Rainforest emissions linked with new particle formation at high altitudes

Why are there so many newly formed aerosol particles in the upper troposphere over tropical regions such as the Amazon? Tropical forests play an important role in global climate regulation. However, the high concentrations of new particles above these areas have puzzled atmospheric scientists for the past 20 years.

An international study, led by researchers at the University of Helsinki, shows that the answer may be isoprene. Isoprene is the most abundant non-methane hydrocarbon emitted into the atmosphere, primarily by vegetation.

The study, published in Nature, investigated the formation of new particles from isoprene in the upper troposphere. The troposphere is the lowest layer of the atmosphere, spanning from ground level to an altitude of 18 km at the equator.

With experiments performed in the CLOUD chamber at CERN, researchers aimed to determine whether isoprene oxygenated organic molecules (IP-OOM), which are compounds that form when isoprene is oxidized in the atmosphere, could form new particles under upper-tropospheric conditions such as temperatures below -30°C.

They also explored how factors, such as temperature, the presence of trace acids, and nitrogen oxides, affect this process.

Isoprene can drive rapid particle formation

The researchers found that isoprene oxygenated organic molecules can rapidly form new particles under upper-tropospheric conditions. Previously, isoprene was thought to have negligible ability to form particles; however, this study showed that isoprene can drive rapid particle formation under certain conditions.

“Our key finding is that the presence of extremely low concentrations of sulfuric acid or iodine oxoacids dramatically enhances particle formation, accelerating it up to 100 times faster compared to when only isoprene oxygenated organics are present. These findings can explain the high particle number concentrations observed at high altitudes over tropical regions such as the Amazon,” explains Jiali Shen, postdoctoral researcher at the Institute for Atmospheric and Earth System Research (INAR), University of Helsinki.

Increasing understanding of cloud formation and climate

Aerosol particles are important for the climate because they scatter and absorb incoming solar radiation and seed cloud droplets by acting as cloud condensation nuclei. These newly published findings could have significant implications for our understanding of cloud formation and climate.

“This research connects the abundant isoprene emissions from tropical rainforests to particle formation in the upper troposphere, highlighting a new aspect of the interaction between forests and the atmosphere. These results may lead to improvements in atmospheric chemistry and climate models, potentially enhancing our ability to predict climate change and its impacts,” says Xu-Cheng He, one of the lead investigators in the study.

“This study underscores the complex interactions between forests, the atmosphere, and climate. This demonstrates how emissions from trees can have far-reaching effects on cloud formation and potentially on the global climate. This type of fundamental research is crucial for improving our understanding of climate processes and our ability to predict and mitigate climate change,” says Professor Katrianne Lehtipalo from the University of Helsinki.