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Abstract. Secondary organic aerosol (SOA) constitutes a largefraction of atmospheric aerosol. To assess its impacts on climate and airpollution, knowledge of the number of phases in internal mixtures ofdifferent SOA types is required. Atmospheric models often assume thatdifferent SOA types form a single phase when mixed. Here, we present visualobservations of the number of phases formed after mixing differentanthropogenic and biogenic SOA types. Mixing SOA types generated inenvironmental chambers with oxygen-to-carbon (O/C) ratios between 0.34 and 1.05, we found 6 out of 15 mixtures of two SOA types to result in two phase particles. We demonstrate that the number of phases depends on thedifference in the average O/C ratio between the two SOA types (Δ(O/C)). Using a threshold Δ(O/C) of 0.47, we can predict the phasebehavior of over 90 % of our mixtures, with one- and two-phase particlespredicted for Δ(O/C)<0.47 and Δ(O/C)≥0.47,respectively. This threshold ΔO/C value provides a simple parameterto predict whether mixtures of fresh and aged SOA form one- or two-phase particles in the atmosphere. In addition, we show that phase-separated SOAparticles form when mixtures of volatile organic compounds emitted from realtrees are oxidized. 

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. 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). SOA from photooxidation of α-pinene was also prepared for comparison. 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. 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. 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. At RH ≤25%, both stressed and healthy plant SOA viscosity exceeded 10 8 Pa s, a value similar to that of tar pitch. 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. The observed peak abundances in the mass spectra were also used to estimate the SOA viscosity as a function of RH and volatility. 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. These findings suggest that plant stress influences the physicochemical properties of biogenic SOA. 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.