Analysis of Daily and Diurnal O3-NOx Relationships and Assessment of Local/Regional Oxidant (OX = O3 + NO2) Levels and Associated Human Health Risk at a Coastal Suburban Site of Sfax (Tunisia)

Abstract

The present study investigated the temporal variability of surface ozone and its nitrogen precursors at the proximity of a traffic crossroad (≈ 22,580 vehicles per day) located in a coastal suburban site of Sfax city (Tunisia). It was performed during January-October, 2010. The study results show that the surface ozone is characterized by a slightly modulated regime between day and night. At traffic-peak hours, the decrease of ozone concentration levels is due to the oxidation reaction of NO into NO₂. Complementary statistical approaches (inter-variable correlation matrix, cluster analysis, representation quality of variables and multiple regression analysis) reveal that the excess of O₃ is mainly affected by the wind speed, temperature, solar radiation and NO₂ with contribution rates of 127, 21, 22 and 12%, respectively. The decrease of O₃ is, however, controlled by NO, relative humidity and boundary layer height with contribution rates of 25, 21 and 16%, respectively. The regional daytime and night-time contributions to O₃ are very different. The daytime intercept which is greater than that of night-time indicates there was a large NO_x independent regional contribution. This could be attributed to the biogenic VOCs effect interfering in the photochemical cycle. It, therefore, implies that the study site is VOC-sensitive. The investigation of the air quality index (AQI) for O₃ and NO₂ showed that more than 86% of the total studied period has a good quality level. Only about 14% of total days are characterized by an acceptable air quality level, however, for a very small number of people are unusually sensitive to air pollution.