Urban influence on increasing ozone concentrations in a characteristic Mediterranean agglomeration

Air quality in cities has been extensively studied due to the high population density potentially exposed to high levels of pollutants. The main problems in urban areas have been related to particulate matter (PM) and NO2. Less attention has been directed towards O-3 because urban levels are generally lower than those recorded in rural areas. The implementation of air quality plans, together with technological improvements, have resulted in reductions of PM and NO2 levels in many European cities. In contrast, urban O-3 levels have experimented increases which may respond to declining NOx emission trends. It is therefore necessary to intensify the study of urban O-3 and its potential relation with NOx variations. In the agglomeration of Zaragoza (NE Spain), traffic circulation through the centre has dropped by 28.3% since 2008 due to several factors such as the implementation of a mobility plan, the completion of major construction projects and the economic crisis in Spain. The study of this case offers a unique opportunity to evaluate the impact of reductions in NOx emissions on the levels of O-3 in a characteristic Mediterranean city. This work analyses the variability and trends of ambient air levels of O-3 and NOx in Zaragoza and the Ebro valley from 2007 to 2012. Results demonstrate that, although the main factor explaining O-3 variability is still linked to meteorology, changes in NOx emissions strongly influence O-3 variability and trends, mainly due to interaction with fresh NOx. Specific analysis of the O-3 weekend effect show a significant correlation (r(2) = 0.81) between the drop of NOx concentrations (associated to emissions) and the increment of O-3 levels during weekends. Moreover, trend analyses reveal that the decline in NOx emissions in Zaragoza from 2007 to 2012 can be associated with significant increments in O-3 levels. (C) 2014 Elsevier Ltd. All rights reserved.