Modelling hygroscopicity-induced gas–aerosol partitioning, organic surface enrichment and cloud droplet formation

The interactions among low and semi-volatile organic compounds, water and other inorganic components within fine-mode aerosols are complex. We show that understanding several features of this complexity can be important in the context of phase separation, a particle surface composition enriched by organics and for related cloud droplet activation modeling.  
Context: oxidized organic compounds contribute to the particle hygroscopicity, yet typically to a lesser extent than dissolved inorganic ions. The overall hygroscopicity of aerosols in turn greatly affects their water uptake in an air parcel experiencing increasing relative humidity. The mechanism acts directly in terms of adding water mass, as well as indirectly via a hygroscopicity-induced feedback leading to enhanced gas–to–particle partitioning of semi-volatile organic components alongside a re-equilibration of the aerosol with inorganic acids, ammonia and further water uptake. Furthermore, non-ideal mixing may induce liquid–liquid phase separation, often leading to an organic-rich phase of relatively low surface tension surrounding an inorganic-rich particle core. This phase separation effect and related surface enhancements of organic component concentrations affect not only the morphology but also the potential for near-surface chemical reactions, as well as the thermodynamics controlling an aerosol particle’s activation into a cloud droplet at realistic water vapour supersaturations.  New experimental techniques and field observations over the past few years have encouraged model development for an improved representation of these processes on a detailed level (see, e.g., discussion in Davies et al., 2019). This has led to a better understanding of the potential role of organic aerosol compounds spanning a range of polarities and an associated evolution of surface tension prior to the cloud condensation nucleus (CCN) activation point. While detailed process models still lack finer details to fully capture these composition and phase effects reliably and predictively, important challenges exist in translating these mechanisms into computationally efficient and feasible reduced-complexity models of use for air quality and chemistry-climate modelling.
In this presentation, we will outline the current state of a relatively complete process-level aerosol thermodynamics model based on AIOMFAC and introduce key features of a recently developed reduced-complexity organic aerosol model that accounts for water content and hygroscopicity-induced feedbacks on composition (Gorkowski et al., 2019). A key advantage of the reduced-complexity model is its ability to use only input typically known from field measurements or data available in large-scale air quality models. Our approach is compatible with a volatility basis set approach and allows for extending it by adding a realistic humidity dependence. In addition, we account for phase separation and related effects on surface tension in a simplified, computationally efficient manner. This approach and its results for aerosol hygroscopicity and cloud droplet activation will be discussed.

References:

Davies, J. F., Zuend, A., and Wilson, K. R.: Technical note: The role of evolving surface tension in the formation of cloud droplets, Atmos. Chem. Phys., 19, 2933–2946, doi:10.5194/acp-19-2933-2019, 2019.

Gorkowski, K., Preston, T. C., and Zuend, A.: Relative-humidity-dependent organic aerosol thermodynamics via an efficient reduced-complexity model, Atmos. Chem. Phys., 19, 13383–13407, 10.5194/acp-19-13383-2019, 2019.