Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
1680-7324
7
16
2007
10.5194/acp-7-4311-2007
http://www.atmos-chem-phys.net/7/4311/2007/
http://www.atmos-chem-phys.net/7/4311/2007/acp-7-4311-2007.html
http://www.atmos-chem-phys.net/7/4311/2007/acp-7-4311-2007.pdf
4311
4326
2007-08-22
Influence of altitude on ozone levels and variability in the lower troposphere: a ground-based study for western Europe over the period 2001–2004
A. Chevalier
F. Gheusi
ghef@aero.obs-mip.fr
R. Delmas
C. Ordóñez
C. Sarrat
R. Zbinden
V. Thouret
G. Athier
J.-M. Cousin
Laboratoire d'Aérologie, Toulouse, France
Centre National de Recherches Atmosphériques, Toulouse, France
The PAES (French acronym for synoptic scale atmospheric pollution) network
focuses on the chemical composition (ozone, CO, NO<sub>x/y</sub> and aerosols) of the
lower troposphere (0–3000 m). Its high-altitude surface stations located in
different mountainous areas in France complete the low-altitude rural MERA
stations (the French contribution to the european program EMEP, European
Monitoring and Evaluation Program). They are representative of pollution at the
scale of the French territory because they are away from any major source of
pollution.
<br><br>
This study deals with ozone observations between 2001 and 2004 at 11 stations
from PAES and MERA, in addition to 16 elevated stations located in mountainous
areas of Switzerland, Germany, Austria, Italy and Spain. The set of stations
covers a range of altitudes between 115 and 3550 m. The comparison between
recent ozone mixing ratios to those of the last decade at Pic du Midi (2877 m),
as well as trends calculated over 14-year data series at three high-altitude
sites in the Alps (Jungfraujoch, Sonnblick and Zugspitze) reveal that ozone
is still increasing but at a slower rate than in the 1980s and 1990s.
<br><br>
The 2001–2004 mean levels of ozone from surface stations capture the ozone
stratification revealed by climatological profiles from the airborne
observation system MOZAIC (Measurement of OZone and water vapour by Airbus
In-service airCraft) and from ozone soundings above Payerne (Switzerland). In
particular all data evidence a clear transition at about
1000–1200 m a.s.l. between a sharp gradient below (of the order of +30 ppb/km) and a gentler
gradient (+3 ppb/km) above. The same altitude (1200 m) is also found to be a
threshold regarding how well the ozone levels at the surface stations agree
with the free-tropospheric reference (MOZAIC or soundings). Below the
departure can be as large as 40%, but suddenly drops within 15% above. For
stations above 2000 m, the departure is even less than 8%. Ozone variability
also reveals a clear transition between boundary-layer and free-tropospheric
regimes around 1000 m a.s.l. Below, diurnal photochemistry accounts for about
the third of the variability in summer, but less than 20% above – and at
all levels in winter – where ozone variability is mostly due to day-to-day
changes (linked to weather conditions or synoptic transport). In summary, the
altitude range 1000–1200 m clearly turns out in our study to be an upper
limit below which specific surface effects dominate the ozone content.
<br><br>
Monthly-mean ozone mixing-ratios show at all levels a
minimum in winter and the classical summer broad maximum in spring and summer
– which is actually the superposition of the tropospheric spring maximum
(April–May) and regional pollution episodes linked to persistent anticyclonic
conditions that may occur from June to September. To complement this classical
result it is shown that summer maxima are associated with considerably more
variability than the spring maximum. This ensemble of findings support the
relevance of mountain station networks such as PAES for the long-term
observation of free-tropospheric ozone over Europe.
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