Observations and implications of liquid–liquid phase separation at high relative humidities in secondary organic material produced by α-pinene ozonolysis without inorganic salts

Abstract

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, solid and so forth) 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 both in the laboratory and with a thermodynamic model over the range of < 0.5 % to 100 % relative humidity (RH) at 290 K. In the laboratory studies, a single phase was observed from 0 to 95 % 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 at which two liquid phases were observed did not depend on the direction of RH change. In the modelling studies at low RH values, the SOM took up hardly any water and was a single organic-rich phase. At high RH values, the SOM underwent LLPS to form an organic-rich phase and an aqueous phase, consistent with the laboratory studies. The presence of LLPS at high RH-values has consequences for the cloud condensation nuclei (CCN) activity of SOM particles. In the simulated Köhler curves for SOM particles, two local maxima are observed. Depending on the composition of the SOM, the first or second maximum can determine the critical supersaturation for activation. The presence of LLPS at high RH-values can explain inconsistencies between measured CCN properties of SOM particles and hygroscopic growth measured below water saturation.

Advanced aerosol optical tweezers chamber design to facilitate phase-separation and equilibration timescale experiments on complex droplets

“…One key property that often impedes thorough experimental or ambient investigations of organic aerosols is the presence of distinct thermodynamic chemical phases in a particle, and the particle's resulting morphological state (or internal structure) in complex multicomponent aerosols (Hallquist et al 2000;McNeill et al 2006;Shen et al 2008;Riemer et al 2009;Zuend et al 2010;Song et al 2012b;Wheeler and Bertram 2012;Schill and Tolbert 2013). Liquid particles are common under atmospheric conditions with three main types of particle morphology (Chang and Pankow 2006;Ciobanu et al 2009;Song et al 2012a;Drozd et al 2013;Schill and Tolbert 2013;Li et al 2015;Qiu and Molinero 2015;Renbaum-Wolff et al 2016;Stewart et al 2015;Hekayati et al 2016). These can be broadly categorized as a homogeneously mixed single-phase structure, and liquid-liquid phase-separated (LLPS) structures (Kwamena et al 2010;Song et al 2013;You et al 2014;Stewart et al 2015;You and Bertram 2015;Metcalf et al 2016).…”

“…The direct observational studies rely on optical microscopy where particles are placed on a hydrophobic substrate, and this can affect the resulting morphology (You et al 2012(You et al , 2014Renbaum-Wolff et al 2016).…”

Relative humidity-dependent viscosity of secondary organic material from toluene photo-oxidation and possible implications for organic particulate matter over megacities

“…This growth and coagulation process resulted in larger but fewer SOM particles on the hydrophobic slides. Details of this procedure are given by Renbaum-Wolff et al (2015) and Song et al (2015). This procedure was used for samples 1, 2, 5, and 6 shown in Table 1.…”

A new method for calculating number concentrations of cloud condensation nuclei based on measurements of a three-wavelength humidified nephelometer system

“…There are two main factors making this conversion necessary. First, closure studies of aerosol hygroscopicity found significant deviations between hygroscopicity at sub-saturated conditions and supersaturated conditions (Wex et al, 2009;Irwin et al, 2010;Good et al, 2010;Renbaum-Wolff et al, 2016). Their difference can be expected to be about 0.1 for accumulation mode aerosol (Wu et al, 2013;Whitehead et al, 2014;Ma et al, 2016).…”

“…In addition, it is important to note that the value of the difference between κ c and κ f is also a rough estimate regardless of the complexity of aerosol hygroscopicity under different conditions, and the influence of κ deviation on N CCN calculation needs to be further examined based on field observation. For fresh aerosol, the actual κ can be too large (about 4 times the κ values for some organic components; Wex et al, 2009;Renbaum-Wolff et al, 2016) or too small (nearly zero for inorganic components and black carbon) and thus is not suitable for the application of this method.…”