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Hygroscopic behavior of organic compounds, includinglevoglucosan, 4-hydroxybenzoic acid, and humic acid, as well as their effects on thehygroscopic properties of ammonium sulfate (AS) in internally mixedparticles are studied by a hygroscopicity tandem differential mobilityanalyzer (HTDMA). The organic compounds used represent pyrolysis products ofwood that are emitted from biomass burning sources. It is found that humicacid aerosol particles only slightly take up water, starting at RH (relative humidity) above~70%. This is contrasted by the continuous water absorption oflevoglucosan aerosol particles in the range 5–90% RH. However, nohygroscopic growth is observed for 4-hydroxybenzoic acid aerosol particles.Predicted water uptake using the ideal solution theory, the AIOMFAC modeland the E-AIM (with UNIFAC) model are consistent with measured hygroscopicgrowth factors of levoglucosan. However, the use of these models withoutconsideration of crystalline organic phases is not appropriate to describethe hygroscopicity of organics that do not exhibit continuous water uptake,such as 4-hydroxybenzoic acid and humic acid. Mixed aerosol particlesconsisting of ammonium sulfate and levoglucosan, 4-hydroxybenzoic acid, orhumic acid with different organic mass fractions, take up a reduced amountof water above 80% RH (above AS deliquescence) relative to pure ammoniumsulfate aerosol particles of the same mass. Hygroscopic growth of mixturesof ammonium sulfate and levoglucosan with different organic mass fractionsagree well with the predictions of the thermodynamic models. Use of theZdanovskii–Stokes–Robinson (ZSR) relation and AIOMFAC model lead to goodagreement with measured growth factors of mixtures of ammonium sulfate with4-hydroxybenzoic acid assuming an insoluble organic phase. Deviations ofmodel predictions from the HTDMA measurement are mainly due to theoccurrence of a microscopical solid phase restructuring at increasedhumidity (morphology effects), which are not considered in the models.Hygroscopic growth factors of mixed particles containing humic acid are wellreproduced by the ZSR relation. Lastly, the organic surrogate compoundsrepresent a selection of some of the most abundant pyrolysis products ofbiomass burning. The hygroscopic growths of mixtures of the organicsurrogate compounds with ammonium sulfate with increasing organics massfraction representing ambient conditions from the wet to the dry seasonalperiod in the Amazon basin, exhibit significant water uptake prior to thedeliquescence of ammonium sulfate. The measured water absorptions ofmixtures of several organic surrogate compounds (including levoglucosan)with ammonium sulfate are close to those of binary mixtures of levoglucosanwith ammonium sulfate, indicating that levoglucosan constitutes a majorcontribution to the aerosol water uptake prior to (and beyond) thedeliquescence of ammonium sulfate. Hence, certain hygroscopic organicsurrogate compounds can substantially affect the deliquescence point ofammonium sulfate and overall particle water uptake

Hygroscopicity of organic compounds from biomass burning and their influence on the water uptake of mixed organic ammonium sulfate aerosols