Water uptake of internally mixed particles containing ammonium sulfate and dicarboxylic acids

There is increasing evidence that organic compounds comprise a significant fraction of tropospheric particles at all altitudes. The presence of these compounds can affect the particles’ ability to take up water and to form ice in the atmosphere. In this paper we present studies that investigate the hygroscopic behavior of internal mixtures of ammonium sulfate and low molecular weight dicarboxylic acids. We find that the ammonium sulfate dominates water uptake behavior for mixtures that contain 100:1 and 10:1 mass ratios of ammonium sulfate:dicarboxylic acid. However, for 1:1 mixtures, the dicarboxylic acids play an important role in determining water uptake characteristics. Observed water contents can be predicted within experimental uncertainties by assuming that each component contributes independently to the total particle water content, in accord with previous measurements for the pure components.