Surface O3 temporal variation, photolysis and accumulation in urban Tunis (North Africa) during January to December, 2016: influence of meteorology and chemical precursors

The present study investigated the temporal variability of surface O 3 and its (NO, NO 2 and CO) precursors at the proximity of a busy trafficked crossroad located in urban Tunis, Tunisia. It was performed during January to December, 2016. The results revealed that the diurnal profiles of selected precursors were characterized by maxima at traffic-peak hours linked to the effect of automobile traffic being a close source. At traffic-peak hours, the decrease of O 3 concentration levels is due to the oxidation reaction of NO into NO 2 . Based on the diurnal profile per each month, the duration of O 3 accumulation was shown close to five hours, with higher accumulation rates in the summer season (2.4 to 3 ppb/h) and also in March (3 ppb/h). The lower accumulation ones were, however, observed in the other months (1.1 to 1.9 ppb/h). The O 3 excess pronounced from the 10 th March to the 1 st April, 2016 was due to the effect of the persistence of a Rex-type blocking anticyclone over Central Europe. Compared to extremely stable conditions occurring in hot seasons (summer and autumn), the recorded moderate stable conditions during such Rex was categorized by a marked deviation between day-night intercepts (41% against 23%). This could be attributed to the cumulative effect of the biogenic VOCs and the seasonal excess of CO interfering in the photochemical cycle. It, therefore, implies that the study site is CO and VOC-sensitive.