Measurement report: 30 years of BTEX monitoring at a suburban site in Switzerland supported by additional urban VOC observations

Abstract. Since 1994, benzene, toluene, ethylbenzene and xylene isomers (BTEX) are monitored in the ambient air at Dübendorf (DUE) in the suburban area of Zurich city in Switzerland. Overall, BTEX concentrations decreased up to 89 % in ambient air in DUE notably due to the introduction of regulations concerning the air quality such as limiting benzene concentrations in car fuel or the introduction of the incentive tax on volatile organic compounds (VOC) in 2000 in Switzerland. While BTEX was one of the major VOCs compound classes in 1994 (33 % of the total non-methane hydrocarbons (NMHC)), BTEX contribution to total NMHC significantly decreased to 6.3 % in 2024. Before 2000, traffic exhaust emissions were the dominant source of BTEX to the ambient air with a toluene-to-benzene (T : B) ratio of 2.4 ± 0.1. Since 2000, the contribution of vehicle emissions to toluene concentrations in ambient air in DUE during summer has decreased from 82 % to 65 %, with the remaining proportion emitted from solvent emissions. As BTEX are important ozone and secondary organic aerosol (SOA) precursors, the long-term measurements in Dübendorf was complemented with VOC data from the urban background station at Kaserne Zurich to assess the contribution of BTEX to ambient VOC composition. While the BTEX contribution to ozone formation potential (OFP) has decreased from 25 % in 2005 to 8 % in 2024, their relative contribution to SOA formation potential remains high, contributing to 80 % of the SOA formation potential of the total VOCs measured in Zurich in 2024.