Abstract
Abstract. Liquid–liquid phase-separated (LLPS) aerosol particles are known to exhibit increased cloud condensation nuclei (CCN) activity compared to well-mixed ones due to a complex effect of low surface tension and non-ideal mixing. The relation between the two contributions as well as the molecular-scale mechanism of water uptake in the presence of an internal interface within the particle is to date not fully understood. Here we attempt to gain understanding in these aspects through steered molecular dynamics simulation studies of water uptake by a vapor–hydroxy-cis-pinonic acid–water double interfacial system at 200 and 300 K. Simulated free-energy profiles are used to map the water uptake mechanism and are separated into energetic and entropic contributions to highlight its main thermodynamic driving forces. Atmospheric implications are discussed in terms of gas–particle partitioning, intraparticle water redistribution timescales and water vapor equilibrium saturation ratios. Our simulations reveal a strongly temperature-dependent water uptake mechanism, whose most prominent features are determined by local extrema in conformational and orientational entropies near the organic–water interface. This results in a low core uptake coefficient (ko/w=0.03) and a concentration gradient of water in the organic shell at the higher temperature, while entropic effects are negligible at 200 K due to the association-entropic-term reduction in the free-energy profiles. The concentration gradient, which results from non-ideal mixing – and is a major factor in increasing LLPS CCN activity – is responsible for maintaining liquid–liquid phase separation and low surface tension even at very high relative humidities, thus reducing critical supersaturations. Thermodynamic driving forces are rationalized to be generalizable across different compositions. The conditions under which single uptake coefficients can be used to describe growth kinetics as a function of temperature in LLPS particles are described.
Highlights
Aerosol–cloud interactions constitute one of the most important sources of uncertainty in assessments of anthropogenic climate change (IPCC, 2021)
Steered molecular dynamics (MD) simulations of water uptake by a model LLPS aerosol particle consisting of a hydroxy-cis-pinonic acid surface layer and a pure aqueous core at two temperatures corresponding to the boundary layer (300 K) and to the top of the troposphere (200 K) were performed to investigate the mechanism of water uptake by LLPS aerosol via detailed analysis of the free-energy profiles of the corresponding transfer process
The following questions were addressed: (i) how does the uptake mechanism depend on temperature? (ii) To what extent and under what conditions can water uptake by particles containing internal interfaces be described with a single uptake coefficient? (iii) What role does the internal interface play in the water uptake mechanism? (iv) How can the relationship between non-ideal mixing in LLPS particles and their increased cloud condensation nuclei (CCN) activity be explained on a molecular level?
Summary
Aerosol–cloud interactions constitute one of the most important sources of uncertainty in assessments of anthropogenic climate change (IPCC, 2021). The number, size and composition characteristics of aerosol influence the number of droplets that can form in cloudy updrafts, which in turn affect the microphysical evolution and radiative properties of clouds and affect climate. Cloud droplets form upon a subset of aerosol, called cloud condensation nuclei (CCN), that becomes unstable and experiences unconstrained growth in the supersaturated water vapor that develops in a cloudy updraft. The dynamics of water uptake on CCN is a critical process that influences the level of supersaturation that can develop in clouds (Raatikainen et al, 2013) and is affected by the interplay of gas-to-particle transport, interfacial mass transfer and diffusion in the particle phase. Particle-phase diffusion dominates in glassy and semisolid aerosol, while interfacial mass transfer can be important when the particle size is comparable to or smaller than the gas-phase mean free path
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