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Impact of transatlantic transport episodes on summertime ozone in Europe

by G Guerova, I Bey, J.-L Attié, R V Martin, J Cui, M Sprenger
Atmos. Chem. Phys. Atmospheric Chemistry and Physics ()
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This paper reports on the transport of ozone (O 3) and related species over the North Atlantic ocean and its im-pact on Europe. Measurements of nitrogen dioxide (NO 2) and carbon monoxide (CO) columns from the GOME and MOPITT satellite instruments, respectively, are used in con-junction with the GEOS-CHEM global model of transport and tropospheric chemistry to identify the major events of long range transport that reach Europe over the course of summer 2000. Sensitivity model simulations are used to analyse observed O 3 distributions with respect to the im-pact of long range transport events. For that purpose, we used in-situ O 3 observations taken at the mountain site of Jungfraujoch as well as O 3 vertical profiles taken in the vicin-ity of central European cities. Over the course of summer 2000, we identified 9 major episodes of transatlantic pol-lution transport; 7 events are associated with transient cy-clones while 2 events occur through zonal transport (e.g. by advection in the strong low-level westerly winds established in summer between the Azores anticyclone and transient cy-clones). We find that on average three episodes occur per month with the strongest ones being in June. The number and frequency of long range transport events that reach Eu-rope are driven by the position and strength of the Azores anticyclone. Model sensitivity simulations indicate that the summer mean North American O 3 contribution ranges from 3 to 5 ppb (7–11%) in the planetary boundary layer and 10 to 13 ppb (18–23%) in the middle and upper troposphere. Dur-ing particular episodes, North American sources can result in O 3 enhancements up to 25–28 ppb in the layer between 800–600 hPa and 10–12 ppb in the boundary layer. The im-pact of the zonal transport events on O 3 distribution over Eu-Correspondence to: G. Guerova ( rope is more clearly seen below 700 hPa as they tend to trans-port pollution at lower levels while the events associated with transient cyclones are more likely to have an impact on the middle and upper troposphere (i.e. above 600 hPa). The air mass origins found in the GEOS-CHEM model are clearly confirmed by back trajectory analyses. During most of the 9 events, a strong contribution in North American O 3 is in general associated with only little European O 3 and vice-versa (in particular at the Jungfraujoch). A substantial North American contribution (e.g., 30% or higher) to O 3 over Eu-rope does not always result in pronounced O 3 enhancements in the observations during our period of study.

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