Abstract
Abstract. We present a theoretical study investigating the cloud activation of multicomponent organic particles. We modeled these complex mixtures using solubility distributions (analogous to volatility distributions in the VBS, i.e., volatility basis set, approach), describing the mixture as a set of surrogate compounds with varying water solubilities in a given range. We conducted Köhler theory calculations for 144 different mixtures with varying solubility range, number of components, assumption about the organic mixture thermodynamics and the shape of the solubility distribution, yielding approximately 6000 unique cloud condensation nucleus (CCN)-activation points. The results from these comprehensive calculations were compared to three simplifying assumptions about organic aerosol solubility: (1) complete dissolution at the point of activation; (2) combining the aerosol solubility with the molar mass and density into a single effective hygroscopicity parameter κ; and (3) assuming a fixed water-soluble fraction ϵeff. The complete dissolution was able to reproduce the activation points with a reasonable accuracy only when the majority (70–80%) of the material was dissolved at the point of activation. The single-parameter representations of complex mixture solubility were confirmed to be powerful semi-empirical tools for representing the CCN activation of organic aerosol, predicting the activation diameter within 10% in most of the studied supersaturations. Depending mostly on the condensed-phase interactions between the organic molecules, material with solubilities larger than about 0.1–100 g L−1 could be treated as soluble in the CCN activation process over atmospherically relevant particle dry diameters and supersaturations. Our results indicate that understanding the details of the solubility distribution in the range of 0.1–100 g L−1 is thus critical for capturing the CCN activation, while resolution outside this solubility range will probably not add much information except in some special cases. The connections of these results to the previous observations of the CCN activation and the molecular properties of complex organic mixture aerosols are discussed. The presented results help unravel the mechanistic reasons behind observations of hygroscopic growth and CCN activation of atmospheric secondary organic aerosol (SOA) particles. The proposed solubility distribution framework is a promising tool for modeling the interlinkages between atmospheric aging, volatility and water uptake of atmospheric organic aerosol.
Highlights
Interactions of atmospheric aerosol particles with ambient water vapor determine to a large extent the influence that aerosols have on climate
Huff Hartz et al (2005) reported effective solubilities of as high as 100 g L−1 for both mono- and sesqui-terpene secondary organic aerosol (SOA) – both are known to consist of a range of compounds with different solubilities. These results demonstrate the importance of knowing the water-soluble fraction of SOA under varying conditions but suggest that its exact speciation is probably not necessary for predictive understanding of the cloud condensation nuclei (CCN) activity of SOA particles (Asa-Awuku et al, 2010; Engelhart et al, 2011)
We introduce a framework for representing the mixture components with a continuous distribution of solubilities, similar to the VBS (Donahue et al, 2006, 2011, 2012). Using this framework in a theoretical model, we investigate the dissolution behavior of complex organic mixtures and their CCN activity, focusing on the impact of mixture solubility on CCN activation
Summary
Interactions of atmospheric aerosol particles with ambient water vapor determine to a large extent the influence that aerosols have on climate. The water content of aerosol particles at atmospheric relative humidity (RH) below 100 % contributes significantly to the direct effect they have on the global radiative balance (Seinfeld and Pandis, 2006; Petters and Kreidenweis, 2007; Swietlicki et al, 2008; Zieger et al, 2011; Rastak et al, 2014). I. Riipinen et al.: Connecting the solubility and CCN activation of complex organic aerosols of aerosol particles on clouds and climate, it is necessary to understand the ways that aerosol constituents interact with water
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