Comparison between weekend and weekday ozone concentration in large citiesin France

This paper examines ozone data from five large French cities (Marseilles, L yon, Paris, Strasbourg and Toulouse) in spring and summer over a three-year period to study the possible influence of local primary pollutant emission s. In these cities the level of traffic emission varies according to the da y of the week. There is a decrease of about 25% in traffic emissions betwee n non-consecutive Tuesdays and Sundays. Traffic emissions on Fridays are ab out 40% more than on non-consecutive Sundays whereas they seem to be simila r for non-consecutive Tuesdays and Thursdays. Despite this variation in tra ffic emissions between Fridays and Sundays, 85% of daily ozone maxima are i dentical for all days compared; in 15% of cases, percentiles of daily ozone maxima vary by about 20% at the most. This difference is observed for the highest values of daily ozone maxima that we can find both in rural and urb an sites. Marseilles is the most pollution-sensitive city; every site of th is area is concerned, which gives a regional origin to ozone variability. I n the less-populated Toulouse area, differences between ozone on Fridays an d Sundays are less significant. Our results show the importance of advectio n phenomena of ozone. It calls into question strategies of local reductions in traffic during ozone episodes. (C) 2001 Elsevier Science Ltd. All right s reserved.