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

► Monthly frequency of nocturnal O3 maxima is lower at rural than at urban ones. ► Nocturnal O3 maxima occurred in general at 5 h in urban costal sites. ► Nocturnal O3 maxima occurred in general at 23 h in rural sites. ► Nocturnal O3 maxima were related with land-sea breeze system at urban coastal sites. ► Nocturnal O3 maxima were related with regional wind system at rural sites.