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The Surprising Role of Semivolatile Organics in the Growth of Ultrafine Particles

 

Authors

Rahul Zaveri — Pacific Northwest National Laboratory
John E Shilling — Pacific Northwest National Laboratory
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Jiumeng Liu — Pacific Northwest National Laboratory
David Bell — Paul Scherrer Institute
Jacqueline Mary Wilson — Pacific Northwest National Laboratory
Emma D'Ambro — University of Washington
Cassandra Gaston — University of Washington
Joel Thornton — University of Washington
Alexander Laskin — Purdue University
Peng Lin — Pacific Northwest National Laboratory
Jian Wang — Washington University in St. Louis
Dick C Easter — Pacific Northwest National Laboratory
John Seinfeld — California Institute of Technology
Allan K Bertram — University of British Columbia
Scot T. Martin — Harvard University
Chongai Kuang — Brookhaven National Laboratory
Daniel James Cziczo — Massachusetts Institute of Technology
Ari Setyan — University of California, Davis
Qi Zhang — University of California, Davis
Timothy B Onasch — Aerodyne Research, Inc.
Douglas R Worsnop — Aerodyne Research, Inc.

Category

Secondary organic aerosol

Description

The indirect effect of atmospheric aerosols still remains one of the most uncertain components in the radiative forcing of climate change over the industrial era. This large uncertainty is partly due to incomplete understanding of the sources and number concentrations of cloud condensation nuclei (CCN). Although atmospheric aerosol particles larger than about 100 nm in diameter act as efficient CCN, they usually originate as ultrafine aerosols (<100 nm), formed from nucleation or combustion processes. Their growth to CCN active sizes occurs largely by condensation of myriad oxidation products of biogenic and anthropogenic volatile organic compounds (VOCs) forming secondary organic aerosol (SOA). While the initial growth is driven by irreversible condensation of nearly nonvolatile oxidation products, the much more abundantly formed semi-volatile oxidation products are traditionally assumed to instantly equilibrate with pre-existing organic aerosol mass, thus favoring the growth of large particles. However, recent studies indicate that SOA exists as a semisolid at low to moderate relative humidity, with the theoretical implication that its reduced bulk diffusivity slows down further growth via condensation of semivolatile organic compounds. In this study, we investigate the role of semivolatile organic vapors from photooxidation of isoprene (the dominant biogenic VOC) in the growth of ultrafine aerosol, under atmospherically relevant conditions in chamber experiments and in a forest during the 2010 CARES field campaign in California. We show that semivolatile vapors do not instantly equilibrate, but rather their time- and size-dependent partitioning is controlled by slow diffusion into the viscous particulate organic phase. Theoretical considerations indicate that diffusion time scale increases with increasing particle size. Consequently, the substantially hindered growth of large organic particles effectively enhances the growth of ultrafine particles that are competing to absorb the semivolatile organics. These results suggest that a significantly greater fraction of ultrafine aerosols may grow to CCN active sizes due to diffusion-controlled condensation of semivolatile organics than predicted by the equilibrium partitioning approach commonly used in climate models. Our findings have direct consequences for the predicted magnitude of the cloud radiative forcing resulting from a global increase in ultrafine aerosol concentrations during the industrial era.