Abstract
Abstract. 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 the LLPS of one-component and two-component organic particles consisting of α-pinene- and β-caryophyllene-derived ozonolysis products and commercially available organic compounds of relevance to atmospheric organic particles. In the experiments involving single-component 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 as well as the nucleation and growth mechanism, and when LLPS occurred, two liquid phases coexisted up to ∼100 % relative humidity (RH). In the experiments involving two-component organic particles, LLPS was observed in 23 out of 25 particles types studied. LLPS almost always occurred when the average was O:C ≤0.67 but never occurred when the average O:C was >0.67. The phase separation occurred by spinodal decomposition as well as the nucleation and growth mechanism. When LLPS occurred, two liquid phases coexisted up to ∼100 % RH. These results provide further evidence that LLPS is likely a frequent occurrence in organic aerosol particles in the troposphere, even in the absence of inorganic salts.
Highlights
Secondary organic aerosol (SOA) is ubiquitous in the atmosphere, comprising up to approximately 80 % of the mass of submicrometer particles (Kanakidou et al, 2005; Jimenez et al, 2009; Heald et al, 2010)
Studies on LLPS in organic aerosol particles free of inorganic salts 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 relative humidity (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 cloud condensation nuclei (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)
We investigate 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. α-Pinene and β-caryophyllene are the most abundant types of monoterpene (C10H16) and sesquiterpenes (C15H24) in the atmosphere, respectively (Guenther, 1995; Sakulyanontvittaya et al, 2008; Henrot et al, 2017)
Summary
Secondary organic aerosol (SOA) is ubiquitous in the atmosphere, comprising up to approximately 80 % of the mass of submicrometer particles (Kanakidou et al, 2005; Jimenez et al, 2009; Heald et al, 2010). Studies on LLPS in organic aerosol particles free of inorganic salts 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). The results from these studies should improve the understanding and modeling of CCN activity of SOA free of inorganic salts
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