Abstract
Abstract. Discontinuities in apparent hygroscopicity below and above water saturation have been observed for organic and mixed organic–inorganic aerosol particles in both laboratory studies and in the ambient atmosphere. However, uncertainty remains regarding the factors that contribute to observations of low hygroscopic growth below water saturation but enhanced cloud condensation nuclei (CCN) activity for a given aerosol population. Utilizing laboratory surrogates for oligomers in atmospheric aerosols, we explore the extent to which such discontinuities are influenced by organic component molecular mass and viscosity, non-ideal thermodynamic interactions between aerosol components, and the combination of these factors. Measurements of hygroscopic growth under subsaturated conditions and the CCN activity of aerosols comprised of polyethylene glycol (PEG) with average molecular masses ranging from 200 to 10 000 g mol−1 and mixtures of PEG with ammonium sulfate (AS) were conducted. Experimental results are compared to calculations of hygroscopic growth at thermodynamic equilibrium conducted with the Aerosol Inorganic Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model, and the potential influence of kinetic limitations on observed water uptake was further explored through estimations of water diffusivity in the PEG oligomers. Particle-phase behavior, including the prevalence of liquid–liquid phase separation (LLPS), was also modeled with AIOMFAC. Under subsaturated relative humidity (RH) conditions, we observed little variability in hygroscopic growth across PEG systems with different molecular masses; however, an increase in CCN activity with increasing PEG molecular mass was observed. This effect is most pronounced for PEG–AS mixtures, and, in fact, an enhancement in CCN activity was observed for the PEG10000–AS mixture as compared to pure AS, as evidenced by a 15 % reduction in critical activation diameter at a supersaturation of 0.8 %. We also observed a marked increase in apparent hygroscopicity for mixtures of higher molecular mass PEG and AS under supersaturated conditions as compared to subsaturated hygroscopic growth. AIOMFAC-based predictions and estimations of water diffusivity in PEG suggest that such discontinuities in apparent hygroscopicity above and below water saturation can be attributed, at least in part, to differences in the sensitivity of water uptake behavior to surface tension effects. There is no evidence that kinetic limitations to water uptake due to the presence of viscous aerosol components influenced hygroscopic growth. For the systems that display an enhancement in apparent hygroscopicity above water saturation, LLPS is predicted to persist to high RH. This indicates a miscibility gap and is likely to influence bulk-to-surface partitioning of PEG at high RH, impacting droplet surface tension and CCN activity. This work provides insight into the factors likely to be contributing to discontinuities in aerosol water-uptake behavior below and above water saturation that have been observed previously in the ambient atmosphere.
Highlights
The extent to which interactions between airborne aerosols and water vapor modulate the Earth’s radiation budget is a source of uncertainty in projections of the impact of aerosols on radiative forcing (Boucher et al, 2013)
hygroscopic diameter growth factors (HGFs) measured here are within ∼ 5 % of those measured for 100 nm particles comprised of polyethylene glycol (PEG) with average molecular masses of 600 and 3400 measured by Petters et al (2006)
We observe substantial enhancements in apparent hygroscopicity based on cloud condensation nuclei (CCN) measurements as compared to HGF measurements conducted below water saturation
Summary
The extent to which interactions between airborne aerosols and water vapor modulate the Earth’s radiation budget is a source of uncertainty in projections of the impact of aerosols on radiative forcing (Boucher et al, 2013). Under supersaturated RH conditions relevant to the activation of cloud condensation nuclei (CCN), aerosol properties influence cloud droplet number, cloud albedo, and, potentially, cloud lifetime. While the water-uptake behavior of inorganic aerosol components is generally well characterized (Seinfeld and Pandis, 2016), a more thorough understanding of the influence of organic compounds on aerosol hygroscopicity and CCN activity is needed. Non-ideal thermodynamic interactions between organic and inorganic particle components can result in liquid–liquid phase separation (LLPS) in which inorganic-dominated and organic-dominated phases coexist (Erdakos and Pankow, 2004; Ciobanu et al, 2009; Zuend et al, 2010; Bertram et al, 2011; Pöhlker et al, 2012; Song et al, 2012; Zuend and Seinfeld, 2012; You et al, 2012, 2013, 2014). Organic aerosol components can exist as viscous liquids, semisolids, and glasses, depending on their composition and ambient conditions (e.g., temperature and RH) (Zobrist et al, 2008, 2011; Mikhailov et al, 2009; Virtanen et al, 2010; Koop et al, 2011; Tong et al, 2011; Saukko et al, 2012; Song et al, 2015; Zhang et al, 2015)
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