The largest atmospheric peroxyacetyl nitrate (PAN) mole fractions at remote surface sites in the Northern Hemisphere are commonly observed during the months April and May. Different formation mechanisms for this seasonal maximum have previously been suggested: hemispheric-scale production from precursors accumulated during the winter months, increased springtime transport from up-wind continents or increased regional-scale production in the atmospheric boundary layer from recent emissions. The two high Alpine research sites Jungfraujoch (Switzerland) and Zugspitze (Germany) exhibit a distinct and consistent springtime PAN maximum. Since these sites intermittently sample air masses of free-tropospheric and boundary layer origin, they are ideally suited to identify the above-mentioned PAN formation processes and attribute local observations to these. Here we present a detailed analysis of PAN observations and meteorological conditions during May 2008 when PAN levels were especially elevated at both sites. The highest PAN concentrations were connected with anticyclonic conditions, which persisted in May 2008 for about 10 days with north-easterly advection towards the sites. A backward dispersion model analysis showed that elevated PAN concentrations were caused by the combination of favourable photochemical production conditions and large precursor concentrations in the European atmospheric boundary layer. The results suggest that the largest PAN values in spring 2008 at both sites were attributable to regional-scale photochemical production of PAN in the (relatively cold) planetary boundary layer from European precursors, whereas the contribution of inter-continental transport or free-tropospheric build-up was of smaller importance for these sites.
Peroxyacetyl nitrate (PAN) is a key compound of reactive nitrogen species
(NO
PAN measurements reported during the period 1987–1988 from three Swiss sites (suburban station, Dübendorf, 431 m a.s.l. (above sea level); forest site, Lägeren, 685 m a.s.l.; Alpine valley site, Davos, 1630 m a.s.l.) showed significant seasonal variation in PAN with elevated monthly mean values in spring for all the stations, suggesting that maximum PAN concentrations were associated with anticyclonic weather types and low wind speeds (Wunderli and Gehrig, 1991). Previous PAN observations at the Swiss high Alpine site Jungfraujoch (JFJ) revealed maximum PAN levels > 1 ppb in spring–summer season and attributed the maxima to thermally induced transport from the PBL (Zellweger et al., 2003).
Based on the above discussion there are three hypotheses for the origin of the PAN spring maximum at remote and high altitude sites in Europe:
increased background concentrations: build-up of precursor during winter in the Northern Hemisphere, active photochemistry producing high PAN concentrations in spring in the free troposphere; boundary layer influence and meteorologically favourable situations: transport from the boundary layer, e.g. by convective transport and/or accumulation in anticyclonic airflow; inter-continental transport: increased background mixing ratios (e.g. for ozone) from inter-continental transport during spring as shown by recent studies (Cooper et al., 2001, 2004, 2010; Fiore et al., 2009).
Fischer et al. (2014) used sophisticated
numerical simulations that described aircraft measurements quite well but
underestimated the PAN spring maxima at the Alpine sites Jungfraujoch
(Switzerland) and Zugspitze (Germany) by more than a factor of 2 (see their
Fig. S2 in the Supplement). The large deviation between these GEOS-Chem (Goddard Earth Observing System Chemistry transport model) simulations (based on a
grid resolution of
The aim of the present study is to use more adequate transport simulations (FLEXPART (flexible particle dispersion model) with finer resolution input and treatment of turbulent and convective vertical transport) combined with a state-of-the-art transport categorisation to verify the tentative interpretation of Pandey Deolal et al. (2013), allowing for a more precise and more detailed description of the involved atmospheric physical processes and their relations. In addition, we extend the analysis by incorporating observations from another European high altitude site (Zugspitze) which showed a similar annual PAN cycle as Jungfraujoch. Here we can show that the PAN formation mechanisms are similar for both sites, allowing for a more generalised view than previously for Jungfraujoch only. We limit the analysis to the month of May 2008, when particularly large PAN mole fractions were reported at both sites. In addition, observations from nearby elevated rural sites (Rigi and Chaumont, Switzerland; Hohenpeissenberg, Germany), which are more representative for the daytime PBL, are used to further interpret the processes responsible for the build-up of large PAN mole fractions.
The main sites used in this study (Jungfraujoch and Zugspitze, but also the
PBL site Hohenpeissenberg) are so-called global stations of the Global
Atmosphere Watch (GAW) programme of the World Meteorological Organization
(WMO), while the two additional Swiss sites are regional (Rigi) and
contributing (Chaumont) stations to GAW. For all sites detailed measurement
and site information can be found in the GAW station information system
(GAWSIS;
The observatory at Jungfraujoch (Sphinx observatory, 3580 m a.s.l.) is situated between the Mönch and the Jungfrau mountains in the Bernese Alps of Switzerland. The site is intermittently influenced by the lower FT and European PBL air and, therefore, provides the opportunity to characterise air masses with very different origin and air mass history. Air arriving from the north is often influenced by surface contact over the Swiss plateau before reaching JFJ, while air masses arriving from the south are often advected from the Po Valley crossing the inner Alpine region (Parker et al., 2009; Zellweger et al., 2003).
The Zugspitze Schneefernerhaus (ZSF, 2670 m a.s.l.) observatory is situated in
southern Germany at the northern rim of the Alps. Therefore, it is suitable
for the detection of air masses advected from the north (Kaiser et al.,
2007). The measurement station is situated on the southern slope of
Zugspitze between the summit and a skiing area. Normally, Zugspitze receives
free-tropospheric air but, similar to JFJ, the site is frequently exposed to
boundary layer air in summer time due to thermally induced flow systems
(Gantner et al., 2003; Reiter et al., 1987). In comparison to JFJ, a
stronger influence of surface emissions on the trace gas observations at ZSF
was deduced and the site was placed into a different category (weakly
influenced) as JFJ (mostly remote) in a study categorising remote
air quality sampling sites in Europe (Henne et al.,
2010). The altitude difference between the two sites is
The Hohenpeissenberg observatory (HPB) is another GAW site located in Germany, about 40 km north of Zugspitze in a hilly area dominated by agriculture and forests. The site is predominantly situated in the daytime PBL and night-time residual layer as it is located on top of a small mountain (985 m a.s.l.) and about 300 m above the surrounding area.
The station Rigi (RIG) (1031 m a.s.l.) is situated on the northern slope of the Rigi mountain, in an elevated rural environment 600 m above Lake Lucerne. The site is surrounded by grassland and forest areas and small cities such as Zug and Lucerne are approximately 12 km away from the site and at considerably lower elevation. Rigi is located about 65 km north-east of JFJ. The Chaumont observatory (CHA) (1136 m a.s.l.) is located about 700 m above Lake Neuchâtel. The area is dominated by meadows and pastures. The city of Neuchâtel is situated about 5 km south of the station, but well below at the lake shore. The station is located about 90 km north-west of JFJ. Both Swiss sites can be expected to be within the daytime PBL during the spring and summer months, while they are more decoupled from lowland influences during night and winter.
Locations of the sites. Red triangles: locations of the two high Alpine stations Jungfraujoch (JFJ) and Zugspitze Schneefernerhaus (ZSF). Blue circles: the additional elevated PBL sites Chaumont (CHA), Rigi (RIG) and Hohenpeissenberg (HPB).
The air was sampled from the main inlet dedicated for trace gas
observations, which has a total length of about 3 m, with 2 m on the roof
top and 1 m inside the laboratory. The inlet consists of stainless steel
tubing with an inner diameter of 90 mm and is constantly heated to 10
PAN measurements were performed at JFJ using a commercial gas chromatograph
(GC) analyser and a calibration unit provided by Meteorologie Consult GmbH
(Metcon). The technique is based on chromatographic separation with
subsequent detection and quantification by an electron capture detector
(ECD); for more details see Pandey Deolal et al. (2013).
Other trace gases such as total reactive nitrogen species (NO
The air was sampled with the UBA steel inlet used for measuring reactive
gases. The total length of the inlet is 3.5 m with 2.25 m on the roof top
and 1.25 m inside the laboratory. A borosilicate glass tube was placed
inside the steel inlet with inner glass diameter of 80 mm. The inlet is
constantly heated to
PAN measurements at ZSF were performed using the same technique and
instrument as described for JFJ measurements. NO
Characteristics of measurement sites: high Alpine sites (Jungfraujoch, JFJ; Zugspitze Schneefernerhaus, ZSF) which are intermittently within the free troposphere (FT) and influenced by boundary layer injections, and elevated rural sites (Hohenpeissenberg, HPB; Rigi, RIG; Chaumont, CHA) which are usually situated within the daytime planetary boundary layer but well above the night-time inversion layer. The given temperature and ambient pressure levels give the range of the observations in May 2008.
PAN measurements at this site were started in the late 1990s and continued
till present, using the same equipment as for JFJ and ZSF. Additionally, the
PAN measurements at HPB have been quality tested by at a blind
inter-comparison experiment at NCAR, Boulder, CO
(Tyndall et al., 2005). Long-term analysis of CO,
nitrogen dioxide (NO
A variety of trace gases and aerosol parameters (such as NO
The Lagrangian particle dispersion model (LPDM) FLEXPART (Version 8.1)
(Stohl et al., 2005) was used to calculate source receptor
relationships (SRRs) for May 2008 measurements for the two high Alpine sites.
For each 3-hourly interval, 50 000 particles were released at each receptor
site (JFJ and ZSF) and traced back in time for 10 days considering the mean
flow, turbulent PBL flow and deep convection. The model was driven by
European Centre for Medium Range Weather Forecast (ECMWF) operational
analyses (00:00, 06:00, 12:00, 18:00 UTC) and forecasts (03:00, 09:00, 15:00, 21:00 UTC) with 91
vertical level and a horizontal resolution of 1
Release heights of 3000 and 2500 m were chosen for JFJ and ZSF, respectively. This is significantly lower than the true altitudes of the observatories and takes into account the limited horizontal resolution of the model, by which the Alpine topography is not well represented, requiring a release height somewhere between the station's real altitude and the model ground (Brunner et al., 2012; Keller et al., 2012).
The simulated SRRs allow directly linking a mass release at a source grid
cell with a mass mixing ratio at the receptor (Seibert and Frank,
2004). SRRs are given in units s m
In order to see the effects of different flow regimes on PAN concentrations,
simulated SRRs were classified into different flow regimes applying
clustering methods to the transport simulations but not to the observed
trace species. A straightforward approach would be to treat the SRR in every
cell of the output grid as an individual time series in the cluster
analysis. However, in that case the number of variables would become too
large to be handled efficiently and arbitrary results might be produced due
to the inclusion of grid cells with very small SRRs. Therefore, we reduced
the number of grid cells by aggregating cells with small average (May 2008)
emission sensitivities to larger grid cells. Starting from grid cells with
0.1
The clustering was applied to the time series of the aggregated emission sensitivities for both high Alpine sites separately. For JFJ the maximal silhouette width was at four clusters, while for ZSF the situation was more complicated. Here the overall maximum silhouette width was obtained for 18 clusters, which gave a much too fine separation of the transport situations. Other local maxima were at three and five clusters. The clustering with five clusters was more similar to the one obtained for JFJ. Hence, five clusters were retained for ZSF.
For each cluster, average surface SRRs were calculated by summation over all
cluster members and division by the number of cluster members
N
PAN mixing ratios.
PAN measurements from different sites are shown in Fig. 2. At JFJ, PAN measurements were performed during campaigns in 2008 for the spring–summer (May and August) and autumn (September and October) months. The monthly mean mixing ratios of PAN for both JFJ and ZSF are presented in Fig. 2, left (left panel). These measurements indicate a strong seasonal cycle in the PAN with peaking mole fractions in late spring (April or May) and minima in the autumn and winter months. Prior to recent measurements, PAN observations at JFJ from April 1997 to May 1998 (black solid line) also revealed a similar annual cycle (Zellweger et al., 2000). PAN measurements at JFJ performed during campaigns between February 2005 and August 2006 (black crosses) by Balzani Lööv et al. (2008) indicated background mole fractions < 0.2 ppb in April and May; however, spring mean mole fractions were found significantly lower than all other reported measurements. Campaign measurements during February–March 2003 at JFJ showed a mean concentration of 0.142 ppb (Whalley et al., 2004), which is in agreement with the observations from other years; Fig. 2. (right panel) shows the PAN measurements at the PBL sites including HPB and the Swiss sites Dübendorf (sub-urban), Lägeren (rural forest) and Davos (Alpine valley, 1630 m a.s.l.) taken from Wunderli and Gehrig (1991). All these measurements in the PBL, also including the Alpine valley site, clearly show the same seasonal behaviour and are in line with previous observations of spring PAN maxima at northern hemispheric mid-latitudes (Penkett and Brice, 1986; Monks, 2000; Zanis et al., 2003, 2007; Fenneteaux et al., 1999). The only exceptions showing a summer maximum are the measurements of HPB in 2003 which are most probably caused by the special conditions of the European heat wave in summer 2003 (e.g. Ordóñez et al., 2010). The larger interannual PAN variability (even excluding 2003) at the PBL sites as compared to the high altitude sites can be explained by the stronger influence of variable meteorology on observed mole fractions close to the precursor emissions. Also note that the site Davos is a relatively remote site compared to the other PBL sites, explaining part of the suggested large interannual variability.
During May 2008 JFJ experienced some of the largest hourly PAN mixing ratios ever recorded at JFJ (Pandey Deolal et al., 2013) and also the monthly mean PAN was among the largest on record (see Fig. 2). PAN at ZSF was comparable to other years. Hence, May 2008 was selected for a more detailed analysis as the variability at the sites can help identifying the potential origin of air masses and meteorological processes involved.
Since high Alpine sites intermittently receive FT and PBL air, it should be possible to attribute high PAN observations to either the FT or PBL, if the air mass contributions can be clarified. Even on typical fair-weather days Jungfraujoch is usually not within the PBL, but it is rather only influenced by intermittent injections of PBL air into a secondary Alpine boundary layer (see Henne et al., 2004). This process can usually be seen by elevated late afternoon concentrations of typical primary PBL tracers like CO. Due to the relatively narrow horizontal extent of this injection layer JFJ comes back under free-tropospheric influence during the night. A strong subsidence during night-time, bringing ozone rich air to the site as observed at other, lower elevation mountain sites (Roberts et al., 1995), is usually not observed at JFJ or ZSF, with the limitation that the ZSF observatory is situated 910 m below JFJ.
The entire month was characterized by an alternation between rather stagnant high and low-pressure systems over Europe. A low-pressure system with its centre over the North Sea and the UK prevailed from the beginning of the month until 5 May. From 6 to 11 May, distinct high pressure conditions developed over southern Scandinavia extending southward and eastward towards Central and Eastern Europe. This resulted in high irradiation and cloud-free conditions in Central Europe and parts of Eastern Europe, and this period was considered as blocking anticyclonic conditions (Hamburger et al., 2011). Towards the end of this period pressure gradients weakened and the deep convection potential increased resulting in local thunderstorms over the Alps and Jura mountains on 12 May. From 13 to 17 May the situation over Central and Western Europe was dominated by a low-pressure system moving from the Gulf of Biscay towards northern Germany. The Alpine area was influenced by the frontal systems embedded in this lower pressure system. As a result, irradiation (cloud cover) was reduced (enhanced) south and west of the Alps. A south foehn situation developed on 15 and 16 May with precipitation on the southern side of the Swiss Alps. From 17 to 31 May, low-pressure conditions persisted over Central Europe leading to a succession of frontal passages.
The time series of trace gas observations at JFJ and ZSF are shown in Fig. 3 for May 2008. The PAN mixing ratios at JFJ were especially high during the period 6–15 May reaching a 3-hourly maximum of 1.2 ppb. PAN was elevated during the same period at ZSF as well, but did not exceed 1 ppb.
In addition to PAN, other trace species such as NO
In general PAN levels were lower (factor 0.7) and NO
O
Observed 3-hourly averages of trace gas mixing ratios (ppbv),
absolute humidity (g kg
Cluster average surface source receptor relationships (SRRs)
for
Cluster average latitude–altitude distribution of source receptor
relationships (SRRs) for
To further analyse the conditions that led to the observed variability in PAN observations and especially the period of elevated PAN, the trace gas time series were split into different categories according to the transport clustering described above (footprint clustering). Since the clustering is based on transport history of the observed air masses only, it is thought to shed light on the main question of this article as to which extent the observed spring maximum in PAN can be related to free-tropospheric, hemispheric background-scale production of PAN from accumulated precursors or regional-scale production from recent emissions.
As described above, the clustering resulted in four and five clusters for JFJ and ZSF, respectively. The results of the clustering in terms of temporal attribution can be seen in Fig. 3, in which the time series were coloured according to the transport cluster. It can directly be seen that the individual clustering at the sites resulted in relatively similar temporal cluster attributions at both sites. When considering that clusters 4 and 5 at ZSF could actually be joined to be comparable to cluster 4 at JFJ, the detected transport regimes match between the sites in 91 % of all cases. Keeping in mind that the clustering relies on the simulated transport history only and not on the in situ observations, it is remarkable that the clustering excellently separates the episode of elevated levels from periods with low PAN. This in itself already indicates that large parts of the PAN variability actually depend on the transport history and may be explained by analysing the conditions during the different transport regimes in detail.
In the following, the observed time series are further interpreted following
the obtained transport clustering. The results of the latter are displayed
in Figs. 4 and 5 as cluster average surface SRRs and cluster average
latitude–altitude SRR distributions for the European domain, respectively.
Similar figures for the larger North Atlantic region can be found in the
Supplement (Figs. S1 and S2). In addition to the observed trace
gas time series, the observations were split by transport cluster and
aggregated to average diurnal cycles (Figs. 6 and 7). Where available,
parameters from the less elevated sites, usually residing in the PBL or
night-time residual layer, were treated in the same way, using the
clustering as obtained for the nearby high Alpine site. For JFJ the average
from the Swiss PBL sites (CHA and RIG) was taken, while for ZSF the
observations from HPB were used as PBL reference. Observed PAN–CO, and
PAN–O
Average diurnal cycles of
Same as Fig. 6, but for
Scatter plots of observed mole fractions of PAN vs. carbon monoxide (top) and PAN vs. ozone (bottom) for Jungfraujoch (JFK, left) and Zugspitze (ZSF, right). Regression lines (obtained using weighted total least-square regression, see text) are only shown if significant correlations between the trace gases exist. The colours represent the correlations within the individual transport clusters, see Fig. 3 and text.
Cluster average for
In a simplified way correlations between CO and PAN can be understood as
follows. Assuming a constant PAN and CO background, emissions will lead to
an initial increase in CO and PAN precursors in an air mass. During
transport from the source to the receptor, PAN may be produced under
favourable conditions while CO can be assumed to remain relatively constant.
During transport the original pollution plume will further mix with
background air masses. The interception of different degrees of mixed air
masses at the receptor will then result in correlation between CO and PAN
spanning the range between the two end points of background conditions and
pollution plume. The stronger the correlation the closer this simplified
view actually matches reality, while weak correlations may indicate both:
minor PAN production and/or ill-defined pollution plumes. In this study we
analysed correlations for well-defined transport regimes and in a relatively
short period of time. Hence, the background for each transport regime is
thought to be relatively constant and correlations should be robust. The
steeper the slope between PAN and CO the more efficient PAN was produced in
the original plume. Similarly, PAN–O
Finally and to foster the interpretation in terms of vertical mixing and local photochemical production, cluster average afternoon (12:00 and 15:00 UTC) PBL heights and daytime (06:00, 09:00, 12:00, and 15:00 UTC) cloud cover maps for Central Europe were derived from ECMWF-IFS operational analysis and forecast data, the same as used for the transport simulations (Fig. 9). PBL heights were derived by applying a critical Richardson number criterion (Vogelezang and Holtslag, 1996).
The individual transport clusters can be described as follows:
Cluster 1 (westerly advection): the SRRs in this cluster indicate a
cyclonic flow from the North Atlantic region passing over the Iberian
Peninsula and France before reaching JFJ and ZSF (Figs. 4 and 5 and
Figs. S1 and S2). Additional influence from the western
Mediterranean was identified for ZSF. While in general, westerly advection
with descending flow dominated this cluster, mainly representing air masses
from lower tropospheric levels, sampled air masses had occasional PBL
contact over France, Switzerland and, in the case of ZSF, southern Germany
(Fig. 4, right panel). The influence of surface emissions was larger for ZSF
than JFJ, as indicated by greater SRRs for the former. This category was
experienced during an uninterrupted period from the beginning of May to 5
May. PAN concentrations were relatively low, in the range between 0.2 and 0.5 ppb
at ZSF but dropped below this range at JFJ from 3 May onwards (Fig. 3).
This coincides with a period of decreased CO and O Cluster 2 (Recirculating north-easterly/south-westerly advection):
this flow regime comprises air masses that mainly arrived from
north-easterly directions with additional surface influence south-west of
the sites (Figs. 4 and 5). This is not the consequence of overlaying two
separate transport regimes at different times, but is also characteristic
for most of the individual SSRs during this period. The core of the backward
plume typically moved slowly south-westward before taking a north-easterly
and ascending path. Finally, part of the plume was recirculated westwards at
higher altitudes and detrained into the PBL again, causing increased surface
emission sensitivities west of the sites. The surface footprint for JFJ in
the Fig. 4 indicates relatively large emission sensitivities over the
western Alps and France, the easterly component is more pronounced for ZSF
with strong sensitivities along the northern flank of the Alps in Austria
and Bavaria. The regime occurred on 5 and 6 May and again between 18 and 25
May. This cluster showed considerable PBL contact in agreement with high
water vapour content as observed at the sites. PAN mole fractions at both
sites remained moderate for most of the category ( Cluster 3 (easterly advection): the SRRs in this cluster describe
typical anticyclonic conditions with easterly to north-easterly advection
and descending air masses. In addition, a southerly component close to the
sites caused enhanced surface emission sensitivities in northern Italy and
Switzerland for JFJ and Austria and to a smaller degree northern Italy for
ZSF (Fig. 4). From Fig. 5 it can be seen that during this period free-tropospheric air masses descended in the anticyclone. However, air masses
had contact with the PBL shortly before reaching the sites, as can be seen
by the relatively focused surface SRRs (Fig. 4). By far the highest
pollutant concentrations were observed in this cluster including maxima for
PAN, NO In summary, these observations point towards a regional-scale production of
PAN in the PBL, which may further be enhanced when PBL air masses are lifted
into the lower FT, where they were sampled at JFJ. Cluster 4 (south-easterly advection): Fig. 4 shows that air masses
combined in this cluster had only weak surface contact, mainly close to the
sites within the Alps and over Italy and the adjacent parts of the
Mediterranean. The respective air masses remained within the lower FT north
of 30 Cluster 5 (southerly advection; ZSF only): this SRR describes
the transport path from the south, with moderate SRRs over the Mediterranean
and Italy and additional boundary layer contact in the Alpine region. While
there are large NO
Figure 8 (bottom) shows PAN vs. ozone correlations for the individual
clusters. Correlations between PAN and O
In our study the highest slopes were found in cluster 3 (Easterly advection)
(see Fig. 8). At JFJ PAN vs. O
In order to further explore the representativeness of the weather conditions encountered in spring 2008, we compared the transport clusters obtained in our study with a long-term weather type classification. The Alpine Weather Statistic (AWS) is a weather classification that was developed to characterise the weather situation at a given time over the Swiss domain (MeteoSwiss, 1985; Wanner et al., 1998). The AWS was previously used to analyse PBL transport to JFJ (Henne et al., 2005b). The AWS types convective–indifferent and convective–anticyclonic were identified as weather types for which PBL transport to JFJ was likely during the afternoon of spring and summer months. Our JFJ cluster 3 largely corresponds with the AWS weather sub-types convective–anticyclonic flat pressure and convective–indifferent easterly advection. The frequency of these two weather types for the years 2001 to 2010 and the months April and May was relatively large in 2008 (> 13 days) but comparable to other years (2007: 15 days; Fig. S4a). When looking at the frequency of all convective–anticyclonic and convective–indifferent weather types, which are likely to allow PBL transport to JFJ, the frequency in 2008 (30 days) was only slightly larger than the average frequency for all years (27 days; Fig. S4b). Hence, our conclusion on the representativeness of our 2008 case study is twofold. On the one hand, the occurrence of strong PBL influence during easterly flow in May 2008 was exceptional in its persistence and continuation for about 10 days. On the other hand, the frequency of weather types with likely PBL transport towards JFJ was not larger in spring 2008 than in other years. Therefore, we are convinced that our findings concerning the origin of the pronounced springtime PAN maximum at high Alpine sites are not restricted to the analysed year but can be interpreted in a more general way.
In agreement with previous studies, PAN measurements from ZSF and JFJ showed a pronounced seasonal cycle with maximum mole fractions in late spring. This indicates that the spring maximum of PAN in background air masses as observed at other northern hemispheric sites is also a typical phenomenon at high Alpine sites. The origin of the springtime maximum at the two Alpine sites was evaluated in more detail for May 2008 when PAN levels at JFJ were especially large. Different transport regimes towards the sites were distinguished using a clustering method on backward dispersion simulations. These show that air masses in May 2008 had recent PBL contact in different parts of Europe before reaching the measurement stations at JFJ and ZSF. At both sites, the highest PAN concentrations of May 2008 were connected with descending air masses in an anticyclonic flow (cluster 3). However, these air masses experienced pronounced contact with the PBL under photochemically favourable (cloud-free) conditions, prior to the arrival at the sites. A comparison with nearby PBL sites reveals that the ZSF observatory was situated within the daytime PBL, while JFJ was influenced by PBL injections during this period. PAN levels were considerably lower during all other flow regimes also for those less influenced by recent PBL contact. Therefore, we conclude from our study that under the conditions as sampled at two high Alpine European sites, PAN spring maxima are mainly caused by the following factors: (1) high pressure conditions lead to an accumulation of trace gases in the PBL and vertical transport from the PBL becomes important for transporting the pollutants to the sites, (2) solar irradiance is already large in May which enhances the photochemistry during cloud-free conditions as encountered during the anticyclonic transport regime and (3) temperatures are still relatively low in the lower free troposphere preventing thermal decomposition of PAN which becomes more important in summer.
The analysis of the high spring PAN mixing ratios at these Alpine sites
clearly suggests that the spring maximum is primarily caused by PAN
production in and export from the regional PBL. The highest PAN observations
during May 2008 were not connected with free-tropospheric conditions, but
with PBL air masses. These results agree well with those of Pandey Deolal et al. (2013) which, based on hemispheric-scale backward
trajectories, indicate that those air masses which had surface contact in
the European boundary layer were associated with the largest PAN and NO
We thank the Swiss National Foundation (SNF) for providing funding of projects no. 200020-117626 and no. 206021_128754 to carry out this research. We also express our gratitude towards international foundation for high Alpine research stations Jungfraujoch and Gornergrat (HFSJG) for providing access to Jungfraujoch facilities and respective custodians for their support. We are thankful to NABEL for their data, which is operated by Empa in collaboration with the Swiss Federal Office for the Environment. We also thank MeteoSwiss for provision of meteorological data from Jungfraujoch. Edited by: A. Pozzer