Air quality data (O₃, NO₂, NO, CO and SO₂) of two Greek coastal cities, Patras and Volos, were analyzed and compared to evaluate: (a) the exceedances of air quality EU threshold values, (b) the diurnal patterns of air pollutants and (c) the “weekend effect” on ozone levels.

High ozone levels, close to the thresholds for human health and clearly above the threshold for the protection of plants and ecosystems, were observed in Volos. O₃ levels in Volos were higher than those in Patras. NO_x levels in Patras were significantly higher than the limits for human health and plants’ protection. Both, NO_x and SO₂ levels were higher in Patras than in Volos. The Patras’ harbor high traffic seems to drive the diurnal pattern of SO₂ in that city.

The examination of the rate of ozone accumulation, during the high O₃ period (Apr.–Sep.), revealed the occurrence of two phases, a fast and a slow one, with different durations in each city. We suggest that the occurrence of such two phases’ patterns should be considered in relevant ozone studies.

In both towns, the O₃ levels were higher during weekends in comparison to midweek days, although NO levels were lower. Our results support the hypothesis that the weekend O₃ effect is due to a combination of VOC sensitivity of the studied areas and the reduced NO_x emissions during weekends. Based on the comparison of the weekend effect in the two cities, we suggest the occurrence of a feedback mechanism between peri-urban natural ecosystems (forests) and the polluting anthropogenic ones (cities).

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