Influence of altitude on ozone levels and variability in the lower troposphere: a ground-based study for western Europe over the period 2001–2004

[摘要] The PAES (French acronym for synoptic scale atmospheric pollution) networkfocuses on the chemical composition (ozone, CO, NO_x/y and aerosols) of thelower troposphere (0–3000 m). Its high-altitude surface stations located indifferent mountainous areas in France complete the low-altitude rural MERAstations (the French contribution to the european program EMEP, EuropeanMonitoring and Evaluation Program). They are representative of pollution at thescale of the French territory because they are away from any major source ofpollution.

This study deals with ozone observations between 2001 and 2004 at 11 stationsfrom PAES and MERA, in addition to 16 elevated stations located in mountainousareas of Switzerland, Germany, Austria, Italy and Spain. The set of stationscovers a range of altitudes between 115 and 3550 m. The comparison betweenrecent ozone mixing ratios to those of the last decade atPic du Midi (2877 m),as well as trends calculated over 14-year data series at three high-altitudesites in the Alps (Jungfraujoch, Sonnblick and Zugspitze) reveal that ozoneis still increasing but at a slower rate than in the 1980s and 1990s.

The 2001–2004 mean levels of ozone from surface stations capture the ozonestratification revealed by climatological profiles from the airborneobservation system MOZAIC (Measurement of OZone and water vapour by AirbusIn-service airCraft) and from ozone soundings above Payerne (Switzerland). Inparticular all data evidence a clear transition at about1000–1200 m a.s.l. between a sharp gradient below (of the order of +30 ppb/km) and a gentlergradient (+3 ppb/km) above. The same altitude (1200 m) is also found to be athreshold regarding how well the ozone levels at the surface stations agreewith the free-tropospheric reference (MOZAIC or soundings). Below thedeparture can be as large as 40%, but suddenly drops within 15% above. Forstations above 2000 m, the departure is even less than 8%. Ozone variabilityalso reveals a clear transition between boundary-layer and free-troposphericregimes around 1000 m a.s.l. Below, diurnal photochemistry accounts for aboutthe third of the variability in summer, but less than 20% above – and atall levels in winter – where ozone variability is mostly due to day-to-daychanges (linked to weather conditions or synoptic transport). In summary, thealtitude range 1000–1200 mclearly turns out in our study to be an upperlimit below which specific surface effects dominate the ozone content.

Monthly-mean ozone mixing-ratios show at all levels aminimum 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 anticyclonicconditions that may occur from June to September. To complement this classicalresult it is shown that summer maxima are associated with considerably morevariability than the spring maximum. This ensemble of findings support therelevance ofmountain station networks such as PAES for the long-termobservation of free-tropospheric ozone over Europe.

[学科分类] 大气科学