Abstract
Abstract. Hygroscopic behavior of organic compounds, including levoglucosan, 4-hydroxybenzoic acid, and humic acid, as well as their effects on the hygroscopic properties of ammonium sulfate (AS) in internally mixed particles are studied by a hygroscopicity tandem differential mobility analyzer (HTDMA). The organic compounds used represent pyrolysis products of wood that are emitted from biomass burning sources. It is found that humic acid aerosol particles only slightly take up water, starting at RH (relative humidity) above ~70%. This is contrasted by the continuous water absorption of levoglucosan aerosol particles in the range 5–90% RH. However, no hygroscopic growth is observed for 4-hydroxybenzoic acid aerosol particles. Predicted water uptake using the ideal solution theory, the AIOMFAC model and the E-AIM (with UNIFAC) model are consistent with measured hygroscopic growth factors of levoglucosan. However, the use of these models without consideration of crystalline organic phases is not appropriate to describe the hygroscopicity of organics that do not exhibit continuous water uptake, such as 4-hydroxybenzoic acid and humic acid. Mixed aerosol particles consisting of ammonium sulfate and levoglucosan, 4-hydroxybenzoic acid, or humic acid with different organic mass fractions, take up a reduced amount of water above 80% RH (above AS deliquescence) relative to pure ammonium sulfate aerosol particles of the same mass. Hygroscopic growth of mixtures of ammonium sulfate and levoglucosan with different organic mass fractions agree well with the predictions of the thermodynamic models. Use of the Zdanovskii–Stokes–Robinson (ZSR) relation and AIOMFAC model lead to good agreement with measured growth factors of mixtures of ammonium sulfate with 4-hydroxybenzoic acid assuming an insoluble organic phase. Deviations of model predictions from the HTDMA measurement are mainly due to the occurrence of a microscopical solid phase restructuring at increased humidity (morphology effects), which are not considered in the models. Hygroscopic growth factors of mixed particles containing humic acid are well reproduced by the ZSR relation. Lastly, the organic surrogate compounds represent a selection of some of the most abundant pyrolysis products of biomass burning. The hygroscopic growths of mixtures of the organic surrogate compounds with ammonium sulfate with increasing organics mass fraction representing ambient conditions from the wet to the dry seasonal period in the Amazon basin, exhibit significant water uptake prior to the deliquescence of ammonium sulfate. The measured water absorptions of mixtures of several organic surrogate compounds (including levoglucosan) with ammonium sulfate are close to those of binary mixtures of levoglucosan with ammonium sulfate, indicating that levoglucosan constitutes a major contribution to the aerosol water uptake prior to (and beyond) the deliquescence of ammonium sulfate. Hence, certain hygroscopic organic surrogate compounds can substantially affect the deliquescence point of ammonium sulfate and overall particle water uptake.
Highlights
The hygroscopicity of aerosol particles plays a critical role in visibility degradation, cloud formation, radiative forcing and thereby regional and global climate (Sloane and Wolff, 1985; Charlson et al, 1992; Pandis et al, 1995; Heintzenberg and Charlson, 2009; Shi et al, 2012)
It has been reported that water-soluble organic compounds from biomass burning, i.e., mono- and carboxylic acids, levoglucosan, and humic acid were quite abundant in the find mode (Dp < 1.1 μm) (Robert et al, 2003; Rissler et al, 2006; Vestin et al, 2007; Agarwal et al, 2010; Claey et al, 2010)
According to field studies reported in the literature, aerosol particles from biomass burning events always contain a variety of inorganic and organic compounds
Summary
The hygroscopicity of aerosol particles plays a critical role in visibility degradation, cloud formation, radiative forcing and thereby regional and global climate (Sloane and Wolff, 1985; Charlson et al, 1992; Pandis et al, 1995; Heintzenberg and Charlson, 2009; Shi et al, 2012). Some organic components that show a small solubility in pure water (i.e., these require a large volume of water to be extracted and labeled as a WSOC), may have a DRH close to 100 % RH, which is not accessible in our hydration experiments (RH probed up to ∼ 90 % RH) All these factors are of great importance in determining the CCN activity of biomass burning particles. Water uptake of surrogate mixtures containing a representative water-soluble organic fraction and inorganic compounds were studied by Svenningsson et al (2006) They used the Zdanovski–Stokes–Robinson (ZSR) relation to compare and successfully explain the observed hygroscopic growth factors for three out of four mixtures. We use the Zdanovskii–Stokes–Robinson (ZSR) relation (Stokes and Robinson, 1966), the Extended Aerosol Inorganic Model (E-AIM) (Clegg et al, 2001; Clegg and Seinfeld, 2006; available online: http://www.aim.env.uea.ac.uk/ aim/aim.php), and the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model (Zuend et al, 2008, 2011) to predict the hygroscopic growth of mixed aerosol particles and provide comparisons to our experimental findings
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