Surface Ozone Pollution: Trends, Meteorological Influences, and Chemical Precursors in Portugal

Surface ozone (O 3 ) is a secondary air pollutant, harmful to human health and vegetation. To provide a long-term study of O 3 concentrations in Portugal (study period: 2009–2019), a statistical analysis of ozone trends in rural stations (where the highest concentrations can be found) was first performed. Additionally, the effect of nitrogen oxides (NO x ) and meteorological variables on O 3 concentrations were evaluated in different environments in northern Portugal. A decreasing trend of O 3 concentrations was observed in almost all monitoring stations. However, several exceedances to the standard values legislated for human health and vegetation protection were recorded. Daily and seasonal O 3 profiles showed high concentrations in the afternoon and summer (for all inland rural stations) or spring (for Portuguese islands). The high number of groups obtained from the cluster analysis showed the difference of ozone behaviour amongst the existent rural stations, highlighting the effectiveness of the current geographical distribution of monitoring stations. Stronger correlations between O 3 , NO, and NO 2 were detected at the urban site, indicating that the O 3 concentration was more NO x -sensitive in urban environments. Solar radiation showed a higher correlation with O 3 concentration regarding the meteorological influence. The wind and pollutants transport must also be considered in air quality studies. The presented results enable the definition of air quality policies to prevent and/or mitigate unfavourable outcomes from O 3 pollution.