TRANSPORT, FORMATION AND SINK PROCESSES BEHIND SURFACE OZONE VARIABILITY IN NORTH EUROPEAN CONDITIONS

Measurements of ozone, nitrogen dioxide and meteorological parameters at the two Finnish EMEP background stations of Ahtari (forested site) and Uto (an offshore island) show clear indications of the influence of the precursor source areas of Western and Eastern Europe on surface ozone behaviour at these higher latitudes. The mean ozone levels are relatively high, with maximum monthly values of 41 and 42 ppb, respectively. These values occur in April at both sites nearly irrespectively of wind direction, pointing to a global feature. Other spring, as well as summer and autumn, months have lower ozone values for wind direction sectors corresponding to clean air masses (north-westerlies and north-easterlies). Surface uptake is an important sink in the local ozone budget, especially during late spring, summer and early autumn. Chemical losses are more efficient in winter, when the ground is covered by snow.