the complex atmosphere in an alpine valley, or the free
troposphere, respectively.
The NABEL network has experienced di!erent
changes in instrumentation and quality assurance
procedures. Shift inhomogeneities are present in the
ozone data before and in 1991, when the network was
modernised and extended to include Chaumont and Rigi.
From 1992 onwards, however, the quality is good and no
further inhomogeneities were found for the sites examined in this study (BroK nnimann et al., 2000; Zanis et al.,
1999; Schuepbach et al., 2000). For this reason, we restrict
our analysis to records from 1992 to 1998. NO measureV
ments at all sites but Jungfraujoch are overestimated due
to cross-sensitivities (BUWAL, 1994).
Regional background conditions are selected with
chemical "lters such as NO (at all sites) and CO concenV
trations (only at Chaumont, since January 1997, and at
Jungfraujoch since June 1996). No procedure was found
to be adequate for all analyses, therefore, di!erent "ltering concepts and thresholds were used and the results
were compared. For brevity's sake, only a part of the
results is presented here. There are two main "ltering
concepts: setting an absolute threshold for daily mean or
median NO concentration enables a comparison of
V
samples at di!erent sites with respect to the chemical
composition of the air masses. In this case, using the same
threshold for all seasons means that the captured spatial
scales vary with season because of the changing lifetime
of NO (or NO ) and hence transport distances. For
V
W
most analyses, a relative threshold based on percentiles is
used, where the percentiles are de"ned separately for
each site and season. This method may be more adequate
for comparing di!erent seasons at a given site. However,
when using the same percentile for all sites the threshold
will capture a di!erent spatial scale at each site. The
5193
cross-sensitivities of NO measurements are not expected
V
to generally disturb their suitability as a "lter because
they tend to increase the lifetime of the measured pollutant mixture. The problem is that the cross-sensitivities
are not exactly known and may be variable and that
NO measurements at Jungfraujoch have less crossV
sensitivities than those at other sites. Nevertheless, the
e!ect of these uncertainties is reduced when relative thresholds are used.
The synoptic situation is analysed using the Alpine
Weather Statistics (AWS) (SMA, 1985; Wanner et al.,
1998). This method was used to study air masses and
transport conditions at high alpine sites before (cf.
Lugauer et al., 1998). We formed six groups: the convective types `higha and `lowa pressure, the advective types
(based on the 500 hPa wind direction) `northa, `westa,
and `south/easta (pooled), and the group `resta with all
other situations. For all analyses, climatological seasons
(MAM, JJA, SON and DJF) are considered, and annual
average denotes the average of the seasonal means.
3. Results
3.1. Ozone concentrations on days with low CO
Regional background air masses are expected to exhibit relatively low CO concentrations. Di!erent authors
report on annual mean concentrations of around 120}
130 ppb for North Atlantic background air and northern
mid-latitude air, respectively, with a distinct seasonal
cycle (Khalil and Rasmussen, 1994; Derwent et al., 1998).
Daily mean CO and ozone values for Chaumont (1997
and 1998) and Jungfraujoch (June 1996}1998) are displayed in Fig. 2. CO shows a spring or winter/spring
Fig. 2. Scatter plots of daily mean values of ozone versus CO at Chaumont (1997 and 1998, top) and at Jungfraujoch (June 1996}1998,
bottom) for climatological seasons.
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S. Bro( nnimann et al. / Atmospheric Environment 34 (2000) 5191}5198
maximum and a summer minimum. Annual averages
of around 125 ppb, comparable to North Atlantic
background air, are obtained at Chaumont (126.1 ppb)
and Jungfraujoch (126.3 ppb) when considering only
days within the lowest 10% (Chaumont) and 50%
(Jungfraujoch) of CO values in each climatological season, respectively. The corresponding annual averages of
NO are 3.13 and 0.24 ppb. This gives a broad indication
V
for the relation between threshold values or percentiles
and a spatial scale.
At low CO concentrations, daily mean ozone concentrations converge to values around 40 ppb (higher for
Jungfraujoch). These situations are supposed to indicate
background ozone levels. In the proximity of precursor
sources, increasing ozone concentrations are expected
with increasing CO in air masses that are subject to
photochemical ozone formation. This is the reason for
the strong positive correlation between CO and O at
Chaumont in summer. On the other hand, decreasing
ozone with increasing CO may indicate air masses that
have undergone ozone titration by NO emissions. In
spring and autumn, it seems that both types of air masses
frequently reach Chaumont. The contribution of titration becomes evident when replacing O with O
V
("O #NO ) (not shown). Many of the data points
that contributed to a negative correlation are shifted
towards a constant function. However, other mechanisms may also contribute (cf. Chin et al., 1994).
The correlation between daily mean values of CO and
NO at Chaumont is excellent in winter (0.88) and spring
V
(0.80) but somewhat worse in summer (0.54) and autumn
(0.66). A "lter based on NO will capture similar conV
ditions than a CO "lter. The situation is di!erent at
Jungfraujoch, where transport processes play a dominant
role, and the correlation between CO and NO is relaV
tively poor and depends on the weather situation (cf.
Forrer et al., 2000).
3.2. Ozone concentrations on days with low NO
V
For a comparison of the sites, we applied two "lters
basing on daily mean NO concentrations using an absoV
lute ()3 ppb) and a relative threshold ()10-percentile).
Fig. 3 (left, middle) shows the seasonal and annual
averages of the "ltered ozone values (daily means) as
a function of altitude. Also, incorporated are logarithmic
least-squares "ts to the annual averages. Compared to
the pro"le, one would obtain with all data (Table 1), we
"nd a smaller ozone increase with height for both "lters.
The calculated ozone values vary with height from
around 33 ppb at the surface (Payerne) to 49 or 50 ppb at
3600 m asl (Jungfraujoch). In previous studies, a background concentration of 40 ppb was estimated for February to April at Chaumont (BroK nnimann, 1999), and
Schuepbach et al. (2000) calculated background levels
around 40 ppb for Jungfraujoch (1988}1997). Simmonds
et al. (1997) report 35 ppb for unpolluted air masses at
ground-level at Mace Head and Scheel et al. (1997) isolated similar values for di!erent European sites.
The sample sizes produced by both "lters are similar in
the middle altitudes but very di!erent at low or high
elevations. Using the absolute threshold, 98.5% of
all days are selected at Jungfraujoch but only 1.5 and
1.2% at Payerne and LaK geren, respectively. While for
Jungfraujoch the results are similar (some high-ozone
days in spring and summer are excluded with the relative
threshold), di!erences are apparent between the two "lters for Payerne and LaK geren. The relative threshold
Fig. 3. Seasonal and annual averages of daily mean ozone concentrations at all sites as a function of altitude and season on days with
daily mean NO concentrations less or equal to the 10-percentile for each site and season (left and less or equal to 3 ppb (middle). `Yeara
V
denotes the annual average, de"ned as the average of the seasonal means. The solid lines are logarithmic "ts on `Yeara. Annual averages
of afternoon (12}16 CET) and nighttime (0}4 CET) mean ozone concentrations for days with daily mean NO )3 ppb and for all days
V
(right).
S. Bro( nnimann et al. / Atmospheric Environment 34 (2000) 5191}5198
produces a larger seasonal cycle, especially at LaK geren.
This is an indication that local to regional pollution
events cannot be fully excluded at all sites with the
10-percentile threshold.
Fig. 3 (right) shows vertical pro"les of annual averages
of afternoon (12}16 CET) and nighttime (0}4 CET) mean
ozone for all days and for the record "ltered with the
absolute threshold (3 ppb). Considering afternoon ozone
values, the di!erence between all days and the "ltered
days is small, with slightly higher ozone values during the
"ltered days in the lowest kilometre. The nighttime
values are lower for all days than for the "ltered days,
leading to a larger diurnal cycle. A possible reason for the
low nighttime values at Payerne for all days is ozone
titration by NO emissions in the stable nocturnal boundary layer. At LaK geren and Davos, the two tower sites on
forested slopes, the diurnal cycle is large for both all days
and the "ltered days. In addition to chemical ozone
destruction, a possible cause is enhanced dry deposition
of ozone within the drainage #ow. At the three sites
Chaumont, Rigi, and Jungfraujoch, which are not on
slopes, the afternoon}nighttime di!erences are small and
almost disappear in the "ltered days.
3.3. Seasonal ozone cycles for diwerent NO levels
V
For investigating the seasonal cycle, we used thresholds based on percentiles. The percentiles 2, 5, 10, 25,
5195
and 50 of daily median NO concentrations were calV
culated for each site and season, and daily mean ozone
concentrations were averaged for the respective fraction
of days below or equal to the di!erent percentiles. The
daily median NO value instead of the daily mean was
V
chosen because the frequency distribution of all halfhourly NO measurements of a day changes with the
V
degree of pollution. Thus, the median is more adequate
for comparing di!erent percentiles. Note that the spatial
scale captured with these "lters decreases with increasing
percentile.
The resulting seasonal cycles are shown in Fig. 4. The
curves are very similar at all sites when daily median
NO is below or equal to its 2-percentile for each season.
V
In agreement with the often cited spring maximum of
photo-oxidants in the unpolluted Northern Hemisphere
(Simmonds et al., 1997), a spring ozone maximum appears at all sites. It is most pronounced at Davos, where it
appears for all thresholds. Jungfraujoch exhibits a summer maximum when using percentiles greater than 2,
mainly due to high-pressure and indi!erent weather
types (cf. Section 3.4). It seems that, at Jungfraujoch,
a NO -based "lter tends to capture some regional ozone
V
episodes. A "lter based on daily average CO concentrations (5- or 10-percentile, 1996}1998) gives somewhat
lower spring and summer ozone levels with a slight
spring maximum. A more detailed discussion about the
shift in the seasonal ozone cycle at Jungfraujoch when
Fig. 4. Seasonal averages of daily ozone mean values at all sites for days with daily median NO values lower or equal to the 2-, 5-, 10-,
V
25-, and 50-percentile for the respective site and season.
5196
S. Bro( nnimann et al. / Atmospheric Environment 34 (2000) 5191}5198
regional background conditions are isolated can be
found in Schuepbach et al. (2000).
Increasing the percentile threshold for daily median
NO levels has a large e!ect upon ozone concentrations
V
in summer, but also in winter. In spring and summer,
ozone concentrations are higher when more NO is adV
mitted due to photochemical ozone production on a local to regional scale. The opposite is the case in winter,
i.e. ozone concentrations are lower when air masses with
more NO are advected. This is possibly indicative of
V
ozone titration due to NO. A similar e!ect on the seasonal cycle was already observed in Fig. 3.
3.4. Dependence of ozone concentrations on synoptic
weather types
Fig. 5 shows the occurrence of the synoptic weather
types as de"ned in Section 2 for days "ltered with the
10-percentile threshold of daily median NO . Regional
V
background conditions occur with di!erent synoptic
situations at each site. With increasing altitude, highpressure situations become more frequent in the sample.
This is especially the case in winter, when high-pressure
situations are characterised by a pronounced inversion
which suppresses convective transport of polluted air to
the sites above around 1000 m asl (Lugauer et al., 1998).
LaK geren and Payerne are then usually below the inversion over the Swiss Plateau. There, clean air situations in
winter occur mainly with advective weather types, when
the polluted air on the Swiss Plateau is replaced by
in#owing fresh air. In summer, when convective mixing
reaches higher up, the di!erences between the three sites
Payerne, LaK geren and Rigi are small.
The daily mean ozone values for each synoptic weather
type, site, and season are displayed in the lower part of
Fig. 5. Westerly situations show the smallest seasonal
variability of background ozone values at most sites.
These situations are probably associated with maritime
air masses with short residence time over the continent
and limited potential for photochemistry upwind of Switzerland because of often overcast skies. Furthermore,
`northa situations appear to be associated with slightly
lower background ozone concentrations than `south/
easta. High-pressure situations in summer are accompanied with high ozone levels at LaK geren, Chaumont,
and Rigi due to pollution episodes. The pronounced
spring maximum at Davos appears for all synoptic
weather types and cannot be explained by circulation
di!erences. Generally, a spring maximum is found for
`lowa and for the advective weather types.
3.5. Interannual variability and trends
Trends of both the "ltered data and all data were
calculated in a three-step approach. To avoid large gaps
in the temporal coverage we used the 25-percentile of
daily median NO rather than the 10-percentile. Note
V
that this threshold does not exclude all local-to-regional
Fig. 5. Upper part, left legend: Frequency of occurrence (left axis) of the synoptic weather types `higha (H), `lowa (L), `westa (W),
`northa (N), `south/easta (SE) and `resta (R) during background conditions for each season (daily median NO below or equal to the
V
10-percentile for each site and season). Lower part, right legend: Daily mean ozone values for the synoptic weather types during
background conditions (see above) for each season. Only averages of classes with at least "ve cases are shown.
S. Bro( nnimann et al. / Atmospheric Environment 34 (2000) 5191}5198
5197
Table 2
Trends of monthly mean ozone values from 1988 to 1998 (Jungfraujoch, 1996) and trends of D and e from 1992 to 1998 for all days and
R
R
for days within the lowest 25% of daily median NO values (de"ned separately for each site and season). D and e are time series of
V
R
R
seasonal averages, based on deseasonalised daily mean ozone values (D ) and the di!erence between the deseasonalised daily mean
R
ozone value and the mean value for the corresponding weather type and season (e ). All trends are in [ppb yr\], given with 95%
R
con"dence interval. [1] from BroK nnimann et al. (2000), calculated from deseasonalised monthly means, the series Payerne and LaK geren
were adjusted for shifts before and in 1991, [2] from Zanis et al. (1999) calculated from monthly means, adjusted for a shift in 1989
Site
Payerne
LaK geren
Rigi
Chaumont
Davos
Jungfraujoch
Longer period trend
1988}1998#0.27$0.21 [1]
1988}1998#0.46$0.21 [1]
1988}1998#0.32$0.17 [1]
1988}1996#0.48$0.48 [2]
1992}1998, all days
1992}1998, "ltered days
D
R
e
R
D
R
e
R
#0.58$0.47
#0.75$0.45
#1.09$0.49
#1.32$0.51
#0.52$0.29
#0.98$0.33
#0.53$0.35
#0.81$0.38
#1.15$0.42
#1.40$0.43
#0.58$0.25
#0.99$0.32
#0.84$0.37
#0.97$0.50
#1.04$0.77
#0.94$0.55
#0.78$0.30
#1.19$0.50
#0.78$0.32
#0.84$0.45
#1.29$0.56
#1.09$0.45
#0.72$0.26
#1.22$0.51
pollution events. First, we removed the seasonal cycle
from a sample by subtracting the function
>"a#b sin (X#d)#c sin (2X#e),
where X has a period of one year, a, b, c, d, and e were
"tted with the method of least-squares. A deseasonalised
series was called D , where i is the observation number in
G
the sample. Second, all elements of a series D were
G
grouped by climatological season s and weather type
w and the averages of the groups de"ned the function
AWS (s, w). A series D was then expressed as
G
D "AWS (s , w )#e .
G
G G
G
D and e were then averaged in consecutive seasons,
G
G
yielding the new series D and e . Third, trends of D and
R
R
R
e were calculated with linear regression, where the "rst
R
(Jan/Feb 1992) and last (December 1998) cases were
weighted and , respectively. All trends are signi"cantly
(p(0.05) positive (Table 2). Except for Chaumont and
Rigi, the trends are stronger positive for the "ltered
record than for all days, even for e , i.e. after the subtracR
tion of the synoptic component. It should be noted that
the AWS weather types are useful for addressing di!erences within background situations but may be inadequate for representing meteorological in#uences on
local chemistry in `all daysa. Table 2 also displays trends
in monthly mean ozone values since 1988 for four of the
sites. There is some indication that the positive ozone
trend has strengthened in recent years; this is in agreement with recent "ndings from Zugspitze (Scheel et al.,
1999) and from Hohenpeissenberg soundings at 500 hPa
(Logan et al., 1999). However, the time series are very
short and it should be noted that, in Switzerland, 1998
was an ozone-rich year with outstanding episodes in
May and in August (BUWAL, 1999), although this is
only to a small degree apparent in D and e . For
R
R
Jungfraujoch, a re"ned statistical analysis of trends and
changes in the seasonal ozone cycle (1988}1997) is given
by Schuepbach et al. (2000).
4. Summary and conclusions
An attempt was made to isolate the most important
features of the temporal and spatial variability of ozone
in Switzerland for conditions without local or strong
regional in#uences, termed regional background. These
features encompass altitudinal and geographical di!erences, diurnal and seasonal cycles, the dependence on
synoptic weather types, and trends. For some of these
features, a di!erent behaviour was found for the regional
background conditions compared to all days. Regional
background ozone shows a weaker altitude dependence,
and diurnal cycles are smaller, mainly due to the exclusion of NO -rich air masses that are depleted in ozone.
V
The seasonal cycles reveal a spring maximum and an
autumn/winter minimum at most sites in regional background situations, but a summer maximum when the
"lter is tuned towards more polluted air masses. Di!erences in ozone under regional background conditions
were also found with respect to synoptic weather types,
which can be explained in terms of di!erent transport
conditions. Trends of daily mean ozone from 1992 to
1998 are higher for background conditions than for all
days. Also, they are stronger positive in the last 7 years
than in the last 11 years. It seems that, with decreasing
local to regional emissions of ozone precursors (a likely
assumption for Central Europe for the last years and
the near future), processes on the regional to large scale
such as transport or large-scale chemical mechanisms
5198
S. Bro( nnimann et al. / Atmospheric Environment 34 (2000) 5191}5198
may become increasingly important in determining the
variability of ozone in a monitoring network.
Acknowledgements
S.B. is funded by the Swiss Agency for Environment,
Forests, and Landscape (SAEFL) under contract
FE/BUWAL/810.98.7. E.S. would like to acknowledge
funding from the Swiss Dept. of Science and Education
(NachwuchsfoK rderungsprogramm). NABEL measurements were provided by SAEFL, CO data by the Swiss
Federal Laboratories for Materials Testing and Research
(EMPA). The Swiss Meteorological Institute (SMI) provided the Alpine Weather Statistics.
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