10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Advancements in Aerosol Thermodynamics for Large-Scale Applications – Bridging the Gap between Structure and Volatility-Based Models
KYLE GORKOWSKI, Andreas Zuend, McGill University
Abstract Number: 508 Working Group: Aerosol Modeling
Abstract The aerosol thermodynamics frameworks used in current chemical transport models (CTMs) do not explicitly account for non-ideality and liquid-liquid phase separation. These effects are arising from the molecular interactions and incomplete miscibility among organic and inorganic aerosol species. These processes are not accounted for due to three main reasons: (1) the necessity for computationally efficient frameworks, which entail some level of simplification, (2) a lack of model availability for the treatment of phase separation in the CTM context, and (3) limited molecular-level information about aerosol composition. We present work on improving the treatment of these effects in CTMs.
The Aerosol Inorganic—Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is a structure-based activity model that explicitly incorporates solution non-ideality, including organic-inorganic interactions. The use of AIOMFAC in our improved phase equilibrium algorithm enables consideration of liquid-liquid phase separation in thermodynamic equilibrium calculations. Here we use this advanced thermodynamic treatment by AIOMFAC to assess the potential for improvements to existing two-product and 1-D volatility basis set approaches with regard to gas-particle partitioning under variable relative humidity (RH). We use an additional term to describe the RH-dependence in the widely used two-product and 1-D volatility basis set representations of aerosol mass concentration and gas-particle partitioning. As RH increases, the characteristic gas-liquid partitioning parameter, C*, of a component tends to decrease, driving additional organic mass from the gas phase to the particle phase. Depending on how liquid-liquid phase separation is treated in simplified models (i.e., complete inorganic-organic split, homogeneous single-phase, etc.), the resulting particulate mass will be either underestimated or overestimated in comparison to a more demanding calculation allowing for partial miscibility at equilibrium. In addition to building extensions to the currently used aerosol thermodynamic models, we have worked on increasing the computational speed of the AIOMFAC-based model. The model uses optimization to high precision as a default, so we performed a systematic assessment of the computational accuracy and speed of the AIOMFAC-based equilibrium model under various simplifying assumptions. We will discuss the challenges and feasibility of implementing an AIOMFAC-based thermodynamic module directly into a CTM.