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