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Compounds emitted by trees, diterpenes, could have a previously unconsidered impact on the formation of particles in the atmosphere.

The study by the Institute for Environmental Diagnosis and Water Studies (IDAEA) of the Spanish National Research Council (CSIC) and the Institute of Atmospheric and Earth System Research (INAR) at University of Helsinki is the first to quantify the total emissions of diterpenes emitted by vegetation worldwide and their potential to form aerosols, which represents a modification of atmospheric models to date. The findings are in Communications Earth & Environment.

Terpenes are volatile natural compounds emitted by plants and are essential for plant communication, pollination, and defense against herbivores. They are, for example, responsible for the characteristic "forest scent" after rain or on a hot day. Once in the atmosphere, terpenes react with other compounds such as ozone, giving rise to particles (also called aerosols). These aerosols affect air quality and respiratory health, reflect solar radiation, and act as seeds for cloud droplets.

The reason diterpenes had not been considered in atmospheric models was they were thought to be non-volatile due to their high molecular weight. However, thanks to more modern and powerful analytical techniques, the scientific team has shown that diterpenes are indeed emitted into the air in appreciable quantities and can contribute to aerosol formation.

The results of laboratory experiments, using the diterpene kaurene, showed that diterpenes can indeed be rapidly transformed into particles upon reactions with ozone, with an efficiency of around 10%.

"This 10% efficiency means that a tenth of the kaurene mass emitted to the atmosphere will form aerosol. Based on our previous chemical knowledge of how molecular structures impact aerosol formation, we expect that most other diterpenes can form aerosol at even higher efficiencies," says Mikael Ehn, professor at University of Helsinki, who led the laboratory experiments of the study. The oxidation and aerosol-forming potential of different terpenes has been a key focus of his research group at INAR over the last decade.

"This study represents a paradigm shift in our understanding of atmospheric composition, as until now, diterpenes were not considered key elements in aerosol formation," says Ana María Yáñez-Serrano, a researcher at Institute of Environmental Assessment and Water Research (IDAEA-CSIC) and first author of the study.