Comparative assessment of ambient air quality in two typical Mediterranean coastal cities in Greece.

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. NOx levels in Patras were significantly higher than the limits for human health and plants' protection. Both, NOx 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 NOx 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).