Temporal distribution, behaviour and reactivities of BTEX compounds in a suburban Atlantic area during a year

Abstract

The temporal distribution of BTEX compounds (benzene, toluene, ethylbenzene and xylene isomers) in ambient air was studied in a suburban Atlantic area in the northwest of Spain. These compounds mean serious risks for public and occupational health and for biological and physical environments. A total of 1200 samples were analysed during a year, and in most of the samples toluene was the most abundant VOC followed by benzene or m+p-xylene. The average concentrations were 1.62, 2.15, 0.60, 0.94 and 0.47 µg m⁻³ for benzene, toluene, ethylbenzene, m+p-xylenes and o-xylene respectively, although they reached values as high as 50.53, 28.57, 21.33, 46.13 and 22.74 µg m⁻³ for the compounds in the same order. An exhaustive study was carried out to establish how both emission sources and the climatic conditions affected the BTEX levels measured in the outskirts of a small city. Wind directions and speeds, daily and seasonal BTEX profiles, ratios among the concentrations measured in the sampling site, in the city centre and in the fuels commonly used by the cars in the city were studied. The pollution roses revealed the presence of diffuse BTEX sources affecting the sampling point, although all the compounds reached their highest levels when the wind blew from an industrial area in the SW of the city. The daily BTEX profiles were well-correlated with the traffic pattern in the accesses to the city and the seasonal profile showed that the highest concentrations were reported in summertime. The ratios T/B, m+p-X/B and o-X/B were 1.32, 0.58 and 0.29 respectively; these values were lower than the ones reported in previous studies carried out in the centre of the same city. This could be caused by the major distance from the sampling point to the city centre and by the changes in the fuels and in the kind of vehicles used in the city. The correlation coefficients among the TEX fraction were always higher than 0.72 but benzene only showed a good correlation with these compounds in winter. The correlations between BTEX concentrations and their contributions to the photochemistry in the sampling area were used in order to evaluate the distance from the road traffic source and to estimate the presence of fresh or old air masses in the sampling point. This study provides data which can be useful to establish the BTEX levels in suburban areas from the coastal Atlantic region and improves the knowledge of the BTEX presence in this kind of area.