1. Atmospheric conditions associated with high and low summer ozone levels in the lower troposphere over the eastern Mediterranean and ship borne observations of ozone over the Western Mediterranean Pavlos Kalabokas 1,2 and Jens Hjorth 2 (1) Academy of Athens, Research Center for Atmospheric Physics and Climatology, Athens, Greece (pkalabokas@academyofathens.gr) (2) European Commission, JRC, Institute for Environment and Sustainability, Air and Climate Unit, Ispra, Italy (jens.hjorth@jrc.ec.europa.eu), 4th International Workshop CHArMEx - the Chemistry-Aerosol Mediterranean Experiment, 20-24 October 2014, Trieste, Italy

2. Synoptic weather conditions over high and low ozone episodes over the Eastern Mediterranean (Aegean Sea) MOZAIC ozone vertical profiles (Kalabokas et al., ACP, 2007) Surface ozone measurements (Kalabokas et al., Atm.Env., 2008) Synoptic weather conditions over high and low ozone episodes over the Eastern Mediterranean (Cairo – Tel-Aviv) MOZAIC ozone vertical profiles (Kalabokas et al., ACP, 2013) Synoptic weather conditions over high and low ozone episodes over the Western Mediterranean (Velchev et al., ACP, 2011) Surface ozone measurements on a cruise ship Surface ozone measurements at EMEP stations

3. Vertical MOZAIC ozone average profiles over Rhodes, during high ozone days (red, 5 profiles) and the low ozone days (blue, 5 profiles) in summer 2002.

4. Percentage contribution of geographical regions after 10- days FLEXPART backward simulation for the 4 high ozone cases and for the 0–2 km air parcels arriving at Rhodes: (a) for the 0–2 km layer, (b) for the 2–4 km layer, (c) for the >4 km layer and d) for the total atmospheric column.

5. Percentage contribution of geographical regions after 10-days FLEXPART backward simulation for the 5 low ozone cases and for the 0–2 km air parcels arriving at Rhodes: (a) for the 0–2 km layer, (b) for the 2–4 km layer, (c) for the >4 km layer and (d) for the total atmospheric column.

6. Composite weather map of geopotential heights at 850 hPa of the days with the highest ozone values observed at 900 hPa (left, 5 profiles) and of the days with the lowest ozone values observed at 900 hPa (right, 5 profiles)

8. Composite weather maps of geopotential heights at 850 hPa of the 7% of the days (18 days) with the highest afternoon (12:00–18:00) ozone values (upper panel, isopleths of 10 m) and of the 7% of the days (18 days) with the lowest afternoon (12:00–18:00) ozone values (lower panel, isopleths of 10 m) in Finokalia, Greece in the summers (JJA) of 1998–2000.

9. Composite weather maps of geopotential heights at 700 hPa of the 7% of the days (18 days) with the lowest afternoon (12:00–18:00) ozone values in Finokalia, Greece (upper panel, isopleths of 15 m) and of the 7% of the days (19 days) with the lowest afternoon (12:00–18:00) ozone values (lower panel,isopleths of 15 m) in Ag. Marina, Cyprus in the summers (JJA) of 1998–2000.

10. (a) (b) (c) (d) (a). Vertical profiles of summer (JJA) ozone over the Eastern Mediterranean airport of Cairo for the 7% highest (red) and the 7% lowest (blue) ozone mixing ratios at 1500-5000m., (b). Same as (a) but for relative humidity, (c). Same as (a) but for carbon monoxide (CO), (d). (Left), Same as (a) but for temperature gradient per km, (Right), Same as (a) but for wind speed. The black line in (a), (b), (c), shows the mean profile of the respective parameter.

11. Composite weather maps of geopotential heights at 850 hPa of the 7% of the days with the highest ozone concentrations at the 1500-5000 m layer (left column) and of the 7% of the days with the lowest ozone concentrations at the 1500-5000 m layer (right column) in Cairo, during the day of the measurement (1st row) and 2-days ago (2nd row).

12. (a) (b) (c) (d) Vertical summer (JJA) ozone profiles over the Eastern Mediterranean airport of Tel-Aviv for the 7% highest (red) and the 7% lowest (blue) ozone mixing ratios at 1500-5000m., (b). Same as (a) but for relative humidity, (c). Same as (a) but for carbon monoxide (CO), (d). (Left), Same as (a) but for temperature gradient per km, (Right), Same as (a) but for wind speed. The black line in (a), (b), (c), shows the mean profile of the respective parameter. In (c) no CO data are available during lowest ozone days.

13. Composite weather maps of geopotential heights at 850 hPa of the 7% of the days with the highest ozone concentrations at the 1500-5000 m layer (left column) and of the 7% of the days with the lowest ozone concentrations at the 1500-5000 m layer (right column) in Tel-Aviv, during the day of the measurement (1st row) and 2-days ago (2nd row).

14. (Upper panel) Daily variability of the 12:00 - 18:00 afternoon ozone and Ox (O3+NO2) on the cruise ship for 2008 (in ppb). (Lower panel) Daily variability of the 12:00 - 18:00 afternoon ozone (in ug/m3) on the EMEP stations surrounding the Western Mediterranean basin: Spain (orange), France (black), Switzerland (green), Italy (red), Malta and coastal Spain (blue) for 2008.

15. (Upper panel): Composite weather maps of the geopotential heights at 850 hPa of a high ozone episode (Aug.28 – Sep.1, 2008) in the Western Mediterranean (left) and 2 days before (right) (Lower panel): Same as in upper panel but for a low ozone episode (Sep.11-15, 2008)

16. (Upper panel) Daily variability of the 12:00 - 18:00 afternoon ozone and Ox (O3+NO2) on the cruise ship for 2011 (in ppb). (Lower panel) Daily variability of the 12:00 - 18:00 afternoon ozone (in ug/m3) on the EMEP stations surrounding the Western Mediterranean basin: Spain (orange), France (black), Switzerland (green), Italy (red), Malta and coastal Spain (blue) for 2011.

17. (Upper panel): Composite weather maps of the geopotential heights at 850 hPa of a high ozone episode (Aug.20–25, 2011) in the Western Mediterranean (left) and 2 days before (right) (Lower panel): Same as in upper panel but for a low ozone episode (Aug.5-10, 2008)

18. CONCLUSION After investigating the characteristic meteorological conditions associated with high and low ozone concentrations in the boundary layer, it appears that an important factor leading to high surface ozone values in the Eastern Mediterranean area is the anticyclonic influence of the high-pressure domination over the Central Mediterranean and the Balkans. On the other hand, the lowest ozone levels are associated with the passing of low-pressure systems or the extension to the west of the Middle-East low associated with weak pressure gradients over the Eastern Mediterranean and an upper air trough in the North Eastern Europe. Similar results come also out from the analysis of MOZAIC ozone vertical profiles in the lower troposphere over the Eastern Mediterranean but also form ship borne ozone measurements and from EMEP stations in the Western Mediterranean although the subsidence seems to be weaker. Based on the above analysis, it comes out that the lower troposphere ozone variability over the Eastern Mediterranean area is mainly controlled by the synoptic meteorological conditions. This applies also to a certain extend to the boundary layer ozone variability in combination with local topographical and meteorological features. In particular, the highest ozone concentrations in the lower troposphere and subsequently in the boundary layer are associated with large scale subsidence of ozone rich air masses from the upper troposphere under anticyclonic conditions while the lowest ozone concentrations are associated with low pressure conditions inducing uplifting of boundary layer air, poor in ozone and rich in relative humidity, to the lower troposphere.