Water uptake by particles containing humic materials and mixtures of humic materials with ammonium sulfate

Recent field studies show that a large fraction of the previously uncategorized organic compounds in atmospheric aerosols are polycarboxylic acids resembling the humic materials (HMs) in soil. The presence of these compounds may alter the water uptake and deliquescence characteristics of particles. We have measured the water uptake by pure HM and by mixed HM/ammonium sulfate particles as a function of relative humidity (RH). We find that the water uptake behavior varies with the type, source, and isolation method of the HM. Two of the three pure humic acids (HAs) and the fulvic acid (FA) studied here exhibited very little water uptake up to 90% RH, while the fourth material, HA from the Fluka chemical company, deliquesced at 70% RH and showed continuous water uptake to 90% RH. For comparison, water uptake by polyacrylic acids of two chain lengths and by phthalic acid were also measured. These species also took up small amounts of water over the range of RHs studied. Particle size and chain length had very little effect on water uptake. The mixed organic/ammonium sulfate particles took up a reduced amount of water relative to pure ammonium sulfate, with the exception of the Fluka HA. We modeled the water uptake of mixed particles assuming that ammonium sulfate and HM took up water independently, and results are consistent with the measured water uptake by mixed particles.