Secondary organic aerosol (SOA) accounts for a large fraction of submicron particles in the atmosphere. SOA can occur in amorphous solid or semi-solid phase states depending on chemical composition, relative humidity (RH), and temperature. The phase transition between amorphous solid and semi-solid states occurs at the glass transition temperature (Tg). We have recently developed a method to estimate Tg of pure compounds containing carbon, hydrogen, and oxygen atoms (CHO compounds) with molar mass less than 450 g mol−1 based on their molar mass and atomic O : C ratio. In this study, we refine and extend this method for CH and CHO compounds with molar mass up to ∼ 1100 g mol−1 using the number of carbon, hydrogen, and oxygen atoms. We predict viscosity from the Tg-scaled Arrhenius plot of fragility (viscosity vs. Tg∕T) as a function of the fragility parameter D. We compiled D values of organic compounds from the literature and found that D approaches a lower limit of ∼ 10 (±1.7) as the molar mass increases. We estimated the viscosity of α-pinene and isoprene SOA as a function of RH by accounting for the hygroscopic growth of SOA and applying the Gordon–Taylor mixing rule, reproducing previously published experimental measurements very well. Sensitivity studies were conducted to evaluate impacts of Tg, D, the hygroscopicity parameter (κ), and the Gordon–Taylor constant on viscosity predictions. The viscosity of toluene SOA was predicted using the elemental composition obtained by high-resolution mass spectrometry (HRMS), resulting in a good agreement with the measured viscosity. We also estimated the viscosity of biomass burning particles using the chemical composition measured by HRMS with two different ionization techniques: electrospray ionization (ESI) and atmospheric pressure photoionization (APPI). Due to differences in detected organic compounds and signal intensity, predicted viscosities at low RH based on ESI and APPI measurements differ by 2–5 orders of magnitude. Complementary measurements of viscosity and chemical composition are desired to further constrain RH-dependent viscosity in future studies.
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Predictions of the glass transition temperature and viscosity of organic aerosols from volatility distributions
Abstract. Volatility and viscosity are important properties of organic aerosols (OA),affecting aerosol processes such as formation, evolution, and partitioning ofOA. Volatility distributions of ambient OA particles have often beenmeasured, while viscosity measurements are scarce. We have previouslydeveloped a method to estimate the glass transition temperature (Tg) ofan organic compound containing carbon, hydrogen, and oxygen. Based onanalysis of over 2400 organic compounds including oxygenated organiccompounds, as well as nitrogen- and sulfur-containing organic compounds, weextend this method to include nitrogen- and sulfur-containing compoundsbased on elemental composition. In addition, parameterizations are developedto predict Tg as a function of volatility and the atomicoxygen-to-carbon ratio based on a negative correlation between Tg andvolatility. This prediction method of Tg is applied to ambientobservations of volatility distributions at 11 field sites. Thepredicted Tg values of OA under dry conditions vary mainly from 290 to 339 Kand the predicted viscosities are consistent with the results of ambientparticle-phase-state measurements in the southeastern US and the Amazonianrain forest. Reducing the uncertainties in measured volatility distributionswould improve predictions of viscosity, especially at low relative humidity.We also predict the Tg of OA components identified via positive matrixfactorization of aerosol mass spectrometer (AMS) data. The predicted viscosity ofoxidized OA is consistent with previously reported viscosity of secondary organic aerosols (SOA) derivedfrom α-pinene, toluene, isoprene epoxydiol (IEPOX), and diesel fuel.Comparison of the predicted viscosity based on the observed volatilitydistributions with the viscosity simulated by a chemical transport modelimplies that missing low volatility compounds in a global model can lead tounderestimation of OA viscosity at some sites. The relation betweenvolatility and viscosity can be applied in the molecular corridor orvolatility basis set approaches to improve OA simulations in chemicaltransport models by consideration of effects of particle viscosity in OAformation and evolution.
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- NSF-PAR ID:
- 10178916
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 20
- Issue:
- 13
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 8103 to 8122
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-20-8103-2020
