| Abstract
| - Water-soluble macromolecular polyacids can play apotentially important role in the hygroscopic properties ofatmospheric aerosols. These acids have molecularstructures similar to natural fulvic acids (FA) (or humicacids) and are referred to as humic-like substances (HULIS).In this study, the hygroscopicity of HULIS and the mixtureof HULIS and sodium chloride (NaCl) and that of HULISand ammonium sulfate (AS) aerosols at a mass ratio of 1:1are studied using two natural FA: the Nordic AquaticFulvic Acid (NAFA) and the Suwannee River Fulvic Acid(SRFA) as model compounds in an electrodynamic balance.NAFA and SRFA both absorbed and desorbed waterreversibly without crystallization and retained water at arelative humidity (RH) < 10%. NAFA and SRFA have a massgrowth ratio of 1.25 and 1.45 from RH = 10% to RH =90%, respectively. However, these results are different fromthose of another natural FA (the Nordic River Fulvic AcidReference) in the literature. The differences are possibly dueto the differences in the chemical composition of thenatural FA, which depends on their sources and the isolationmethods. These results suggest that a standardization ofthe isolation methods of HULIS is needed for betterunderstanding of their atmospheric properties andenvironmental impacts. In general, the deliquescence andcrystallization RH of FA−inorganic mixtures are comparablewith those of their respective pure inorganic species. SinceFA are less hygroscopic than NaCl and AS, all mixturesabsorb less water compared to their respective pure inorganicspecies of equal particle mass. The FA−AS mixtureshave a larger water uptake than the sum of those of theFA and AS individually following a simple additivity rule asnoninteracting species at RH = 90%. This enhancementeffect increases as the RH decreases. There is no suchenhancement effect for the FA−NaCl mixtures until RHis below 90%. These results reveal that the effect of theinteractions between FA and inorganic species on the wateruptake of the mixtures, in general, is a function of RH.
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