November 6, 2023
Research Highlight

Mixing and Aging of Atmospheric Aerosols

New research identifies an aerosol process that will impact atmospheric model predictions of organic aerosol concentration and size

Ultraviolet lights inside the Atmospheric Measurements Laboratory environmental chamber

Experiments in the PNNL Environmental Chamber found that fresh organic vapors established equilibrium with fresh organic particles but were unable to equilibrate with the same particles after they were aged for as little as 20 minutes.

(Photo by Andrea Starr | Pacific Northwest National Laboratory)

The Science

Atmospheric aerosols play a significant role in Earth’s climate. Understanding the formation of organic particles in the atmosphere is important for unraveling future climate change.

Isoprene, an organic compound, is produced by many plants and has a large impact on atmospheric chemistry and composition. As described in an earlier research project, the ways isoprene converts to a secondary organic aerosol (SOA) and how anthropogenic, or man-made, pollutants affect this process are researched extensively because it affects Earth’s climate and local air quality.

Typically, atmospheric models assume that fresh organic material, generated as a result of chemical reactions in the atmosphere, can dissolve in the existing organic particulate matter. This study revealed that fresh organic vapors are indeed soluble in particulate organics that are actively growing in size. However, if growth halts and the particulate matter ages, fresh organic vapors can no longer mix with the particulate organic matter.

The Impact

This new study shows that the way most large-scale models predict organic aerosol concentration and size is not accurate. The unexpectedly short aging timescale necessary for observing non-mixing behavior will impact model predictions of aerosol particle size distributions.

Most atmospheric models will need to be updated to include this mixing limitation. The observed behavior will cause small particles to grow faster and large particles to grow slower than would be otherwise predicted. This will allow more small particles to grow to sizes capable of forming cloud droplets and, thus, impact cloud formation and precipitation and the prediction of climate change.


A rapid segregation of fresh and aged organic particulate matter, derived from tree emissions, has a significant impact on particle growth.

Recent studies have shown that instantaneous gas-particle equilibrium partitioning assumptions fail to predict SOA formation, even at high relative humidity (~85%). Photochemical aging seems to be one driving factor.

In this study, we probed the minimum aging timescale required to observe non-equilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas- and aerosol-phase at ~50% RH. Seed isoprene SOA was generated by photo-oxidation in the presence of effloresced ammonium sulfate seeds at < 1 ppbv NOx, aged photochemically or in the dark for 0.3–6 hours, and subsequently exposed to fresh isoprene SVOCs.

Our results show that the equilibrium partitioning assumption is accurate for fresh isoprene SOA but breaks down after isoprene SOA has been aged for as short as 20 minutes, even in the dark. Modeling results showed that a semi-solid SOA phase state was necessary to reproduce the observed particle size distribution evolution. The observed non-equilibrium partitioning behavior and inferred semi-solid phase state were corroborated by offline mass spectrometric analysis on the bulk aerosol particles showing the formation of organosulfates and oligomers. The unexpectedly short timescale for the phase transition within isoprene SOA has important implications for the growth of atmospheric ultrafine particles to climate-relevant sizes.

The research team believes the next step is to design experiments that test whether SOA from mixtures of anthropogenic and biogenic VOCs behaves in a similar manner.

PM Contacts

Shaima Nasiri

Jeff Stehr



This research was supported by the Atmospheric System Research (ASR) program as part of the Department of Energy (DOE), Office of Science, Biological and Environmental Research program under Pacific Northwest National Laboratory project 57131. The nano-desi mass spectrometer analysis was performed on a project award (DOI: 10.46936/lser.proj.2020.51418/60000197) from the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by the Biological and Environmental Research program under Contract No. DE-AC05-76RL01830. Pacific Northwest National Laboratory is operated for DOE by the Battelle Memorial Institute under Contract DE-A06-76RLO1830.

Published: November 6, 2023

Chen, Yuzhi, et al. “Nonequilibrium Behavior in Isoprene Secondary Organic Aerosol.” Environmental Science & Technology, vol. 57, no. 38, 2023, pp. 14182–93, [DOI: 10.1021/acs.est.3c03532]