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

Liquid–liquid phase separation (LLPS) in organic aerosol particles can impact several properties of atmospheric particulate matter, such as cloud condensation nuclei (CCN) properties, optical properties, and gas-to-particle partitioning. Yet, our understanding of LLPS in organic aerosols is far from complete. Here, we report on LLPS of one-component and two-component organic particles consisting 20 of α-pineneand β-caryophyllene-derived ozonolysis products and commercially-available organic compounds of relevance to atmospheric organic particles. In the experiments involving singlecomponent organic particles, LLPS was observed in 8 out of 11 particle types studied. LLPS almost always occurred when the oxygen-to-carbon elemental ratio (O:C) was ≤ 0.44, but did not occur when O:C was > 0.44. The phase separation occurred by spinodal decomposition, and when LLPS occurred, 25 two liquid phases co-existed up to ~100% relative humidity (RH). In the experiments involving twohttps://doi.org/10.5194/acp-2020-318 Preprint. Discussion started: 9 April 2020 c © Author(s) 2020. CC BY 4.0 License.

. More recently, studies on LLPS in organic aerosol particles free of inorganic salts 55 have shown that LLPS occurs in SOA generated in environmental chambers when the average O:C of the organic material is smaller than roughly 0.5 across the RH range of ~95% to ~100% (Renbaum-Wolff et al., 2016;Rastak et al., 2017;Song et al., 2017;Ham et al., 2019) with implications for the CCN properties of the SOA (Petters et al., 2006;Hodas et al., 2016;Renbaum-Wolff et al., 2016;Ovadnevaite et al., 2017;Rastak et al., 2017;Liu et al., 2018;Ham et al., 2019). 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.
In the following, we investigated LLPS in particles containing one and two organic species generated from ozonolysis products of α-pinene and β-caryophyllene, which are atmospherically relevant, and commercially-available organic compounds. These results provide additional insight into the O:C range required for LLPS in organic particles free of inorganic salts.  Table 1 presents the physical properties of the organic compounds investigated. In this study, 11 organic species were used, including seven products from the ozonolysis of α-pinene and β-caryophyllene and 75 four commercially-available organic compounds. These species covered an O:C range of 0.13 -1.00 (Table 1). All species were liquid at room temperature.
Seven of the products from the ozonolysis of α-pinene and β-caryophyllene were synthesized. The detailed synthesis methods for these species are described in Bé et al. (2017). Using 1 H NMR, 13 C NMR, and IR spectroscopy, the ozonolysis products were characterized to confirm their identity and purity. 80 All products contained a carboxylic acid, ketone, and/or aldehyde, which are abundant organic functional groups in the atmosphere (Hallquist et al., 2009;Nozière et al., 2015). The O:C range of the https://doi.org/10.5194/acp-2020-318 Preprint. Discussion started: 9 April 2020 c Author(s) 2020. CC BY 4.0 License. ozonolysis products was between 0.13 and 0.44 (Table 1). To achieve O:C ratios up to 1.00, we used commercially-available organic compounds (Sigma-Aldrich, purities ≥ 97%) (Table 1). 85

Preparation of particles consisting of one and two organic species
Particles consisting of either one or two organic compounds were prepared at room temperature without the addition of a solvent. Particles consisting of the commercially-available organic compounds were nebulized directly on siliconized hydrophobic glass slides (Hampton Research, Canada). Particles consisting of ozonolysis products were slightly viscous. To form particles on a substrate, these 90 ozonolysis products were picked up with the tip of a pipette, and the pipette was then flicked towards a hydrophobic glass slide.
Particles consisting of two organic compounds were prepared using mixtures (1:1 mass ratio) of pure organic species without addition of a solvent. To prepare the mixtures with 1:1 mass ratio, each organic species was weighed in a vial and then combined. After mixing, the solutions were homogenous based 95 on visual inspection. Particles were generated from these mixtures and deposited on hydrophobic slides either by nebulization (for the mixtures involving commercially-available organic compounds) or by the flicking method via the tip of a pipette as described above (for the ozonolysis products). This method of producing two-component organic particles did not work for α-pinene ozonolysis products and βcaryophyllinic acid due to the stickiness of these material. Hence, these materials were not included in 100 the systems used to generate two-component organic particles.

Optical microscopy for observation of liquid-liquid phase separation
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. The temperature and RH were monitored by a humidity and temperature sensor (Sensirion, SHT 71, Switzerland). RH inside the flow-cell was calibrated by measuring the deliquescence RH of four different pure inorganic 110 https://doi.org/10.5194/acp-2020-318 Preprint. Discussion started: 9 April 2020 c Author(s) 2020. CC BY 4.0 License. salts (potassium carbonate, sodium chloride, ammonium sulfate, and potassium nitrate) (Winston and Bates, 1960). The RH uncertainty from the calibration was ±1.5%.
At the beginning of LLPS experiments, organic particles inside the flow-cell were equilibrated at ~100% RH for 15-20 min. If LLPS was observed, the RH was decreased from ~100% to ~5-10% lower than the RH at which the two liquid phases merged into one phase followed by an increase in RH to 115 ~100%. If LLPS was not observed, the RH was decreased from ~100% to ~0% RH, followed by an increase to ~100% RH. During all experiments, the RH was adjusted at a rate of 0.1 -0.2% RH min −1 .
The optical images during experiments were recorded every 5 s using a CMOS (complementary metaloxide-semiconductor) detector (DigiRetina 16, Tucsen, China). Organic particles were selected in the diameter range of 30-100 µm, which was required for LLPS experiments.

Liquid-liquid phase separation in particles containing one organic species
Eleven different types of particles containing one organic species were investigated for LLPS at 291 ± 1 K. Out of the eleven different types of one-component organic particles studied, eight underwent LLPS 125 during humidity cycles (Table S1). LLPS occurred in all one-component organic particles containing αpinene and β-caryophyllene ozonolysis products. Figure 1 and Movies S1-S7 are optical images recorded while the RH was decreased for all the cases where LLPS was observed in one-component organic particles. For these cases, two liquid phases were always observed at ~100% RH. As the RH was decreased, the two liquid phases merged 130 into one liquid phase at ~95% RH, except for particles of β-caryophyllinic acid ( Fig. 1e and Movie S5).

Shown in
For β-caryophyllinic acid particles, the two liquid phases merged into one liquid phase at 83.7% RH ( Fig. 1e and Movie S5). Particles of β-caryophyllonic acid and β-nocaryophyllonic acid had a partially engulfed morphology after LLPS (Fig. 1b, d and Movies S2, S4) (Kwamena et al., 2010;Reid et al., 2011;Song et al., 2013) while the others particles had a core-shell morphology after LLPS. We expect 135 that the inner phase consisted mainly of water while the outer phase consisted mainly of organic molecules because the amount of the inner phase reduced in size as the RH was decreased (Renbaum-Shown in Figure 2 and Movies S8-S14 are optical images of the same seven particles shown in Fig. 1 and Movies S1-S7, except the images were recorded while the RH was increased, rather than decreased. 140 At low RH-values, the particles contained one phase. As the RH increased, LLPS occur at ~95% RH for all cases exception for β-caryophyllinic acid particles, which underwent LLPS at 84.9% RH ( Fig. 2e and Movie S12). At the onset of LLPS, many small inclusions formed in the particles. As the RH was further increased, the small inclusions coagulated and coalesced, and the particles continued to grow ( Fig. 2 and Movies S8-S14). At ~100% RH, all particles contained two liquid phases. 145 The mechanism for LLPS in the single-component organic particles was likely spinodal decomposition based on the formation of many small inclusions at the onset of LLPS. Spinodal decomposition is a phase transition that occurs within a liquid without an energy barrier (Shelby, 1995;Papon et al., 1999;Ciobanu et al., 2009;Song et al., 2012a). Previous studies also observed LLPS by spinodal decomposition in α-pinene-derived SOA, β-caryophyllene-derived SOA, and limonene-derived  (Tables S1 and S2). In addition, no dependence on particle size was observed for LLPSlower and LLPSupper within the size range investigated (30-100 μm). difference in organic compounds used in the current study and used to generate the BAT model. In the current study, we investigated compounds with more than one type of functional group while the BAT 170 model used a single type of functional group to generate miscibility boundaries.

Liquid-liquid phase separation in particles containing two organic species
To better mimic the complexity of real aerosol compositions, we also studied LLPS in particles containing two organic species. Table S2 lists the 25 different mixtures investigated using combinations 175 of β-caryophyllene ozonolysis products and commercially-available organic compounds. In total, 23 out of the 25 two-component organic particle types investigated underwent LLPS ( Fig. 3b and Table S2). Fig. 5 and Movies S15-S19 are examples of images of two-component organic particles that underwent LLPS during a decrease in RH. Shown in Fig. 6 and Movies S20-S24 are the same five particles, but images recorded as the RH was increased. 180 Out of the 23 particles types that underwent LLPS, 19 of the particle types formed a core-shell morphology with decreasing RH. Only four particle types (β-caryophyllonic acid/suberic acid, βcaryophyllonic acid/polyethylene glycol-400(PEG-400), β-caryophyllene aldehyde/β-caryophyllonic acid, and β-caryophyllonic acid/β-nocaryophyllonic acid) formed a partially engulfed morphology with decreasing RH. 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.

Shown in
As RH was decreased, the two liquid phases merged into one phase. For example, particles of βcaryophyllene aldehyde/PEG-400 merged into one phase at 39.9% RH (Fig. 5e and Movie S19).
In the experiments with two-component organic particles and increasing RH, in most cases (19 out of the 23 particle types that underwent LLPS), phase separation began with the abrupt formation of many 190 small inclusions (e.g. Fig. 6a, b, e and Movies S20, 21, 24). This behavior suggests spinodal decomposition as the mechanism for LLPS. In contrast, in experiments with particles containing ozonolysis products mixed with pyruvic acid, phase separation began with the growth of a second phase at the surface of the particle as the RH increased (Figs. 6c, d and Movies S22, 23). This type of https://doi.org/10.5194/acp-2020-318 Preprint. Discussion started: 9 April 2020 c Author(s) 2020. CC BY 4.0 License. mechanism was previously observed in organic/inorganic aerosol particles (Ciobanu et al., 2009;Song 195 et al., 2012a). Fig. 3b

Atmospheric implications
O:C of organic materials has been used to interpret and parameterize hygroscopicity (Jimenez et al., 2009), oxidation (Heald et al., 2010;Kroll et al., 2011), and mixing thermodynamics of organic aerosol particles (Donahue et al., 2011;Hodas et al., 2016). 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. 3a   215 and b). Considering the chemical complexity and the O:C ratio of organic particles in the troposphere (0.20 < O:C < 1.00) (Zhang et al., 2007;Hallquist et al., 2009;Jimenez et al., 2009;Heald et al., 2010;Ng et al., 2010), our result provided additional evidence that LLPS is likely a common feature of organic aerosols free of inorganic salts. A caveat is that the mixing ratio of 1:1 for two organic species and the chemical complexity used in our experiments is rather simple compared to the chemical 220 complexity found in the atmosphere (Zhang et al., 2007;Hallquist et al., 2009;Jimenez et al., 2009Jimenez et al., ). https://doi.org/10.5194/acp-2020 Preprint. Discussion started: 9 April 2020 c Author(s) 2020. CC BY 4.0 License.
Further studies are needed to confirm LLPS in organic aerosols comprising of more complex mixtures with different mixing ratios.
The occurrence of LLPS in organic aerosol particles at high RH, as observed in the current studies, is important since LLPS at high RH can lower the barrier to CCN activation by decreasing the surface 225 tension of the particles Rastak et al., 2017;Liu et al., 2018). A decrease in surface tension and lowering of the barrier to CCN, can lead to an increase in cloud droplets numbers in the atmosphere, with implications for modelling the indirect effect of aerosols on climate Rastak et al., 2017). 230 Data availability. Underlying material and related items for this paper are located in the Supplement.