Identification and origin of nocturnal ozone maxima at urban and rural areas of Northern Portugal – Influence of horizontal transport

Nocturnal ozone maxima have been reported by several authors in different environments. The aim of this study was to evaluate the influence of horizontal transport on the occurrence of nocturnal ozone maxima. Frequency, concentration and time of the nocturnal ozone maxima were investigated during three consecutive years (2005–2007) in two urban and two rural sites of Northern Portugal using hourly ozone concentrations. These concentrations were further related with temperature, relative humidity, wind direction and speed. Results showed that urban sites had frequencies of nocturnal ozone maxima between 40% and 50% and rural sites of 15%. Rural sites were those with higher concentrations being in average 117 ± 35 μg m −3 , with minimum and maximum concentrations of, respectively, 51 μg m −3 and 283 μg m −3 ; information and alert thresholds were exceeded ( Directive 2002/3/EC ). At urban sites, the nocturnal ozone maxima occurred most at 5 h and at rural sites at 23 h/24 h. Among the meteorological parameters analysed, wind direction was the only one that had influence on the occurrence of nocturnal O 3 maxima. When nocturnal O 3 maxima occurred it was possible to observe at the urban sites, prevailing winds from ESE, related with land–sea breezes, meaning that land–sea breeze influenced the occurrence of nocturnal O 3 maxima trough the transport of O 3 from places where its concentrations were higher. At rural sites similar behaviour has been observed, nevertheless with different wind directions according to meteorological characteristics of the sites; frequent wind directions were from WNW for LO and from the North quadrant for SM. Additionally, between 70% and 99% of the nocturnal O 3 maxima for urban and rural sites occurred when there was a shift in the wind direction. Results allowed concluding that horizontal transport might have high importance on the occurrence of those maxima. Nevertheless, other factors, such as vertical transport, may also be important, as have been suggested by other authors. ► Monthly frequency of nocturnal O 3 maxima is lower at rural than at urban ones. ► Nocturnal O 3 maxima occurred in general at 5 h in urban costal sites. ► Nocturnal O 3 maxima occurred in general at 23 h in rural sites. ► Nocturnal O 3 maxima were related with land–sea breeze system at urban coastal sites. ► Nocturnal O 3 maxima were related with regional wind system at rural sites.