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. Secondary organic aerosol (SOA) accounts for a large fraction of submicronparticles in the atmosphere. SOA can occur in amorphous solid or semi-solidphase states depending on chemical composition, relative humidity (RH), andtemperature. The phase transition between amorphous solid and semi-solidstates occurs at the glass transition temperature ( Tg) . We haverecently developed a method to estimate Tg  of pure compoundscontaining carbon, hydrogen, and oxygen atoms (CHO compounds) with molar massless than 450 g mol −1 based on their molar mass and atomic O : Cratio. In this study, we refine and extend this method for CH and CHOcompounds with molar mass up to ∼  1100 g mol −1 using the numberof 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 massincreases. We estimated the viscosity of α -pinene and isoprene SOA asa function of RH by accounting for the hygroscopic growth of SOA and applyingthe Gordon–Taylor mixing rule, reproducing previously published experimentalmeasurements very well. Sensitivity studies were conducted to evaluateimpacts of Tg , D , the hygroscopicity parameter ( κ) , andthe Gordon–Taylor constant on viscosity predictions. The viscosity oftoluene SOA was predicted using the elemental composition obtained byhigh-resolution mass spectrometry (HRMS), resulting in a good agreement withthe measured viscosity. We also estimated the viscosity of biomass burningparticles using the chemical composition measured by HRMS with two differentionization techniques: electrospray ionization (ESI) and atmospheric pressurephotoionization (APPI). Due to differences in detected organic compounds andsignal intensity, predicted viscosities at low RH based on ESI and APPImeasurements differ by 2–5 orders of magnitude. Complementary measurementsof viscosity and chemical composition are desired to further constrainRH-dependent viscosity in future studies.

Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition