Surface ozone (O-3) is one of the most important pollutant in the atmosphere, causing several damages on human health, climate, vegetation and materials. This pollutant presents different behaviours in urban and rural environments. High levels of nitrogen oxides (O-3 precursor) at urban sites have an important role in the photochemical O-3 production in the atmosphere. However, O-3 concentrations tend to be higher in rural areas. To better understand this phenomenon, this study aims to apply statistical methods to evaluate the O-3 behaviour at 13 rural sites in Portugal. The results showed that: (i) the O-3 concentrations presented the highest values in 2003 and 2005; (ii) the standard values for human health protection was surpassed at almost all monitoring sites; (iii) the maximum concentrations usually occurred at the same time at all monitoring sites (the highest frequency at 16 h), with exception of the monitoring site located in Azores archipelago which had an almost uniform distribution; (iv) three groups of monitoring sites were identified using cluster analysis; and (v) similar daily average profiles of O-3 concentrations were observed at sites belonging to the same group.
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