Atmospheric BTEX at a Mediterranean city (Heraklion, Crete, Greece): levels, sources and contribution to secondary aerosol formation

BTEX (benzene, toluene, ethylbenzene and xylenes) are key atmospheric pollutants due to their toxicity, as benzene is classified as a carcinogen, and their role in atmospheric chemistry. This study reports on the levels and sources of BTEX, cyclohexane and styrene in a Mediterranean urban site (Heraklion, Crete, Greece), along with their potential contribution to secondary organic aerosol (SOA) formation. From April 2023 to October 2024, continuous monitoring was performed by using an on-line gas chromatograph. On average, toluene (31%) was found to be the dominant BTEX, followed by xylenes (28% for m/p xylenes and 11% for o xylene), while benzene remained below the EU’s annual limit of 5 μg m -3 . Most compounds exhibited distinct seasonal variability with maxima in winter and autumn and minima in summer. A pronounced bimodal diurnal pattern was observed, with peaks during morning and evening rush hours, hinting traffic as their main source. The night-time maximum was enhanced during winter indicating an additional contribution from biomass burning, estimated to account for nearly half of the benzene levels. Styrene showed no clear seasonal or diurnal patterns. Source attribution was supported by specific tracers such as fossil fuel and wood burning black carbon, along with diagnostic ratios of combined BTEX, demonstrating permanent dominance of vehicular emissions and enhancement of biomass burning during winter night-time. SOA formation potential from the oxidation of locally emitted BTEX was investigated during winter, resulting to 2.8 μg m -3 of SOA, corresponding to approximately 50% of the locally measured organic aerosol mass.