title: Lability of secondary organic particulate matter.
abstract: The energy flows in Earth's natural and modified climate systems are strongly influenced by the concentrations of atmospheric particulate matter (PM). For predictions of concentration, equilibrium partitioning of semivolatile organic compounds (SVOCs) between organic PM and the surrounding vapor has widely been assumed, yet recent observations show that organic PM can be semisolid or solid for some atmospheric conditions, possibly suggesting that SVOC uptake and release can be slow enough that equilibrium does not prevail on timescales relevant to atmospheric processes. Herein, in a series of laboratory experiments, the mass labilities of films of secondary organic material representative of similar atmospheric organic PM were directly determined by quartz crystal microbalance measurements of evaporation rates and vapor mass concentrations. There were strong differences between films representative of anthropogenic compared with biogenic sources. For films representing anthropogenic PM, evaporation rates and vapor mass concentrations increased above a threshold relative humidity (RH) between 20% and 30%, indicating rapid partitioning above a transition RH but not below. Below the threshold, the characteristic time for equilibration is estimated as up to 1 wk for a typically sized particle. In contrast, for films representing biogenic PM, no RH threshold was observed, suggesting equilibrium partitioning is rapidly obtained for all RHs. The effective diffusion rateDorgfor the biogenic case is at least 103times greater than that of the anthropogenic case. These differences should be accounted for in the interpretation of laboratory data as well as in modeling of organic PM in Earth's atmosphere.
authors: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.
authors: Pengfei
authors: P
authors: Liu
authors: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.|Department of Civil and Environmental Engineering, Faculty of Science and Technology, University of Macau, Macau 999078, China.
authors: Yong Jie
authors: YJ
authors: Li
authors: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138.|T. H. Chan School of Public Health, Harvard University, Boston, MA 02115.
authors: Yan
authors: Y
authors: Wang
authors: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
authors: Mary K
authors: MK
authors: Gilles
authors: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99354.
authors: Rahul A
authors: RA
authors: Zaveri
authors: Department of Chemistry, University of British Columbia, Vancouver, BC, Canada, VT6 1Z1.
authors: Allan K
authors: AK
authors: Bertram
authors: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; scot_martin@harvard.edu.|Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138.
authors: Scot T
authors: http://orcid.org/0000-0002-5372-6214
authors: ST
authors: Martin
keywords: atmospheric chemistry; evaporation; secondary organic aerosol