Liquid–liquid phase separation in organic particles containing one and two organic species: importance of the average O : C

Abstract

Abstract. Recently, experimental studies have shown that liquid–liquid phase separation (LLPS) can occur in organic particles free of inorganic salts. Most of these studies used organic particles consisting of secondary organic materials generated in environmental chambers. To gain additional insight into LLPS in organic particles free of inorganic salts, we studied LLPS in organic particles consisting of one and two commercially available organic species. For particles containing one organic species, three out of the six particle types investigated underwent LLPS. In these cases, LLPS was observed when the O : C was ≤ 0.44 (but not always) and the relative humidity (RH) was between ∼ 97 % and ∼ 100 %. The mechanism of phase separation was likely nucleation and growth. For particles containing two organic species, 13 out of the 15 particle types investigated underwent LLPS. In these cases, LLPS was observed when the O : C was ≤ 0.58 (but not always) and mostly when the RH was between ∼ 90 % RH and ∼ 100 % RH. The mechanism of phase separation was likely spinodal decomposition. In almost all cases when LLPS was observed (for both one-component and two-component particles), the highest RH at which two liquids was observed was 100±2.0 %, which has important implications for the cloud condensation nuclei (CCN) properties of these particles. These combined results provide additional evidence that LLPS needs to be considered when predicting the CCN properties of organic particles in the atmosphere.

Liquid–liquid phase separation in organic particles consisting of α-pinene and β-caryophyllene ozonolysis products and mixtures with commercially-available organic compounds

“…Previous studies have shown LLPS in mixed organic and inorganic aerosol particles often occurs for O:C < 0.8 (Bertram et al, 2011;Krieger et al, 210 2012;Song et al, 2012aSong et al, , 2012bSchill and Tolbert, 2013;You et al, 2013You et al, , 2014. Even in the absence of inorganic salts, the occurrence of LLPS was dependent on the O:C of organic materials (Renbaum-Wolff et al, 2016;Song et al, 2017;Song et al, 2018;Ham et al, 2019). Our results show that as compositional complexity increased from one organic species to two organic species, LLPS occurred over a wider range of average O:C values of organic materials (increasing from 0.44 to 0.67) ( Figs.…”

“…The organic particles on hydrophobic glass slides were placed into a RH and temperature controlled flow-cell coupled to an optical microscope (Olympus BX43, 40× objective, Japan) (Parsons et al, 2004;105 Pant et al, 2006;Bertram et al, 2011;Song et al, 2012aSong et al, , 2018Ham et al, 2019). During all experiments, the temperature inside the flow-cell was maintained at 291 ± 1 K. The RH was controlled by a continuous flow of a wet and dry N2 mixture with a total flow rate of 500 sccm.…”

“…As discussed in Sect. 3.1, the inner phase is expected to be mainly water while the outer 185 phase is expected to be mainly organic material (Renbaum-Wolff et al, 2016;Song et al, 2017Song et al, , 2018.…”

“…Consistent with these 60 laboratory studies, a recently introduced binary activity thermodynamic (BAT) model, with reduced complexity for atmospheric modelling, predicts that LLPS can occur when the O:C of the organic material is < 0.5 (Gorkowski et al, 2019), when considering different types of functional groups. In addition, Song et al (2018) showed that LLPS occurs in organic particles containing one commerciallyavailable organic compound when the O:C is smaller than 0.44 while LLPS occurs in organic particles 65 containing two commerically available organic species when the average the O:C is smaller than ≤ 0.58.…”

“…shows the occurrence of LLPS in single-component organic particles as a function of O:C and molar mass from the current study and as well as that ofSong et al (2018). For comparison purposes, also included inFig.…”