1Department of Chemistry and iNANO, Aarhus University, Aarhus, Denmark
2Department of Physics, University of Helsinki, Helsinki, Finland
3Finnish Meteorological Institute, Helsinki, Finland
4Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
Received: 13 May 2015 – Published in Atmos. Chem. Phys. Discuss.: 25 Jun 2015
Abstract. Organosulfates have been observed as constituents of atmospheric aerosols in a wide range of environments; however their hygroscopic properties remain uncharacterised. Here, limonene-derived organosulfates with a molecular weight of 250 Da (L-OS 250) were synthesised and used for simultaneous measurements with a hygroscopicity tandem differential mobility analyser (H-TDMA) and a cloud condensation nuclei counter (CCNC) to determine the hygroscopicity parameter, κ, for pure L-OS 250 and mixtures of L-OS 250 with ammonium sulfate (AS) over a wide range of humidity conditions. The κ values derived from measurements with H-TDMA decreased with increasing particle dry diameter for all chemical compositions investigated, indicating that κH-TDMA depends on particle diameter and/or surface effects; however, it is not clear if this trend is statistically significant. For pure L-OS 250, κ was found to increase with increasing relative humidity, indicating dilution/solubility effects to be significant. Discrepancies in κ between the sub- and supersaturated measurements were observed for L-OS 250, whereas κ of AS and mixed L-OS 250/AS were similar. This discrepancy was primarily ascribed to limited dissolution of L-OS 250 at subsaturated conditions. In general, hygroscopic growth factor, critical particle diameter and κ for the mixed L-OS 250/AS particles converged towards the values of pure AS for mixtures with ≥ 20 % w / w AS. Surface tension measurements of bulk aqueous L-OS 250/AS solutions showed that L-OS 250 was indeed surface active, as expected from its molecular structure, decreasing the surface tension of solutions with 24 % from the pure water value at a L-OS 250 concentration of 0.0025 mol L−1. Based on these surface tension measurements, we present the first concentration-dependent parametrisation of surface tension for aqueous L-OS 250, which was implemented to different process-level models of L-OS 250 hygroscopicity and CCN activation. The values of κ obtained from the measurements were compared with κ calculated applying the volume additive Zdanovskii–Stokes–Robinson mixing rule, as well as κ modelled from equilibrium Köhler theory with different assumptions regarding L-OS 250 bulk-to-surface partitioning and aqueous droplet surface tension. This study is to our knowledge the first to investigate the hygroscopic properties and surface activity of L-OS 250; hence it is an important first step towards understanding the atmospheric impact of organosulfates.
Revised: 18 Nov 2015 – Accepted: 04 Dec 2015 – Published: 21 Dec 2015
Hansen, A. M. K., Hong, J., Raatikainen, T., Kristensen, K., Ylisirniö, A., Virtanen, A., Petäjä, T., Glasius, M., and Prisle, N. L.: Hygroscopic properties and cloud condensation nuclei activation of limonene-derived organosulfates and their mixtures with ammonium sulfate, Atmos. Chem. Phys., 15, 14071-14089, doi:10.5194/acp-15-14071-2015, 2015.