Evaluation of group contribution methods to model the viscosity of organic aerosols

Description

The viscosity of organic aerosols influences intraparticle mixing time. In turn, viscosity may modulate the uptake of water, uptake of trace gases, heterogeneous chemical reaction rates, and warm or cold cloud nucleation properties. Thus, accurate parametrization of organic aerosol viscosity would support more accurate numerical modeling of critical aerosol processes within the atmosphere. Currently available group contribution methods offer reasonable viscosity estimation for many common organic compounds, but were not constructed with an atmospheric context in mind. Here, we evaluate model estimates of viscosity against available viscosity data from the literature. Gaps in data available for model evaluation and gaps in model-defined functional groups for atmospheric chemistry use are identified. We assess the sensitivity of the model scheme to the addition of one or more functional groups to hydrocarbon chains between five and twenty carbon atoms. These results are combined with experimental data to identify necessary refinements to the model for development of a more general parametrization of organic aerosol viscosity. We anticipate that the model can be incorporated into scale-bridging testbeds such as the Master Chemical Mechanism or the Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere to track the evolution of viscosity for a complex mix of organic compounds.