![]() Furthermore, a complex mixture of VOCs resulted in a higher SOA viscosity compared to SOA generated from α-pinene alone at ≥25% RH, highlighting the importance of studying properties of SOA generated from more realistic multi-component VOC mixtures. These findings suggest that plant stress influences the physicochemical properties of biogenic SOA. The predicted viscosity of the healthy plant SOA was lower than that of the stressed plant SOA driven by both the higher glass transition temperatures and lower hygroscopicity of the organic molecules making up stressed plant SOA. The observed peak abundances in the mass spectra were also used to estimate the SOA viscosity as a function of RH and volatility. At 40% and 50% RH, stressed plant SOA had the highest viscosity, followed by healthy plant SOA and then α-pinene SOA in descending order. At RH ≤25%, both stressed and healthy plant SOA viscosity exceeded 10 8 Pa s, a value similar to that of tar pitch. LLPS occurred in both the healthy and stressed plant SOA however, stressed plant SOA exhibited phase separation over a broader humidity range than healthy plant SOA, with LLPS persisting down to 23 ± 11% RH. The stressed plant SOA had increased abundance of higher molecular weight species, reflecting a greater fraction of sesquiterpenes in the stressed VOC mixture compared to the healthy plant VOC mixture. Molecular composition was determined with high resolution mass spectrometry, viscosity was determined with the poke-flow technique, and liquid–liquid phase separation was investigated with optical microscopy. SOA from photooxidation of α-pinene was also prepared for comparison. Model “healthy plant SOA” and “stressed plant SOA” were generated in a 5 m 3 environmental smog chamber by photooxidation of the mixtures at 50% relative humidity (RH). ![]() Molecular composition, viscosity, and liquid–liquid phase separation (LLPS) were investigated for secondary organic aerosol (SOA) derived from synthetic mixtures of volatile organic compounds (VOCs) representing emission profiles for Scots pine trees under healthy and aphid-herbivory stress conditions.
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