Appendix 1 Characterisation of the ozone phenomenology

Appendix 1 Characterisation of the ozone phenomenology

This appendix consists of the tables and figures mentioned in section 5.5 and subsequent sections.

I.1 Ozone exceedances and the weather type over Middle Europe

Table I.1: Definition of the GWL weather types⁽¹⁾ observed over the April-August 1994,1995 and 1996 periods.

⁽¹⁾ classes that contribute less than 7% to the exceedances for each Member State are not considered
⁽²⁾ bold and normal characters refer to anticyclonic and cyclonic weather types, respectively.

Table I.2: Total ozone exceedances⁽¹⁾ (N) and percentage per GWL weather type⁽²⁾ reported over the April-August 1994,1995 and 1996 periods (456 days).

⁽¹⁾ August 1996 is not included for France
⁽²⁾ classes that contribute less than 7% to the exceedances for each Member State are not considered. Bold and normal characters refer to anticyclonic and cyclonic weather types, respectively.

The second column refers to the occurrence of the weather types (in % days). The three highest values in bold and the maximum is underlined. Dash symbol means that no exceedance was recorded.

Table I.3: Total ozone exceedances⁽¹⁾ (N) and average number per day of GWL weather type⁽²⁾ reported over the April-August 1994, 1995 and 1996 period (456 days).

⁽¹⁾ August 1996 is not included for France
⁽²⁾ classes that contribute less than 7% to the exceedances for each Member State are not considered. Bold and normal characters refer to anticyclonic and cyclonic weather types, respectively.

The second column refers to the occurrence of the weather types (in % days). The three highest values in bold and the maximum is underlined. Dash symbol means that no exceedance was recorded.

I.2. Ozone conducive meteorological parameters

The eight following characteristic wide-spread ozone episodes were selected during the 1994, 1995 and 1996 growing seasons, in order to characterise the meteorological situations which are conductive of high ozone levels: (1) 23 to 30 June 1994; (2) 19 July to 2 August 1994; (3) 3 to 9 May 1995; (4) 27 June to 3 July 1995; (5) 23 to 28 July 1995; (6) 18 to 24 April 1996; (7) 3 to 9 June 1996; (8) 10 to 14 June 1996. A total of 4347 ozone exceedances of the population information threshold were reported during the 62 corresponding days (i.e. 45 % of the total exceedances recorded over the April-August 1994, 1995 and 1996 periods).

Table I.4: The selected episodes: duration (days), exceedances of the population information threshold (N_exc. ), Member States where they were recorded, and daily GWL weather types (days)

Figure I.1: Examples of the daily Europe scale thematic maps performed for the 62 days of the eight selected 1994, 1995 and 1996 episodes for (a) the topology of the 850 pressure surface at 12 UTC (dm), (b) the horizontal wind at 12 UTC for the 850 mb (not shown) and 1000 mb levels, (c) the daily maximum surface temperature (°C) and (d) the 12-18 UTC average surface solar radiation (W.m^-2.s). Stations which reported ozone exceedance(s) of 180 mg/m³ threshold are shown in red points.

Data sources are (a) the European Meteorological Bulletin of the Deutscher Wetterdienst (Offenbach, Germany), (b, d) the 0.5° resolution analyses from the European Centre of Meteorological Weather Forecast (Reading, United-Kingdom) and (c) the MARS-JRC meteorological station-database (Ispra, Italy).

I.3 Photochemical formation of ozone

Figure I.2 Stations which reported exceedance(s) of 180 mg/m³ ozone concentration (black points) and topology of the 850 mb pressure surface at 12 UTC (dm) derived from the European Meteorological Bulletin of the Deutscher Wetterdienst (Offenbach, Germany) over the 4-9 June 1996 period (selected episode n°7), which was characterised by a ridge of high pressure over Middle Europe (BM GWL weather type).

Table I.5: Annual and seasonal average ⁽¹⁾ daily mean and maximum O₃ and Ox (O₃ + NO₂ ) concentrations (mg.m^-3) for selected urban and rural stations in the EU, over the 1994-1995 period ⁽²⁾.

⁽¹⁾ Only complete available data (i.e. 24 hourly values per day) are taken into account ⁽²⁾ O₃ ( and Ox) complete available data: Athens: 421 (343) days, Barcelona: 363 (265) days, London: 480 (461) days, Berlin: 721 (678) days, Veciana: 580 days, Agullana: 570 days, Lady Bower Reservoir: 520 days, Spessart: 640 days.

Figure I.3: Monthly average⁽¹⁾ daily maximum O_x (O₃ + NO₂ ) concentrations (mg.m^-3) in Athens, Barcelona, London and Berlin, over the 1994-1995 period.

(1) No data for April 1995 in Barcelona and for July and August 1995 in London.

Figure I.4a: Hourly ozone (O₃), temperature (T°) and solar radiation (SR) measurements, at Veciana elevated (726m) rural station (Spain), from the 10 to the 14 June 1996. An atypical daily evolution of the concentrations is observed, with high concentrations also reported during the night.

Figure I.4b: 96h-backward trajectories initialised on the 14 June 1996, at 10 UTC and at 1000 m (solid line), 1500 m (dotted line), 3000 m (dashed line) altitude levels for Veciana reception site, respectively (Koffi, 1997). The Pyrénées altitude range is given by the PERIDOT 10km -resolution model (Imbard et al., 1986). Veciana is located in a sheltered zone south of the Pyrenees where stagnation and vortex circulation of the air masses in the low troposphere induced a particular daily evolution and an increase of the ozone concentration level.

Figure I.5: Mean diurnal ozone variations at several locations in the greater Athens area, as a function of the wind regime. These typical diurnal variations were summarised from a time series of 24 days during August/September 1994 MEDCAPHOT campaign (Suppan et al., 1998).

I.4. Long-range transport

Figure I.6a: Hourly O_x (O₃ + NO₂ ) measurements at Lullington Heath rural station during episode n°5.

Figure I.6b: horizontal wind field at 850 hPa (left) and backward trajectories at (a) 500, (b) 1500 and (c) 3000 m altitude, for Lullington Heath (England) reception point (right), on the 25 and 26 July 1995 (Koffi, 1997). The air flows changed from maritime to continental direction from the 24 to the 25 July 1995 and were again maritime on the 26. The air in the boundary layer in South England came from the Benelux on the 25 July, where it had remained for more than 12 hours in a highly polluted environment. The increase of ozone levels observed in Lullington Heath is obviously due to long-range transport from the continent.

Figure I.7a: Exceedances of the population information threshold and topology of the 850 mb surface at 12 UTC (dm), on the 20, 21 and 22 April 1996 (episode n° 6).

Figure I.7b: 48-h backward trajectories initialised at 14 UTC on the 22/04/96 at (a) 500, (b) 1500 and (c) 3000 m altitude, for Spessart, Salzwedel and Berlin reception sites, in Germany (Koffi, 1997). Episode of 18-24 April 1996 occurred during an anticyclonic situation over Middle Europe with predominantly south-westerly flows. The centre of the anticyclone quickly moved from Germany to East-Southeast on the three first days, but ozone exceedances where still recorded in that country on the following days. Long-range transport from North-east of France and from Austria is shown for three reception sites in Germany, over the 20-22 April 1996 period.

I.5 Transport down from the upper troposphere/lower stratosphere

Figure I.8: Vertical velocity isolines (mm.s^-1) on the 5 May 1995, at 12 UTC for the (a) 1000 mb and (b) the 850 mb pressure levels, derived from the 0.5° lat/long resolution analyses from the European Centre for Medium range Weather Forecasts (ECMWF, UK)

The wind vertical velocity calculated over Europe for the 850 and 1000 mb pressure levels during episode n°3 shows a subsidence of the air masses during the three first days, with its centre over Benelux on the 5 May 1995.

Figure I.9: Weekly average ⁷Be surface concentrations at Luxembourg (from the REM data bank; De Cort, 1997) and 180 mg.m^-3 ozone exceedances at Luxembourg, Esch/Alzette, Elvange and Vianden stations in Luxembourg during 1995.

Beryllium 7 (a tracer of air coming from the free troposphere) surface concentrations measured in Luxembourg in 1995, show a significant peak of the concentrations during the May ozone episode (episode n°3) which indicates down transport of ozone from the upper troposphere/lower stratosphere. Moreover, the 1995 two main ozone summer episodes (episodes n° 4 and n° 5) coincide with peaks of ⁷Be concentrations, which also suggest a contribution of ozone coming from the free troposphere, in addition to local photochemical formation.