Atmospheric Chemistry and Physics Discussions
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2007
10.5194/acpd-7-1327-2007
http://www.atmos-chem-phys-discuss.net/7/1327/2007/
http://www.atmos-chem-phys-discuss.net/7/1327/2007/acpd-7-1327-2007.html
http://www.atmos-chem-phys-discuss.net/7/1327/2007/acpd-7-1327-2007.pdf
1327
1356
2007-01-29
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
amandine.chevalier@aero.obs-mip.fr
F. Gheusi
R. Delmas
C. Ordó\ nez
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 with those of the last decade found in the literature
for two high-elevation sites (Pic du Midi, 2877 m and Jungfraujoch, 3580 m)
leads to a trend that has slowed down compared to old trends but remains
positive. This could be attribuable to the reduction of ozone precursors at
European scale, that however do not compensate an ozone increase at the global
scale. Averaged levels of ozone increase with elevation in good
agreement with data provided by the airborne observation system MOZAIC
(Measurement of OZone and water vapour by Airbus In-service airCraft), showing a
highly stratified ozone field in the lower troposphere, with a transition at
about 1000 m asl between a sharp gradient (30 ppb/km) below but a gentler
gradient (3 ppb/km) above. Ozone variability also reveals a clear transition between boundary-layer
and free-tropospheric regimes at the same altitude. 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). 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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