Aerial view of the Amazon Basin.
Aerial view of the Amazon Basin. Carbon-based chemical emissions from biological sources create many small particles above the Amazon rainforest.
Credit: Department of Energy Atmospheric Radiation Measurement (ARM) user facility

The Science

The aerosol particles that serve as seeds for cloud formation are major drivers of global climate change. One of the world’s largest reservoirs of aerosol particles is at high altitudes above tropical regions. However, the sources and chemical processes behind the formation of these particles are unclear. A collaborative team of researchers addressed this problem by integrating insights from laboratory measurements, chemical transport modeling, and field measurements. They found that carbon-based compounds from biological sources drive the formation of new particles. These sources play key roles in producing the large number of small particles in the atmosphere above the Amazon rainforest.

The Impact

This study untangles the long-standing mystery of the source of large numbers of particles at high altitudes above the Amazon rainforest. Natural environments like the Amazon are a baseline for understanding how human activity contributes to climate change. This new finding will help researchers better quantify the effect of aerosol particles from human activity. This is one of the greatest uncertainties in climate change predictions.


Researchers systematically investigated new-particle formation mechanisms by integrating insights from laboratory measurements, chemical transport modeling, and field measurements to elucidate the sources of the numerous small particles in the Amazon free troposphere. To account for new-particle formation from organic compounds—the most complicated and least understood component of particle formation—researchers developed a comprehensive model that represents both the temperature-dependent formation chemistry and the thermodynamics of extremely low volatility organic compounds, elaborating their roles in new-particle formation processes. Pure-organic new-particle formation driven by natural biogenic emissions is the dominant pathway in the uppermost troposphere (more than 13 kilometers above the Earth), accounting for 65 to 83 percent of the column total new-particle formation rate under relatively pristine conditions. Ternary new-particle formation that involves both organics and sulfuric acid dominates between 8 and 13 kilometers above the surface. The high organic new-particle formation rates at high altitudes primarily stems from the decreased volatility of organics and increased new-particle formation efficiency at low temperatures. These are somewhat counterbalanced by the reduced formation rate of extremely low volatility organic compounds in pristine environments. This particle formation mechanism was likely also prevalent during preindustrial times and is a critical component of understanding deviations from the preindustrial baseline caused by anthropogenic aerosol forcing.


Jerome D. Fast
Pacific Northwest National Laboratory

Shaima Nasiri
Department of Energy Office of Science
Atmospheric System Research

Jeff Stehr
Department of Energy Office of Science
Atmospheric System Research


This research was funded by the Department of Energy Office of Science, Office of Biological and Environmental Research, through the Atmospheric System Research (ASR) program via the Integrated Cloud, Land-surface, and Aerosol System Study (ICLASS) Science Focus Area.


Zhao, B. et al., High concentration of ultrafine particles in the Amazon free troposphere produced by organic new particle formation. Proceedings of the National Academy of Sciences, 117 (41) (2020). [DOI: 10.1073/pnas.2006716117]