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Ambient ozone (O₃) concentrations tend to be higher on weekends than on weekdays in urban areas, a phenomenon known as the weekend effect. Nevertheless, its downwind influence could be diverse. The MM5-CMAQ model has been used to assess the causes of weekday/weekend O₃ differences in the north-eastern Iberian Peninsula during an episode of photochemical pollution covering the whole Western Mediterranean Basin (13–16 August 2000). Emissions were estimated with EMICAT2000 model. The key differences between weekday and weekend on-road vehicle emissions are in magnitude and timing. On-road traffic accounts for ∼57% of the anthropogenic NO_x and VOC emissions in the modeling domain, and are principally located along the coast and mainly in the Barcelona Geographical Area. The drop of heavy-duty tracks traffic on weekends involves a 22% reduction of NO_x emissions and just …