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

Abstract

A useful aerosol model must be able to adequately resolve the chemical complexity and phase state of the wide particle size range arising from the many different secondary aerosol growth processes to assess their environmental and health impacts. Over the past two decades, significant advances in understanding of gas-aerosol partitioning have occurred, particularly with respect to the role of organic compounds, yet aerosol representations have changed little in air quality and climate models since the late 1990s and early 2000s. The gas-aerosol partitioning models which are still commonly used in air quality models are separate inorganics-only thermodynamics and secondary organic aerosol (SOA) formation based on absorptive partitioning theory with an assumption of well-mixed liquid-like particles that continuously maintain equilibrium with the gas phase. These widely used approaches in air quality models for secondary aerosol composition and growth based on separated inorganic and organic processes are inadequate. This review summarizes some of the important developments during the past two decades in understanding of gas aerosol mass transfer processes. Substantial increases in computer performance in the last decade justify increasing the process detail in aerosol models. Organics play a central role during post-nucleation growth into the accumulation mode and change the hygroscopic properties of sulfate aerosol. At present, combined inorganic-organic aerosol thermodynamics models are too computationally expensive to be used online in 3-D simulations without high levels of aggregation of organics into a small number of functional surrogates. However, there has been progress in simplified modeling of liquid-liquid phase separation (LLPS) and distinct chemical regimes within organic-rich and inorganic-rich phases. Additional limitations of commonly used thermodynamics models are related to lack of surface tension data for various aerosol compositions in the small size limit, and lack of a comprehensive representation of surface interaction terms such as disjoining pressure in the Gibbs free energy which become significant in the small size limit and which affect both chemical composition and particle growth. As a result, there are significant errors in modeling of hygroscopic growth and phase transitions for particles in the nucleation and Aitken modes. There is also increasing evidence of reduced bulk diffusivity in viscous organic particles and, therefore, traditional secondary organic aerosol models, which are typically based on the assumption of instantaneous equilibrium gas-particle partitioning and neglect the kinetic effects, are no longer tenable.

Enhanced Gas Uptake during α-Pinene Ozonolysis Points to a Burying Mechanism

“…While tropospheric concentrations are much lower than those used in these laboratory experiments, a large number of biogenic and anthropogenic VOCs are available in air that also contribute to particle growth. The majority of air quality models employ equilibrium gas–particle partitioning to determine gas and particle interactions, assuming the particles are low viscosity liquids . However, improvements in particle growth predictions have been reported when kinetically controlled condensation is included with equilibrium partitioning, especially under conditions of low RH and temperature .…”

“…The majority of air quality models employ equilibrium gas−particle partitioning to determine gas and particle interactions, assuming the particles are low viscosity liquids. 18 However, improvements in particle growth predictions have been reported when kinetically controlled condensation is included with equilibrium partitioning, especially under conditions of low RH and temperature. 99 The burying effect presented here over a wide range of initial AP concentrations leads to enhancements in equilibrium partitioning coefficients by factors of ∼4−10 under dry conditions at varied initial AP concentrations.…”

“…9−17 Although great progress has been made in understanding how gases are incorporated into particles to contribute to particle growth, these processes are still not completely understood or parameterized by models. 18 During growth of a liquid droplet, gases diffuse into and out of the particle quickly, establishing a relatively rapid quasiequilibrium. 19−24 However, many SOA particles have been shown to be more viscous in nature.…”

“…Oxidation of volatile organic compounds (VOCs) in the atmosphere can result in low volatility species that either condense together to form new particles or condense onto and grow existing particles. , These secondary organic aerosol (SOA) particles play an important role in Earth’s radiative budget directly through light scattering and indirectly by acting as cloud condensation nuclei. − They have also been shown to have deleterious effects on human health. − Although great progress has been made in understanding how gases are incorporated into particles to contribute to particle growth, these processes are still not completely understood or parameterized by models …”

Model reduction in Smoluchowski-type equations

“…The range of phenomena modelled via Smoluchowski kinetic equations has also expanded over time, from molecular scales [3][4][5][6] to astronomical [7][8][9]. More detailed information about possible applications of aggregation-based models can be found in extensive reviews [10,11] and references therein.…”