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 NOx 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.