December 3, 2021

The Water Content of Organic Aerosol Predicts Its Viscosity

Simulations show strong correlation between water content and viscosity of organic aerosol over Amazon rainforest

Photograph of clouds over the Amazon rainforest

An organic aerosol’s phase affects its formation, chemistry, lifetime, and growth to sizes where it can activate as cloud droplets.

The Science

Small organic aerosols (OA) are mixtures of thousands of species produced from natural biogenic, anthropogenic, or biomass burning sources. The viscosity of OA determines its phase in the atmosphere. Directly measuring the viscosity of fine particulate OA in the atmosphere is challenging. To determine which parameters could accurately predict the viscosity of OA, researchers implemented a well-defined relationship between OA composition, volatility, and water content into the Weather Research and Forecasting Model coupled to chemistry (WRF-Chem) and simulated the variations in OA viscosity over the Amazon rainforest. By analyzing WRF-Chem simulated variations in OA viscosity throughout the atmospheric column over the Amazon, they determined that water associated with OA shows a strong logarithmic-linear relationship with OA viscosity.

The Impact

OA water content depends on easily measurable parameters: ambient relative humidity and OA hygroscopicity. The researchers found that this water content can be used to predict the OA phase state and viscosity in field studies that do not measure viscosity. OA viscosity affects the lifetime of particles in the atmosphere. Predicting the variations of OA viscosity in time and space is also important for understanding how ultrafine particles grow to sizes where they seed clouds. It enhances our understanding of the ability of these particles to scatter/absorb sunlight. The effect of fine OA particles on clouds and sunlight is one of the least understood aspects of climate systems.


In this work, researchers incorporated a well-defined relationship between the volatility and viscosity of OA particles within the WRF-Chem regional model. They simulated the phase state and viscosity of OA particles in the Amazon rainforest during the dry-to-wet transition season (September–October). Their results show that water associated with OA is a governing factor that explains variations in OA viscosity over several orders of magnitude. At high altitudes with low relative humidity (RH) and temperatures, the simulations indicate that OA exists in a semi-solid/solid like phase state. Near the surface, OA mostly exists in a liquid like state due to high RH. At an urban site near surface, where the day-night RH differences are significant, RH drives the phase state. In contrast, at a background forested site with small day-night RH differences, biomass burning influenced-OA exists as a semi-solid. This work develops a new capability in WRF-Chem to predict variations in the phase state of OA. The capability will be critical for predicting the atmospheric formation and removal of climate relevant OA particles as well as their interactions with clouds and sunlight in future studies.

The Environmental Molecular Sciences Laboratory’s computational resources were used for the simulations.

PNNL Contact

Manish Shrivastava, Pacific Northwest National Laboratory,


This research is supported by the U.S. Department of Energy Office of Science, Office of Biological and Environmental Research through the Early Career Research Program, the Atmospheric System Research Program and ARM.

Published: December 3, 2021

Rasool, Q. Z. et al. Modeling Volatility-Based Aerosol Phase-state Predictions in the Amazon Rainforest.” ACS Earth and Space Chemistry, 5(10), 2910–2924, (2021), [DOI: 10.1021/acsearthspacechem.1c00255].