PNNL

The Science

As carbon-containing vapors escape from trees, fossil-fuel burning, and forest fires, the atmosphere "cooks" these airborne emissions to form particles called secondary organic aerosols, or SOA. Most atmospheric models assume that all SOA undergo a process called equilibrium partitioning, where organic aerosol readily mix and enhance SOA formation. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory put this assumption to the test in experiments that varied both the aging time between SOA formation steps and the experimental relative humidity (RH). They found the equilibrium partitioning assumption was accurate for freshly formed SOA, but that it broke down after SOA had been photochemically aged for modest amounts of time. They also found that high RH—thought to promote mixing—had no effect on the results. The findings have implications for model assumptions regarding the growth of small aerosols to sizes that can affect atmospheric processes such as clouds, precipitation, heating, and cooling.

The Impact

Aerosols influence many atmospheric processes, so knowing how they grow and change over time is important for improving models that simulate Earth’s climate. This study showed that a common assumption for predicting SOA concentrations was accurate for fresh isoprene and α-pinene SOA—commonly studied SOA precursors—but not after the particles were moderately aged, even up to 85 percent RH. The results help to resolve divergent conclusions that exist in the literature regarding the timescale required for mixing and will be useful for improving model predictions about aerosol processes.

Summary

In this study, researchers investigated the ability of equilibrium partitioning to describe laboratory SOA formation experiments. The team conducted two types of experiments: 1) co-condensation experiments in which isoprene and α-pinene were simultaneously oxidized to form SOA, and 2) sequential condensation experiments in which fresh isoprene SOA was formed in the presence of aged, pre-existing isoprene or α-pinene-derived SOA particles. In the co-condensation experiments, equilibrium partitioning successfully predicted the time-dependent SOA concentrations, suggesting that the SOA from both precursors rapidly formed a well-mixed phase. However, in the sequential condensation experiments, equilibrium partitioning assumptions significantly overpredicted the observed SOA yield. The lower yield indicates that freshly formed isoprene SOA did not rapidly partition into either of the aged SOA particles to form a well-mixed phase over the 4-hour experimental timescale, even at relative humidity as high as 85 percent. This study showed that the equilibrium partitioning assumption was accurate for freshly formed SOA, but that it broke down after SOA had been photochemically aged for modest amounts of time (15-18 hours).

PNNL Contacts

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

John Shilling, Pacific Northwest National Laboratory, john.shilling@pnnl.gov

Funding

This research is based on work supported by the U.S. Department of Energy (DOE) Office of Science, Office of Biological and Environmental Research, Atmospheric Systems Research (ASR) program. Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute.