Diterpenes: A New Piece in the Atmospheric Puzzle

In our recent study, led by the Institute for Environmental Assessment and Water Research (IDAEA-CSIC), the Centre of Ecological Research and Forestry Applications, and the University of Helsinki, we discovered that diterpenes—natural compounds emitted by trees—may have a previously unrecognized role in forming particles in the atmosphere. Our work is the first to estimate global vegetation emissions of diterpenes and to assess their potential to form aerosols, which has important implications for how we model the atmosphere.

Terpenes, including isoprene, monoterpenes, and sesquiterpenes, are known volatile compounds released by plants. They play key roles in plant communication, pollination, and defence. Once in the atmosphere, these compounds react with ozone and other pollutants to form aerosols—tiny particles that can influence respiratory health, reflect sunlight, and act as seeds for cloud droplets.

Until now, diterpenes, another group of terpenes, had been largely excluded from atmospheric models. Despite being common in trees, they were assumed to be too heavy and not volatile enough to enter the atmosphere in meaningful quantities. Thanks to recent advancements in detection methods, we were able to show otherwise.

Our Discovery

Our interest in diterpenes began in 2018, when we detected an unexpected signal while studying Mediterranean vegetation. Using a mass spectrometry technique (PTR-TOF-MS), we identified a compound that we couldn’t immediately recognize. After cross-validating with gas chromatography, we confirmed it was kaurene, a type of diterpene.

This finding raised an exciting question: if plants are emitting kaurene, could it also be influencing the atmosphere like other terpenes? No study had yet quantified the atmospheric impact of diterpenes, so we set out to investigate.

Collaborating Across Borders

To explore this, we teamed up with colleagues at the Institute for Atmospheric and Earth System Research (INAR) at the University of Helsinki. We sent them a solid kaurene standard, and they carried out the first laboratory experiments to measure how efficiently kaurene forms particles under atmospheric conditions. Their results showed SOA (secondary organic aerosol) yields of between 1.8% and 17%, suggesting kaurene could contribute significantly to aerosol formation.

With this new data, we collaborated with the Barcelona Supercomputing Center to estimate the global impact of these emissions. Using the MONARCH atmospheric chemistry model, we simulated global diterpene emissions and evaluated their contribution to aerosol levels.

Gathering the Data

A crucial step was determining how much diterpene different types of vegetation emit. To do this, we conducted a comprehensive literature review and gathered all available field measurements of diterpene emissions. In total, we identified seven relevant studies and extracted emission factors across plant types such as broadleaf trees, needleleaf trees, shrubs, and crops.

Since different studies reported emissions in different ways—some focusing on specific compounds like kaurene or cembrene—we standardized the data by assuming these dominant compounds reflected the total diterpene output. Where emission rates rather than emission factors were given, we used empirical conversion methods to estimate them. To strengthen our assumptions, we consulted researchers from Finland, Cyprus, and the U.S. who are experts in plant emissions and atmospheric modeling.

What We Found

Using this data, we ran six model simulations with varying emission factors and SOA yields. The results showed that global diterpene emissions in 2018 likely totaled around 11.5 Tg per year, with a range between 0.1 and 94.3 Tg/year. These emissions contributed an estimated 0.63 Tg/year to SOA formation, resulting in an aerosol burden of 0.008 Tg.

When compared to other terpenes, diterpenes correspond to 13% of the aerosol burden from isoprene, 6.4% from monoterpenes, and 19% from sesquiterpenes. These results clearly show that diterpenes can be a significant and previously overlooked source of atmospheric particles.

Limitations and Challenges

Despite the exciting results, we recognize the uncertainties involved. Only a handful of diterpene emission studies exist—just six in the last decade—and emission rates vary widely between species, sometimes by up to four orders of magnitude. Our current SOA yield estimates are based only on kaurene, which is considered less reactive than many other diterpenes. That means we may still be underestimating their full atmospheric impact.

Geographically, most data comes from temperate regions. There is very little information from tropical forests, where warmer temperatures and higher biodiversity could mean even more significant emissions. In fact, our results suggest that diterpene emissions increase with temperature, making them potentially more relevant in a warming climate.

Why This Matters

Aerosols play a central role in climate regulation, cloud formation, and air quality. Better understanding the sources of these particles helps us improve predictions about how our environment is changing. Our study also contributes to solving the mystery of the “missing reactivity”—a well-known discrepancy between predicted and observed chemical reactions in the atmosphere, which may be partially explained by previously unaccounted-for compounds like diterpenes.

What’s Next

We see this as a first step toward fully integrating diterpenes into atmospheric science. More research is urgently needed, particularly in tropical regions where data is scarce. Laboratory work should explore the SOA yields of other diterpenes besides kaurene, and model results must be compared with actual measurements of diterpene concentrations in the air.

By providing the first experimental SOA yield for a diterpene and estimating its global impact, we hope to pave the way for future studies and more accurate atmospheric models. If diterpenes are confirmed as major aerosol precursors, many current models may be underestimating the biogenic contribution to air pollution and climate processes.