Deliquescence and crystallization of ammonium sulfate particles internally mixed with water-soluble organic compounds

The deliquescence and crystallization of ammonium sulfate particles internally mixed with water-soluble organic material have been studied, restricted to an organic mass fraction of less than 0.6. The organic species used were malonic acid, glycerol, levoglucosan (1,6-anhydro-beta-D-glucopyranose), and Suwannee River fulvic acid. Our deliquescence results for systems with malonic acid and fulvic acid are in agreement with existing literature values. Glycerol deliquescence results are slightly lower than previous measurements. The levoglucosan results are the first of this kind. Total deliquescence relative humidities for the different systems are the same within the uncertainty of the measurements when the organic mole fraction is less than approximately 0.35. At an organic mole fraction of 0.6, the maximum deviation of total deliquescence relative humidities between the systems is approximately 10% relative humidity. We show that thermodynamic calculations based on a simplified version of a model recently proposed by Clegg et al. (J. Aerosol Sci. 2001, 32, 713)1 are in agreement with measured values of deliquescence relative humidity up to an organic mole fraction of approximately 0.4 for most of the systems studied. The crystallization relative humidity (CRH) of mixed systems of ammonium sulfate with malonic acid, glycerol, or levoglucosan decreases significantly from the CRH of pure ammonium sulfate when the organic mole fraction is greater than about 0.25. This is in contrast to our previous study with glutaric acid where CRH remained close to CRH of pure ammonium sulfate up to a glutaric acid mole fraction of 0.4. CRH values are shown to vary depending on the type of,organic present. In terms of atmospheric implications, we estimate that organics, on average, are only a minor perturbation on the deliquescence relative humidity of the pure inorganic particles, whereas the organics, on average, may decrease the CRH of pure inorganic particles significantly and this effect depends on the type of organic material.