Articles | Volume 17, issue 9
Research article
03 May 2017
Research article |  | 03 May 2017

Influence of organic compound functionality on aerosol hygroscopicity: dicarboxylic acids, alkyl-substituents, sugars and amino acids

Aleksandra Marsh, Rachael E. H. Miles, Grazia Rovelli, Alexander G. Cowling, Lucy Nandy, Cari S. Dutcher, and Jonathan P. Reid

Abstract. Hygroscopicity data for 36 organic compounds, including amino acids, organic acids, alcohols and sugars, are determined using a comparative kinetics electrodynamic balance (CK-EDB). The CK-EDB applies an electric field to trap-charged aqueous droplets in a chamber with controlled temperature and relative humidity (RH). The dual micro dispenser set-up allows for sequential trapping of probe and sample droplets for accurate determination of droplet water activities from 0.45 to > 0.99. Here, we validate and benchmark the CK-EDB for the homologous series of straight-chain dicarboxylic acids (oxalic–pimelic) with measurements in better agreement with Universal Quasichemical Functional Group Activity Coefficients (UNIFAC) predictions than the original data used to parametrise UNIFAC. Furthermore, a series of increasingly complex organic compounds, with subtle changes to molecular structure and branching, are used to rigorously assess the accuracy of predictions by UNIFAC, which does not explicitly account for molecular structure. We show that the changes in hygroscopicity that result from increased branching and chain length are poorly represented by UNIFAC, with UNIFAC under-predicting hygroscopicity. Similarly, amino acid hygroscopicity is under-predicted by UNIFAC predictions, a consequence of the original data used in the parametrisation of the molecular subgroups. New hygroscopicity data are also reported for a selection of alcohols and sugars and they show variable levels of agreement with predictions.

Short summary
The response of aerosol particles to changes in relative humidity (RH) is dependent on the hygroscopicity of the constituent chemical components. We present accurate measurements of the hygroscopic response of aerosols over a wide range in RH, extending up to the saturation RH. The chemical compounds studied include dicarboxylic acids, amino acids, sugars and polyols. These data provide a comprehensive source of data to help improve functional group activity models of aerosol hygroscopicity.
Final-revised paper