1Department of Chemistry, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
2Marcolli Chemistry and Physics Consulting GmbH, Zurich, Switzerland
3Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
4School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
5Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
6Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
anow at: Aerodyne Research, Inc, Billerica, MA 01821 and Boston College, Chestnut Hill, MA 02467, USA
bnow at: Department of Earth and Environmental Sciences, Chonbuk National University, Jeollabuk-do, Republic of Korea
*These authors contributed equally to this work.
Received: 18 Oct 2015 – Discussion started: 26 Nov 2015
Abstract. Particles consisting of secondary organic material (SOM) are abundant in the atmosphere. To predict the role of these particles in climate, visibility and atmospheric chemistry, information on particle phase state (i.e., single liquid, two liquids and solid) is needed. This paper focuses on the phase state of SOM particles free of inorganic salts produced by the ozonolysis of α-pinene. Phase transitions were investigated in the laboratory using optical microscopy and theoretically using a thermodynamic model at 290 K and for relative humidities ranging from < 0.5 to 100 %. In the laboratory studies, a single phase was observed from 0 to 95 % relative humidity (RH) while two liquid phases were observed above 95 % RH. For increasing RH, the mechanism of liquid–liquid phase separation (LLPS) was spinodal decomposition. The RH range over which two liquid phases were observed did not depend on the direction of RH change. In the modeling studies, the SOM took up very little water and was a single organic-rich phase at low RH values. At high RH, the SOM underwent LLPS to form an organic-rich phase and a water-rich phase, consistent with the laboratory studies. The presence of LLPS at high RH values can have consequences for the cloud condensation nuclei (CCN) activity of SOM particles. In the simulated Köhler curves for SOM particles, two local maxima were observed. Depending on the composition of the SOM, the first or second maximum can determine the critical supersaturation for activation. Recently researchers have observed inconsistencies between measured CCN properties of SOM particles and hygroscopic growth measured below water saturation (i.e., hygroscopic parameters measured below water saturation were inconsistent with hygroscopic parameters measured above water saturation). The work presented here illustrates that such inconsistencies are expected for systems with LLPS when the water uptake at subsaturated conditions represents the hygroscopicity of an organic-rich phase while the barrier for CCN activation can be determined by the second maximum in the Köhler curve when the particles are water rich.
Revised: 14 Apr 2016 – Accepted: 06 May 2016 – Published: 01 Jul 2016
Renbaum-Wolff, L., Song, M., Marcolli, C., Zhang, Y., Liu, P. F., Grayson, J. W., Geiger, F. M., Martin, S. T., and Bertram, A. K.: Observations and implications of liquid–liquid phase separation at high
relative humidities in secondary organic material produced by α-pinene ozonolysis without inorganic salts, Atmos. Chem. Phys., 16, 7969-7979, doi:10.5194/acp-16-7969-2016, 2016.