Weekend-Weekday Effect Assessment for O3, NOx, CO and PM10 in Andalusia, Spain (2003-2008)

Day-of-week variations in O-3, NOx, CO and PM10 were analysed in the southwestern region of Europe (specifically Andalusia, Spain) using hourly concentrations collected at 43 stations (representing five typologies) over 6 years (2003-2008). This area has air pollution problems, and the study of the weekend effect is used as a tool to plan future strategies for emissions reductions. Maximum daily 8 h averages for O-3, daily 90th percentiles for NO, NO2 and PM10 and mean daily values for CO were calculated and used to assess the weekend effect by applying three different metrics: weekly evolution, weekend-weekday differences and average daily difference between weekends minus workdays. Based on daily parameters and weekend-weekday differences, all the measured air pollutants exhibited clearly reduced concentrations (oscillating between 25 to 85%) on weekends, mainly at urban and suburban stations, with the exception of O-3 (less than 10 mu g/m(3)). This decrease on weekend days was mostly observed for NO, NO2 and PM10 at urban stations, while differences at industrial and rural stations were low or null. CO showed a low reduction. Using the daily cycle of differences (weekend minus workdays) as a reference, positive deviations were observed at night and negative differences were observed during the daytime for NO, NO2 and PM10 at urban traffic, urban background and suburban background sites. A reduction in morning rush hour traffic during the weekends was reflected in the data from urban stations, which showed a decrease of up to 50, 15 and 12 mu g/m(3) for NO, NO2 and PM10, respectively. An opposite daily behaviour was found for ozone, which showed an increase at urban area sites of up to 15 mu g/m(3) in the early morning.