Scientists have discovered that a class of chemical compounds emitted by trees, previously dismissed as too heavy to be significant, plays a substantial role in forming atmospheric particles. This finding, detailed in a study led by the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) and the University of Helsinki, fundamentally alters the understanding of how forests influence air quality and climate, necessitating a revision of current atmospheric models. The research is the first to quantify the global emissions of these compounds, called diterpenes, and demonstrates their efficient conversion into the aerosols that can affect human health, reflect sunlight, and seed clouds.
Diterpenes are a type of terpene, which are volatile organic compounds responsible for the characteristic scent of a forest. While scientists have long known that smaller terpenes like monoterpenes and sesquiterpenes react in the air to form secondary organic aerosols (SOAs), diterpenes were largely ignored in atmospheric chemistry. Researchers had assumed their high molecular weight made them non-volatile, meaning they were thought to be too heavy to be released into the atmosphere in meaningful amounts. The new study overturns this long-held assumption, revealing that diterpenes are indeed emitted in significant quantities and are an important, previously unaccounted-for source of atmospheric aerosols with implications for biosphere-atmosphere interactions.
An Overlooked Emission Source
The primary reason diterpenes were excluded from atmospheric models was a decades-old assumption about their physical properties. Due to their large molecular size, the scientific consensus was that these compounds had very low volatility, effectively keeping them grounded and out of the atmosphere. This meant that their potential contribution to aerosol formation was considered negligible and they were not included in models simulating atmospheric composition, air quality, or climate. This omission persisted for years, creating a gap in the scientific understanding of biogenic, or plant-based, emissions.
Advanced Detection Reveals Presence
This paradigm shifted with the application of more powerful and sensitive analytical technologies. Researchers first detected an unexpected signal while studying Mediterranean vegetation in 2018. Using advanced mass spectrometry techniques, the team identified a compound they later confirmed to be kaurene, a type of diterpene. These modern methods are sensitive enough to detect gaseous diterpenes in the air, proving they are emitted in appreciable quantities from forests. This technological advancement was the critical step that allowed the scientific team to challenge the old assumptions and begin quantifying the true atmospheric role of these overlooked molecules.
From Forest to Atmosphere
Terpenes are vital for plants, used for communication, pollination, and as a defense against herbivores. When released, these volatile compounds react with oxidants in the atmosphere, such as ozone, in a process that transforms them from a gas into solid or liquid particles. These newly formed particles are known as secondary organic aerosols. This well-understood process is what links the “forest scent” to tangible impacts on the atmosphere, as the resulting aerosols influence everything from local air quality to global climate patterns.
Chamber Experiments Confirm Conversion
To verify that diterpenes could efficiently form aerosols, the research team conducted laboratory experiments in controlled atmospheric chambers. They focused on kaurene, the diterpene they had successfully identified in the field. When they introduced kaurene into the chamber with ozone, they observed its rapid transformation into particles. The experiments established a conversion efficiency of approximately 10%, meaning that for every 10 grams of kaurene emitted, 1 gram becomes aerosol mass. Mikael Ehn, a professor at the University of Helsinki who led the laboratory work, noted the significance of this figure. “This 10 percent efficiency means that a tenth of the kaurene mass emitted to the atmosphere will form aerosol,” Ehn stated. He added that based on existing chemical knowledge, most other diterpenes are expected to form aerosols at even higher efficiencies.
Quantifying a Global Impact
Armed with experimental data, the researchers integrated their findings into a global chemistry transport model, MONARCH, to estimate the worldwide impact of diterpene emissions. The simulations provided the first-ever global estimate for this class of compounds, revealing their surprisingly large contribution to atmospheric processes. The model incorporated all available emission data along with the newly derived aerosol yields to assess how diterpenes move through the atmosphere and how they contribute to particle loading on a global scale.
The results were striking. The model estimated that total global diterpene emissions amount to approximately 11.5 teragrams (Tg) per year, with a potential range between 0.1 and 94.3 Tg. These emissions contribute an estimated 0.63 Tg to the formation of secondary organic aerosols annually. To put this in perspective, the study compared the resulting aerosol burden from diterpenes to that of other well-known terpenes. The analysis showed the diterpene aerosol burden is equivalent to 13% of the burden from isoprene, 6.4% from monoterpenes, and 19% from sesquiterpenes. These figures firmly establish diterpenes as a significant and previously hidden source of atmospheric particles.
Revising Atmospheric Models
The discovery that diterpenes are a notable source of aerosols has profound implications for climate and air quality models. Aerosols play a dual role in the climate system: they can scatter and reflect incoming solar radiation, creating a cooling effect, and they also serve as the “seeds” around which cloud droplets form, influencing cloud properties and precipitation patterns. By not accounting for the significant contribution from diterpenes, current models may be underestimating the total amount of biogenic aerosols, potentially affecting the accuracy of climate projections. The authors of the study emphasize the need to incorporate their findings into existing atmospheric and climate models to improve their predictive power.
Future Research Directions
While this study represents a major breakthrough, the researchers highlight it as a foundational step with much more work to be done. They call for more research to refine the understanding of diterpenes, particularly in tropical regions where biogenic emissions are highest but measurement data remains scarce. The scientific team also notes the urgent need for laboratory studies on other types of diterpenes beyond kaurene to see if their aerosol yields are indeed higher as suspected. Finally, model predictions must be rigorously compared with direct, real-world measurements of diterpene concentrations in the atmosphere to validate and fine-tune the simulations. Confirming the role of diterpenes as major aerosol precursors will be essential for creating more accurate models of air pollution and climate change.
