Relative-humidity-dependent organic aerosol thermodynamics via an efficient reduced-complexity model

Abstract

Abstract. Water plays an essential role in aerosol chemistry, gas–particle partitioning, and particle viscosity, but it is typically omitted in thermodynamic models describing the mixing within organic aerosol phases and the partitioning of semivolatile organics. In this study, we introduce the Binary Activity Thermodynamics (BAT) model, a water-sensitive reduced-complexity model treating the nonideal mixing of water and organics. The BAT model can process different levels of physicochemical mixture information enabling its application in the thermodynamic aerosol treatment within chemical transport models, the evaluation of humidity effects in environmental chamber studies, and the analysis of field observations. It is capable of using organic structure information including O:C, H:C, molar mass, and vapor pressure, which can be derived from identified compounds or estimated from bulk aerosol properties. A key feature of the BAT model is predicting the extent of liquid–liquid phase separation occurring within aqueous mixtures containing hydrophobic organics. This is crucial to simulating the abrupt change in water uptake behavior of moderately hygroscopic organics at high relative humidity, which is essential for capturing the correct behavior of organic aerosols serving as cloud condensation nuclei. For gas–particle partitioning predictions, we complement a volatility basis set (VBS) approach with the BAT model to account for nonideality and liquid–liquid equilibrium effects. To improve the computational efficiency of this approach, we trained two neural networks; the first for the prediction of aerosol water content at given relative humidity, and the second for the partitioning of semivolatile components. The integrated VBS + BAT model is benchmarked against high-fidelity molecular-level gas–particle equilibrium calculations based on the AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficient) model. Organic aerosol systems derived from α-pinene or isoprene oxidation are used for comparison. Predicted organic mass concentrations agree within less than a 5 % error in the isoprene case, which is a significant improvement over a traditional VBS implementation. In the case of the α-pinene system, the error is less than 2 % up to a relative humidity of 94 %, with larger errors past that point. The goal of the BAT model is to represent the bulk O:C and molar mass dependencies of a wide range of water–organic mixtures to a reasonable degree of accuracy. In this context, we discuss that the reduced-complexity effort may be poor at representing a specific binary water–organic mixture perfectly. However, the averaging effects of our reduced-complexity model become more representative when the mixture diversity increases in terms of organic functionality and number of components.

A predictive group-contribution model for the viscosity of aqueous organic aerosol

“…As a result, the SOA particle that was given more time to oxidize may have a higher mixture viscosity (particularly at low RH). Grayson et al (2016) also provide evidence that production aerosol mass concentrations are inversely proportional to the SOA viscosity. From gas-particle partitioning theory and experimental evidence, a higher abundance of less-oxidized components in the SOA is expected for high aerosol loading chamber experiments.…”

“…Although, the adjustments made to achieve this agreement results in an average O : C of 0.51. If SOA constituent concentrations are modified to produce an average mixture O : C of ∼ 0.4, then the model is in agreement with the measurements from Grayson et al (2016) but not with those of Renbaum-Wolff et al (2013). By choosing to fit the model to the data of Renbaum-Wolff et al (2013), the general shape of the AIOMFAC-VISC prediction curve appears reasonable and ensures most of the experimental data fall within the uncertainty in T g values.…”

“…In order to fully understand the implications of viscous SOA, we must be able to quantify how frequently SOA precursors and atmospheric conditions, namely relative humidity (RH) and temperature, favour their formation. Semi-solid anhydrous SOA can be formed from biogenic precursors, like monoterpenes (Renbaum-Wolff et al, 2013;Grayson et al, 2016) and isoprene (Song et al, 2015) or from anthropogenic precursors, like polycyclic aromatic hydrocarbons (Zelenyuk et al, 2017). The type of precursor as well as the degree of oxidation governs the degree of functionalization of the resulting SOA species.…”

Current State of Atmospheric Aerosol Thermodynamics and Mass Transfer Modeling: A Review

“…Currently, only offline comprehensive coupled organic-inorganic models are available. However, some attempts have been made towards reducing computational cost through the use of chemical surrogates [112,165,166] or developing simplified models which could be incorporated into 3-D AQ models [167,168]. Table 2 summarizes the models presented in this section.…”

“…A reduced complexity organics thermodynamics model has been developed by Gorkowski et al [168]. The Binary Activity Thermodynamics (BAT) model can use explicit (composition-resolved) or aggregated (limited chemical composition information) organic species characteristics such as the oxygen to carbon ratio, hydrogen to carbon ratio, molar mass and vapor pressure.…”