June 29, 2021

Understanding How Urban Pollution Affects Ultrafine Particle Concentrations in the Amazon

Small airplane on a bright blue sky

During the Green Ocean Amazon campaign, the Gulfstream 1 aircraft sampled aerosol particles in and out of the pollution plumes downwind of Manaus, an isolated metropolis in the central Amazon.

(Photo: U.S. Department of Energy Atmospheric Radiation Measurement user facility)

The Science

The aerosol particles that serve as seeds for cloud formation are major drivers of global climate change. Understanding how human activities change aerosol concentrations and distributions relative to preindustrial conditions presents a major challenge in assessing the impact of aerosols on climate change. A collaborative team of researchers used a unique natural laboratory—an isolated metropolis located in the preindustrial-like Amazon rainforest—to address this problem. They found that urban pollution substantially increases particle number concentrations over the Amazon. This primarily occurs through new particle formation driven by anthropogenic sulfuric acid and biologically-derived trace organic gases, rather than direct particulate emissions.

The Impact

This study identifies for the first time how urban pollution increases particle number concentrations over the Amazon. Pristine environments like the Amazon form a baseline for deriving anthropogenic causes of climate change, making this finding key to better quantifying the influence of anthropogenic aerosols on climate—one of the greatest uncertainties in climate change predictions.


Researchers combined chemical transport simulations and field measurements from the Green Ocean Amazon (GOAmazon) campaign to investigate the effect of anthropogenic pollution from an isolated metropolis on particle number concentration over the preindustrial-like Amazon rainforest. They particularly focused on the effects of new particle formation (NPF) mechanisms and primary particle emissions. To represent organic-mediated NPF, researchers employed a state-of-the-art model that systematically simulates the formation chemistry and thermodynamics of extremely low volatility organic compounds as well as their roles in NPF processes. They further updated the model to improve organic NPF simulations under human-influenced conditions. The results show that urban pollution from the metropolis increases particle number concentration by a factor of 5–25 over the downwind region (within 200 km of the city center) compared to background conditions. The model indicates that NPF contributes over 70% of the total particle number in the downwind region not immediately adjacent to the sources. Among different NPF mechanisms, the NPF involving organics and sulfuric acid overwhelmingly dominates. The improved understanding of particle formation mechanisms will help better quantify how anthropogenic aerosol has perturbed the climate from pristine-preindustrial times to the present day.


PNNL Contact

Jerome Fast, Pacific Northwest National Laboratory, Jerome.Fast@pnnl.gov


This research was funded by the U.S. Department of Energy Atmospheric System Research program via the Integrated Cloud, Land-surface, and Aerosol System Study Science Focus Area.

Published: June 29, 2021

B. Zhao, J. D. Fast, N. M. Donahue, M. Shrivastava, et al.Impact of urban pollution on organic-mediated new-particle formation and particle number concentration in the Amazon rainforest.” Environ. Sci. Technol., 55, 4357 (2021). [DOI: 10.1021/acs.est.0c07465]