AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Predictions of Viscosity of Organic Aerosols by Volatility Distributions: Applications to Field Observations
YING LI, Douglas Day, Harald Stark, Jose-Luis Jimenez, Manabu Shiraiwa, University of California, Irvine
Abstract Number: 347 Working Group: Aerosol Physics
Abstract Volatility and viscosity are important properties of organic aerosols (OA), affecting aerosol processes such as gas–particle partitioning, new particle formation and evolution of size distribution. Chemical composition of OA is complex and molecular specificity is often unavailable, which makes viscosity predictions of OA challenging. Laboratory experiments have found that viscosity of pure compounds is inversely correlated with vapor pressure. The molecular corridor-based analysis of hundreds of OA components shows that compounds with lower volatility have higher glass transition temperature (Tg), at which the phase transition between amorphous solid and semi-solid states occurs (Shiraiwa et al., 2017). Motivated by the evidence of close relations between volatility and viscosity, in this study we develop parameterizations to predict Tg as a function of pure compound saturation mass concentration for compounds including oxygenated organic compounds as well as nitrogen- and sulfur-containing organic compounds. The new parameterization is applied to field observations of volatility distributions to predict viscosity. The prediction shows that organic particles are mostly liquid in background conditions of the Amazonian tropical forest and semi-solid at low relative humidity conditions in the southeastern US, consistent with the results of ambient particle phase state measurements. Predictions for the other seven global observation sites show the frequent occurrence of non-liquid phase state in organic aerosols. The relation between volatility and viscosity established in this study is helpful to estimate viscosity from volatility distributions measured in field observations and laboratory experiments. It can also be applied in the molecular corridor or volatility basis set approaches to improve OA simulations in chemical transport models, by consideration of effects of particle viscosity in OA formation and evolution.
References: Shiraiwa, et al. (2017), Global distribution of particle phase state in atmospheric secondary organic aerosols, Nat. Commun., 8:15002.