Particles containing secondary organic material (SOM) are ubiquitous in theatmosphere and play a role in climate and air quality. Recently, researchhas shown that liquid–liquid phase separation (LLPS) occurs at high relativehumidity (RH) (greater than ∼ 95 %) in -pinene-derivedSOM particles free of inorganic salts, while LLPS does not occur inisoprene-derived SOM particles free of inorganic salts. We expand on thesefindings by investigating LLPS at 290 ± 1 K in SOM particles free ofinorganic salts produced from ozonolysis of -caryophyllene,ozonolysis of limonene, and photo-oxidation of toluene. LLPS was observed atgreater than ∼ 95 % RH in the biogenic SOM particlesderived from -caryophyllene and limonene while LLPS was not observedin the anthropogenic SOM particles derived from toluene. This work combinedwith the earlier work on LLPS in SOM particles free of inorganic saltssuggests that the occurrence of LLPS in SOM particles free of inorganicsalts is related to the oxygen-to-carbon elemental ratio (O : C) of theorganic material. These results help explain the difference between thehygroscopic parameter of SOM particles measured above and belowwater saturation in the laboratory and field, and have implications forpredicting the cloud condensation nucleation properties of SOM particles.
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