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

Abstract

Abstract. The PAES (French acronym for synoptic scale atmospheric pollution) network focuses on the chemical composition (ozone, CO, NOx/y 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. 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.

Surface ozone at the Caucasian site Kislovodsk High Mountain Station and the Swiss Alpine site Jungfraujoch: data analysis and trends (1990–2006)

“…Le Casset (1750 m a.s.l. ), another station which is even lower than KHMS has slightly higher average mixing ratio but it is still within the limits of 1 standard deviation of the average estimate (Chevalier et al, 2007). Comparing KHMS ozone levels with the data reported for USA sites by Jaffe this study * Selected subset (overlapping time period); * * For American sites daytime data (10:00-18:00 LST) are reported; * * * Similar to the American sites the day hours only (10:00-18:00 LST) are selected.…”

“…In Europe distinct vertical gradients are mostly reported for the measurements up to 1200 m a.s.l. (Chevalier et al, 2007) while the picture is less consistent for more elevated sites (see Table 2). …”

The influence of European pollution on ozone in the Near East and northern Africa

“…Ozone at high-altitude stations in Europe tends to be higher on average than ozone at stations below 1000-1200 m due to titration and deposition; however, variability is higher at stations below 1000-1200 m, especially in polluted regions in summer (e.g. Reiter et al, 1987;Wunderli and Gehrig, 1990;Chevalier et al, 2007). The largest overpredictions tend to occur at stations near coasts (e.g.…”

Investigating ozone episodes in Portugal: a wavelet-based approach

“…Regarding the high-altitude sites, it is often claimed that they are "above the boundary layer" or "representative of the free troposphere" by sole consideration of their elevation (Chevalier et al 2007). However, mountains appear to considerably enhance atmospheric turbulence and affect circulation for many reasons (roughness, synoptic lifting, hydraulic effects, thermally induced circulations, etc), and thus, it can be hardly stated that even a high-altitude station is free from the influence of the surface without further investigation (Lefhon 1992;Sanz and Millán 2000).…”