Atmospher Sci & Global Chg
Research Highlights

March 2018

How Some Aerosols Transform From Atmospheric Bystanders Into Climate Actors

New research improves the understanding of how ultrafine particles in the atmosphere grow into climatically active sizes.

Many plants and trees release a volatile organic compound known as isoprene, which plays a key role in ozone formation. Enlarge Image.

The Science

The dynamics of aerosol growth and size distribution evolution, and how they affect Earth's climate, are poorly understood. New research on secondary organic aerosols formed from oxidation of isoprene—a volatile organic compound released from many plants and trees—provided quantitative insights into the effects of slow diffusion inside viscous organic particles on their growth kinetics.

A research team led by Pacific Northwest National Laboratory scientists discovered that bulk diffusion-limited partitioning of semivolatile organic compounds into large viscous organic particles effectively promotes the growth of smaller particles that have shorter diffusion timescales.

The Impact

Physicochemical processes governing secondary organic aerosol formation are more complex than atmospheric models currently represent. The results of this study will enable more accurate predictions of secondary organic aerosol formation by including a better representation of the growth of ultrafine aerosol particles to climatically active sizes in atmospheric models. This will improve simulations of how aerosols affect Earth's energy balance.

Summary

Secondary organic aerosols (SOAs) in the atmosphere are produced when oxidation products from volatile organic compounds condense from the gas phase to the particle phase. These SOAs constitute a major fraction of the submicron aerosol in Earth's atmosphere, and they play a crucial role in the growth of ultrafine particles to sizes larger than ~80 nanometers. At this size, the particles begin to efficiently scatter light and can activate as cloud condensation nuclei. Under dry to moderate relative humidity, SOAs can be highly viscous such that slow diffusion of condensing compounds inside these semisolid particles can prolong the gas-particle equilibration timescale.

Researchers investigated the effects of low bulk diffusivity on the growth and evaporation kinetics of SOAs formed in PNNL's environmental chamber from photooxidation of isoprene. Mass spectrometric analysis was performed using the FIGAERO-CIMS of the University of Washington and the nanoDESI and miniSPLAT II of EMSL, the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy Office of Science user facility.

The researchers found that isoprene SOA was composed of several semivolatile organic compounds, with some reversibly reacting to form high molecular weight compounds called oligomers. Model analysis revealed that hindered partitioning of semivolatile organic compounds into large viscous particles is responsible for the observed growth of the smaller particles that have shorter diffusion timescales.

This effect has important implications for the growth of atmospheric ultrafine particles to climatically active particles via SOA formation under relatively dry conditions.

Acknowledgments

Sponsors: This research is based on work supported by the U.S. Department of Energy (DOE) Office of Science, Biological and Environmental Research (BER), as part of the Atmospheric System Research program and by the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by BER and located at Pacific Northwest National Laboratory.

Research Area: Climate and Earth Systems Science

Research Team: Rahul A. Zaveri, John E. Shilling, Alla Zelenyuk, Jiumeng Liu, Jacqueline Wilson, and Richard C. Easter, PNNL; David M. Bell, PNNL/Paul Scherrer Institute (Switzerland); Emma L. D'Ambro and Joel A. Thornton, University of Washington; Cassandra J. Gaston, University of Washington/University of Miami; Alexander Laskin and Peng Lin, PNNL/Purdue University; Jian Wang, Brookhaven National Laboratory; Allan K. Bertram, University of British Columbia; Scot T. Martin, Harvard University; John H. Seinfeld, California Institute of Technology; and Douglas R. Worsnop, Aerodyne Research

Reference: R.A. Zaveri, J.E. Shilling, A. Zelenyuk, J. Liu, D.M. Bell, E.L. D'Ambro, C.J. Gaston, J.A. Thornton, A. Laskin, P. Lin, J. Wilson, R.C. Easter, J. Wang, A.K. Bertram, S.T. Martin, J.H. Seinfeld, D.R. Worsnop, "Growth Kinetics and Size Distribution Dynamics of Viscous Secondary Organic Aerosol." Environmental Science & Technology 52, 1191-1199 (2018). [DOI: 10.1021/acs.est.7b04623]