How Atmospheric Conditions Transform Some Aerosols
New research improves the understanding of how ultrafine particles in the atmosphere grow into climatically active sizes.
How individual aerosol particles grow, how a distribution of aerosol sizes changes over time, and how these changes affect Earth’s climate are all poorly understood. New research on secondary organic aerosols (SOA), a broad class of atmospheric particles that comprises a large fraction of the smallest particles in the atmosphere, shows that under dry atmospheric conditions when relative humidity is low and SOA is semi-solid, smaller SOA particles are actually able to grow faster than larger SOA particles, potentially raising the number of cloud condensation nuclei formed.
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.
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 photo-oxidation of isoprene, a volatile organic compound released from many plants and trees. Mass spectrometric analysis was performed using the FIGAERO-CIMS capability at the University of Washington and the nanoDESI and miniSPLAT II capabilities at EMSL, the Environmental Molecular Sciences Laboratory, an Office of Science user facility. The researchers found that isoprene SOA was composed of several semi-volatile organic compounds, with some reversibly reacting to form high molecular weight compounds called oligomers. Model analysis revealed that hindered partitioning of semi-volatile 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.
Rahul A. Zaveri
Pacific Northwest National Laboratory
This research was supported by the Office of Science of the U.S. Department of Energy (DOE) as part of the Atmospheric System Research program and by the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL). PNNL is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. Data used in this work are available from the corresponding author (firstname.lastname@example.org).
Zaveri, R.A., 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.M. Wilson, R.C. Easter, J. Wang, A.K. Bertram, S.T. Martin, J.H. Seinfeld, and 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]