-
A study by IDAEA-CSIC and CREAF shows that trees emit toluene, a volatile organic compound, during periods of extreme heat and drought
-
Its presence contributes to the formation of ground-level ozone, a major summertime air pollutant
Forests in the Montseny Natural Park emit toluene in response to water stress and heat. | Ana Yáñez-Serrano
Toluene, a volatile organic compound primarily known for its association with traffic emissions, could become a new indicator of environmental stress in Mediterranean forests. This is the conclusion of a new study by the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) and CREAF, which detected significant emissions of this compound from Mediterranean holm oak forests, particularly under extreme climatic conditions such as heatwaves and droughts.
The study, published in Environmental Science Atmospheres, is based on data collected during the summers of 2021, 2022 and 2023 in the Montseny Natural Park (Barcelona), coinciding with one of the most severe drought episodes recorded in Catalonia in recent decades. During these periods, the research team observed recurring increases in toluene concentrations in forested areas, especially following episodes of extreme heat.
“We observed that, approximately two days after heatwaves, peaks of toluene appeared in the morning. This led us to suspect that the forest itself was responding to these stressful conditions,” explains Ana Yáñez-Serrano, researcher at IDAEA and first author of the study.
Statistical analyses showed that these episodes were clearly defined. Temperatures were around 5°C higher than on non-heatwave days, while the air was significantly drier, indicating that the vegetation was experiencing water stress.
A potential tracer of ecosystem stress
The findings open the possibility of using toluene as an indicator of the physiological condition of Mediterranean forests. The researchers’ next step will be to compare these results with observations from other Mediterranean forests, such as those in Marseille and Cyprus, to determine whether the same pattern occurs across different regions.
“If we confirm similar behaviour in other ecosystems, toluene could become a useful tool for monitoring climate-related stress in forests,” says Roger Seco, IDAEA researcher and co-author of the study.
Implications for air quality
Toluene belongs to the group of volatile organic compounds (VOCs), which play a key role in the formation of ground-level ozone and airborne particulate matter, particularly during summer. Ozone is not always beneficial: in the stratosphere, it forms the ozone layer that protects life from harmful ultraviolet radiation. However, in the troposphere, the atmospheric layer closest to the Earth’s surface, it acts as a pollutant that can irritate the respiratory system, damage vegetation and contribute to global warming.
An increase in atmospheric VOCs such as toluene therefore promotes ozone formation and may worsen summertime pollution episodes. In areas such as Montseny, these volatile compounds react with nitrogen oxides from urban traffic, enhancing ozone production.
“Until now, toluene was mainly considered an anthropogenic pollutant emitted by traffic and industry. The fact that Mediterranean forests release significant amounts of it under heat and drought conditions opens new opportunities to improve air-quality assessments in an increasingly warmer climate,” says Joan Llusià Benet, researcher at CREAF and co-author of the study.
Most volatile organic compounds are currently not specifically regulated. However, in light of these findings, the researchers highlight the importance of incorporating VOCs into air-quality management strategies because of their significant role in atmospheric chemistry.
M. Yáñez-Serrano, M. Lampadariou, I. Filella, J. Llusià, R. Seco, M. Fernández-Martínez, M. Portillo Estrada, N. Mihalopoulos, J. Massagué, N. Pérez, M. Savadkoohi, M. Pandolfi, J. Peñuelas, X. Querol and A. Alastuey. Toluene early morning peaks as potential biogenic stress signals of Mediterranean forests. Environmental Science Atmospheres, 2026, DOI: 10.1039/D5EA00168D
More news
