Temporal variations of surface regional background ozone over Crete Islandin the southeast Mediterranean

The first year-round observations of seasonal and diurnal variations of bac kground ozone at a coastal site on Crete island in the southeast Mediterran ean area are presented here. They point out (1) the existence of a well-def ined seasonal cycle with maximum during summer months, (2) the presence of elevated O-3 levels (up to 80 ppbv) during daytime and over time periods of several days, and (3) the dependence of O-3 mixing ratios on air mass orig in. Comparison with three-dimensional chemistry transport model results sho ws that during summer the measured O-3 values exceed the calculated by 10-2 0 ppbv. Inclusion of biomass burning and biogenic volatile organic emission s in the model could partly offset the discrepancy between model results an d observations.