Seasonal and diurnal variations of BTEX and their potential for ozone formation in the urban background atmosphere of the coastal city Jeddah, Saudi Arabia

Abstract

Past measurements of volatile organic compound (VOC) concentrations from Middle Eastern countries are very few, and this study assesses the concentrations and processes affecting benzene, toluene, ethylbenzene and xylenes (BTEX) in the atmosphere of an urban background area of Jeddah, a coastal city in Saudi Arabia, and their potential for ozone formation. The measurements were carried out for a year (from December 2011 to November 2012) and include hourly BTEX and meteorological parameters. The annual average concentrations of BTEX species were 0.41 ppb for benzene, 1.40 ppb for toluene, 0.49 ppb for ethylbenzene, 1.56 ppb for m,p-xylene and 0.94 ppb for o-xylene. The annual mean benzene level (0.41 ppb, ∼1.31 μg m−3) did not exceed the annual threshold level (5 μg m−3) set by the European Union but still represents a small risk to human health. BTEX showed a seasonal variation, with higher concentrations during the spring and lower concentrations during the autumn. The diurnal variation of BTEX concentrations followed a commonly observed pattern, with two peaks associated with high traffic volumes. m,p-Xylene was the largest contributor to ozone formation potential followed by o-xylene, toluene and benzene. The significantly positive correlation between BTEX compounds as well as the ratio of toluene/benzene (average = 4.03) suggested that vehicle emissions were the major source of BTEX during the whole investigated period. m,p-Xylene-to-ethylbenzene ratios showed an annual mean of 3.18 with little variability during the different seasons indicating that the photochemical age in the study area is relatively young due to the continual fresh emissions experienced in Jeddah city.