Abstract
Abstract. Hygroscopic growth factors of organic surrogate compounds representing biomass burning and mixed organic–inorganic aerosol particles exhibit variability during dehydration experiments depending on their chemical composition, which we observed using a hygroscopicity tandem differential mobility analyzer (HTDMA). We observed that levoglucosan and humic acid aerosol particles release water upon dehumidification in the range from 90 to 5 % relative humidity (RH). However, 4-Hydroxybenzoic acid aerosol particles remain in the solid state upon dehumidification and exhibit a small shrinking in size at higher RH compared to the dry size. For example, the measured growth factor of 4-hyroxybenzoic acid aerosol particles is ∼ 0.96 at 90 % RH. The measurements were accompanied by RH-dependent thermodynamic equilibrium calculations using the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model and Extended Aerosol Inorganics Model (E-AIM), the Zdanovskii–Stokes–Robinson (ZSR) relation, and a fitted hygroscopicity expression. We observed several effects of organic components on the hygroscopicity behavior of mixtures containing ammonium sulfate (AS) in relation to the different mass fractions of organic compounds: (1) a shift of efflorescence relative humidity (ERH) of ammonium sulfate to higher RH due to the presence of 25 wt % levoglucosan in the mixture. (2) There is a distinct efflorescence transition at 25 % RH for mixtures consisting of 25 wt % of 4-hydroxybenzoic acid compared to the ERH at 35 % for organic-free AS particles. (3) There is indication for a liquid-to-solid phase transition of 4-hydroxybenzoic acid in the mixed particles during dehydration. (4) A humic acid component shows no significant effect on the efflorescence of AS in mixed aerosol particles. In addition, consideration of a composition-dependent degree of dissolution of crystallization AS (solid–liquid equilibrium) in the AIOMFAC and E-AIM models leads to a relatively good agreement between models and observed growth factors, as well as ERH of AS in the mixed system. The use of the ZSR relation leads to good agreement with measured diameter growth factors of aerosol particles containing humic acid and ammonium sulfate. Lastly, two distinct mixtures of organic surrogate compounds, including levoglucosan, 4-hydroxybenzoic acid, and humic acid, were used to represent the average water-soluble organic carbon (WSOC) fractions observed during the wet and dry seasons in the central Amazon Basin. A comparison of the organic fraction's hygroscopicity parameter for the simple mixtures, e.g., κ ≈ 0.12 to 0.15 for the wet-season mixture in the 90 to 40 % RH range, shows good agreement with field data for the wet season in the Amazon Basin (WSOC κ ≈ 0.14±0.06 at 90 % RH). This suggests that laboratory-generated mixtures containing organic surrogate compounds and ammonium sulfate can be used to mimic, in a simplified manner, the chemical composition of ambient aerosols from the Amazon Basin for the purpose of RH-dependent hygroscopicity studies.
Highlights
IntroductionIt is well established that biomass burning, as an important source of atmospheric aerosol particles, has a wide range of climate effects that can be classified into direct radiative effects through light-absorbing carbon aerosol particles and indirect effects by impact on cloud condensation nuclei (CCN) and cloud microphysics (Andreae and Gelencsér, 2006; Moosmüller et al, 2009; Hecobian et al, 2010; Rizzo et al, 2011; Rose et al, 2011; Cheng et al, 2012; Engelhart et al, 2012; Lack et al, 2012; Jacobson, 2014; Liu et al, 2014; Saleh et al, 2013, 2014)
The model-predicted GF and solid–liquid phase transition of ammonium sulfate (AS) are in relatively good agreement with the experimental data and, in particular, the efflorescence of AS is captured by the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) and Extended Aerosol Inorganics Model (E-AIM) models
A number of field-based hygroscopicity studies about biomass burning aerosol focus on the growth factors of mixtures at high relative humidity (RH) (e.g., 90 % RH)
Summary
It is well established that biomass burning, as an important source of atmospheric aerosol particles, has a wide range of climate effects that can be classified into direct radiative effects through light-absorbing carbon aerosol particles and indirect effects by impact on cloud condensation nuclei (CCN) and cloud microphysics (Andreae and Gelencsér, 2006; Moosmüller et al, 2009; Hecobian et al, 2010; Rizzo et al, 2011; Rose et al, 2011; Cheng et al, 2012; Engelhart et al, 2012; Lack et al, 2012; Jacobson, 2014; Liu et al, 2014; Saleh et al, 2013, 2014). 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 liquid– liquid 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. Many independent studies suggest that the occurrence of solid–liquid and/or liquid– liquid phase separations, as well as related (temperaturedependent) RH levels of phase transitions (DRH, ERH, and onset of RH of liquid–liquid phase separation, SRH), depend on the relative amounts of organic and inorganic aerosol components and their nonideal mixing behavior
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