On the spatial distribution and seasonal variation of lower-troposphere ozone over Europe

Surface ozone data from 25 European low-altitude sites and mountain sites located between 79 degrees N and 28 degrees N were studied. The analysis covered the time period March 1989-February 1993. Average summer and winter O-3 concentrations in the boundary layer over the continent gave rise to gradients that were strongest in the north-west to south-east direction and west-east direction, respectively. Wintertime O-3 ranged from 19 to 27 ppb over the continent, compared to about 32 ppb at the western border, while for summer the continental O-3 values ranged between 39 and 56 ppb and the oceanic mixing ratios were around 37 ppb. In the lower free troposphere average wintertime O-3 mixing ratios were around 38 ppb, with only an 8 ppb difference between 28 degrees N and 79 degrees N. For summer the average O-3 levels decreased from about 55 ppb over Central Europe to 32 ppb at 79 degrees N. In addition, O-3 and O-x (= O-3 + NO2) in polluted and clean air were compared. The amplitudes of the seasonal ozone variations increased in the north-west to southeast direction, while the time of the annual maximum was shifted from spring (at the northerly sites) to late summer (at sites in Austria and Hungary), which reflected the contribution of photochemical ozone production in the lower parts of the troposphere.