Article

Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol – Part 2: Theoretical approaches

Leibniz-Institute for Tropospheric Research, Leipzig, Germany
ATMOSPHERIC CHEMISTRY AND PHYSICS (Impact Factor: 5.05). 01/2008; 9(12). DOI: 10.5194/acp-9-3999-2009
Source: DOAJ

ABSTRACT

We examine the hygroscopic properties of secondary organic aerosol particles generated through the reaction of alpha-pinene and ozone using a continuous flow reaction chamber. The water activity versus composition relationship is calculated from measurements of growth factors at relative humidities up to 99.6% and from measurements of cloud condensation nuclei activity. The observed relationships are complex, suggesting highly non-ideal behavior for aerosol water contents at relative humidities less than 98%. We present two models that may explain the observed water activity-composition relationship equally well. The first model assumes that the aerosol is a pseudo binary mixture of infinitely water soluble compounds and sparingly soluble compounds that gradually enter the solution as dilution increases. The second model is used to compute the Gibbs free energy of the aerosol-water mixture and shows that the aerosol behaves similarly to what can be expected for single compounds that contain a certain fraction of oxygenated and non-polar functional groups.

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    • "The normalized Gibbs free energy of the mixture, defined as the actual Gibbs free energy divided by the thermal energy, is needed to compute the number of thermodynamically stable phases in the system. For a binary system consisting of water (w) and a single solute (s), Gibbs energy is calculated from the activity coefficients via standard thermodynamic relationships (Prausnitz et al., 1999;Petters et al., 2009a) "
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    ABSTRACT: A wealth of recent laboratory and field experiments demonstrate that organic aerosol composition evolves with time in the atmosphere, leading to changes in the influence of the organic fraction to cloud condensation nuclei (CCN) spectra. There is a need for tools that can realistically represent the evolution of CCN activity to better predict indirect effects of organic aerosol on clouds and climate. This work describes a model to predict the CCN activity of organic compounds from functional group composition. The model combines Köhler theory with semi-empirical group contribution methods to estimate molar volumes, activity coefficients and liquid-liquid phase boundaries to predict the effective hygroscopicity parameter, kappa. Model evaluation against a selected database of published laboratory measurements demonstrates that kappa can be predicted within a factor of two. Simulation of homologous series is used to identify the relative effectiveness of different functional groups in increasing the CCN activity of weakly functionalized organic compounds. Hydroxyl, carboxyl, aldehyde, hydroperoxide, carbonyl, and ether moieties promote CCN activity while methylene and nitrate moieties inhibit CCN activity. The model can be incorporated into scale-bridging testbeds such as the Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere to evaluate the evolution of kappa for a complex mix of organic compounds and to develop suitable parameterizations of CCN evolution for larger scale models.
    Full-text · Article · Sep 2015 · Geoscientific Model Development Discussions
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    • ") . Previous work [ Petters et al . , 2009 ] suggests that the observed κ HGF < κ CCN discrepancy is caused either by a strongly composition - dependent activity coefficient of water or by a mixture of infinitely water - soluble compounds and sparingly soluble compounds that dissolve in solution at RH > 90% . Our present work suggests that κ HGF < κ CCN because of adsorption - d"
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    ABSTRACT: Aerosol climate effects are intimately tied to interactions with water. Here we combine hygroscopicity measurements with direct observations about the phase of secondary organic aerosol (SOA) particles to show that water uptake by slightly oxygenated SOA is an adsorption-dominated process under subsaturated conditions, where low solubility inhibits water uptake until the humidity is high enough for dissolution to occur. This reconciles reported discrepancies in previous hygroscopicity closure studies. We demonstrate that the difference in SOA hygroscopic behavior in sub- and supersaturated conditions can lead to an effect up to about 30% in the direct aerosol forcing – highlighting the need to implement correct descriptions of these processes in atmospheric models. Obtaining closure across the water saturation point is therefore a critical issue for accurate climate modeling.
    Full-text · Article · Mar 2015
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    • "Several previous studies have compared the κ values derived from the methods employing hygroscopicity tandem differential mobility analyzer (h-TDMA) and cloud condensation nuclei (CCN) [Juranyi et al., 2009; Petters et al., 2009; Wex et al., 2009; Good et al., 2010; Henning et al., 2010; Massoli et al., 2010; Snider et al., 2010; Stratmann et al., 2010]. It has been suggested that the HGF derived κ values are lower than those derived from the CCN method, possibly attributable to gradual dissolution of aerosol components when the aerosol solution becomes more diluted at high RH [Petters et al., 2009; Wex et al., 2009; Good et al., 2010]. In addition, the relationship between κ values (hygroscopicity) and the oxidation state of organic aerosols from the aerosol mass spectrometer data has been evaluated, showing that κ (hygroscopicity) increases with the oxidation level of organic aerosols [Massoli et al., 2010; Sorooshian et al., 2010; Duplissy et al., 2011]. "
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    ABSTRACT: The hygroscopic growth factor (HGF) and cloud condensation nuclei (CCN) activity are measured for surrogates that mimic atmospherically relevant oligomers, including glyoxal trimer dihydrate, methyl glyoxal trimer dihydrate, sucrose, methyl glyoxal mixtures with sulfuric acid and glycolic acid, and 2,4-hexandienal mixtures with sulfuric acid and glycolic acid. For the single-component aerosols, the measured HGF ranges from 1.3 to 1.4 at a relative humidity of 90%, and the hygroscopicity parameter (κ) is in the range of 0.06 to 0.19 on the basis of the measured CCN activity and 0.13 to 0.22 on the basis of the measured HGF, compared to the calculated values of 0.08 to 0.16. Large differences exist in the κ values derived using the measured HGF and CCN data for the multi-component aerosols. Our results reveal that, in contrast to the oxidation process, oligomerization decreases particle hygroscopicity and CCN activity and provide guidance for analyzing the organic species in ambient aerosols.
    Full-text · Article · Sep 2014
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