Influence of particle-phase state on the hygroscopic behavior of mixed organic–inorganic aerosols

Abstract

Abstract. Recent work has demonstrated that organic and mixed organic-inorganic particles can exhibit multiple phase states depending on their chemical composition and on ambient conditions such as relative humidity (RH). To explore the extent to which water uptake varies with particlephase behavior, hygroscopic growth factors (HGFs) of nine laboratory-generated, organic and organic-inorganic aerosol systems with physical states ranging from well-mixed liquids to phase-separated particles to viscous liquids or semi-solids were measured with the Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe at RH values ranging from 40 to 90 %. Water-uptake measurements were accompanied by HGF and RH-dependent thermodynamic equilibrium calculations using the Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model. In addition, AIOMFAC-predicted growth curves are compared to several simplified HGF modeling approaches: (1) representing particles as ideal, well-mixed liquids; (2) forcing a single phase but accounting for non-ideal interactions through activity coefficient calculations; and (3) a Zdanovskii-Stokes-Robinson-like calculation in which complete separation of the inorganic and organic components is assumed at all RH values, with water uptake treated separately in each of the individual phases. We observed variability in the characteristics of measured hygroscopic growth curves across aerosol systems with differing phase behaviors, with growth curves approaching smoother, more continuous water uptake with decreasing prevalence of liquidliquid phase separation and increasing oxygen : carbon ratios of the organic aerosol components. We also observed indirect evidence for the dehydration-induced formation of highly viscous semi-solid phases and for kinetic limitations to the crystallization of ammonium sulfate at low RH for sucrosecontaining particles. AIOMFAC-predicted growth curves are generally in good agreement with the HGF measurements. The performances of the simplified modeling approaches, however, differ for particles with differing phase states. This suggests that no single simplified modeling approach can be used to capture the water-uptake behavior for the diversity of particle-phase behavior expected in the atmosphere. Errors in HGFs calculated with the simplified models are of sufficient magnitude to produce substantial errors in estimates of particle optical and radiative properties, particularly for the assumption that water uptake is driven by absorptive equilibrium partitioning with ideal particle-phase mixing.

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

“…The CPOT has been constructed as a complement to the Caltech 24 m 3 batch chambers (Bates et al, , 2016Schilling et al, 2015;Hodas et al, 2015;Loza et al, 2013Loza et al, , 2014McVay et al, 2014McVay et al, , 2016Nguyen et al, 2014Nguyen et al, , 2015Schwantes et al, 2015;Yee et al, 2013;Zhang et al, 2014; in carrying out studies of SOA formation resulting from the oxidation of volatile organic compounds (VOCs) by oxidants OH, O 3 and NO 3 over timescales not accessible in a batch chamber. Due to its steady-state operation, the CPOT also affords the capability to collect sufficient quantities of SOA generated in the reactor for comprehensive composition determination by offline mass spectrometry.…”

Hygroscopicity of organic surrogate compounds from biomass burning and their effect on the efflorescence of ammonium sulfate in mixed aerosol particles

“…Studies have shown that water-soluble organic matter from biomass burning (approximately 70 % of total organic matter) can significantly suppress, enhance, or have no effect on the deliquescence (e.g., the RH at which deliquescence occurs at a certain temperature, the DRH) and efflorescence processes (e.g., the efflorescence RH, ERH) of present inorganic electrolytes. The effect depends predominantly on the type of organics, mass fraction of organics relative to inorganics, and particle size (Zawadowicz et al, 2015;Hodas et al, 2015;Gupta et al, 2015). Whole particles, individual phases within particles, or specific chemical compounds can undergo a range of phase transitions including crystallization-efflorescence, dissolution-deliquescence, and liquid-liquid phase separation as the RH varies in the atmosphere.…”

“…For example, equilibrium between the variable environmental water vapor mixing ratio and aerosol particles may lead to substantial changes in particle size and chemical composition, all of which can influence light absorption and scattering (Seinfeld and Pandis, 2006;Zhang et al, 2016). Transitions between solid and liquid (aqueous) phases that are dependent on relative humidity (RH) are also important in determining optical properties (Martin et al, 2013;Wang et al, 2010;Kim et al, 2016;Wu et al, Denjean et al, 2015Hodas et al, 2015;Atkinson et al, 2015). Studies have shown that water-soluble organic matter from biomass burning (approximately 70 % of total organic matter) can significantly suppress, enhance, or have no effect on the deliquescence (e.g., the RH at which deliquescence occurs at a certain temperature, the DRH) and efflorescence processes (e.g., the efflorescence RH, ERH) of present inorganic electrolytes.…”

“…Whole particles, individual phases within particles, or specific chemical compounds can undergo a range of phase transitions including crystallization-efflorescence, dissolution-deliquescence, and liquid-liquid phase separation as the RH varies in the atmosphere. A number of laboratory studies have focused on liquid-liquid phase separations within particles consisting of inorganic and organic fractions (Svenningsson et al, 2006;Carrico et al, 2008;Dusek et al, 2011;Hodas et al, 2015). For example, studies about liquid-liquid separation occurring in mixed organic-inorganic aerosols were performed by Song et al (2012a, b) and You et al (2013) using Raman and optical microscopy, establishing that liquidliquid phase separation typically occurs in mixed organics + ammonium sulfate (AS) particles with an average elemental oxygen-to-carbon (O : C) ratio of the organic fraction of less than 0.6 and in some cases for 0.6 < O : C < 0.8.…”

Physicochemical Characteristics of Individual Aerosol Particles during the 2015 China Victory Day Parade in Beijing

“…For example, various "bulk" sample analyses conducted in the laboratory have confirmed the liquid-liquid phase separation do exist for the secondary particles [41,46,47]. The presence of the organic core and inorganic shell in individual particles have been confirmed in the laboratory by using optical microscopy and Raman spectroscopy [38,41,42], atomic force microscopy with infrared spectroscopy [48], and environmental scanning electron microscopy and scanning transmission X-ray microscopy [49].…”