Toluene early morning peaks as potential biogenic stress signals of Mediterranean forests

Mediterranean forests emit biogenic volatile organic compounds (BVOCs) that significantly influence atmospheric chemistry and ecosystem functioning. Due to high solar radiation and the important anthropogenic emission of atmospheric pollutants in the Western Mediterranean Basin, photochemical activity is enhanced, favouring the formation of ozone and secondary organic aerosols. We measured ambient mixing ratios of volatile organic compounds (VOCs) using Proton Transfer Reaction Mass Spectrometry (PTR-MS) at Montseny Natural Park, a Mediterranean forest 60 km from Barcelona, during summers 2021–2023. Surprisingly, during heat wave conditions, a toluene early morning peak of 0.23 (interquartile range 0.18) ppbv was observed. These early morning peaks were associated with prolonged high temperatures (approximately 5 °C higher as compared to non-peak days) and persistently higher vapour pressure deficit (approximately 877 Pa higher as compared to non-peak days), suggesting a potential link to vegetation drought stress. Cross-validation of measurements with Gas Chromatography-Mass Spectrometry (GC-MS) enhanced data reliability. Analysis of the meteorology and the variability of other atmospheric pollutants at the site, allowed to attribute these peaks to a local biogenic origin. These findings suggest that early morning toluene peaks could serve as an indicator of Mediterranean forest vulnerability to climate-induced stressors, with potentially more frequent and intense peaks occurring in the future.