Hygroscopicity of aerosol particles composed of surfactant SDS and its internal mixture with ammonium sulfate at relative humidities up to 99.9%

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For atmospheric aerosol particles, the ubiquitous surfactants can significantly influence their hygroscopicity. However, the hygroscopicity of nanoscale particles containing surfactants are not well understood under subsaturated conditions, especially at high relative humidity (RH = 90–100%), due to the limitation of traditional instruments and the diversity of surfactants. Here, the hygroscopicity of nanoscale particles composed of a strong surfactant sodium dodecyl sulfate (SDS) and its mixture with ammonium sulfate (AS) was measured using the high humidity tandem differential mobility analyzer (HHTDMA) at RH up to about 99.9%. Three different models based on Köhler theory but different representations of surfactant characteristics were employed to reproduce the measurement outcomes. Results show that SDS particles start gradual hygroscopic growth after RH reaching 97.5%. For SDS-AS mixed particles with 25% w/w SDS, prompt deliquescence and gradual deliquescence are observed for smaller (20 and 40 nm) and larger particles (100 and 200 nm), respectively. For particles with ≥50% w/w SDS, the onset deliquescence is difficult to found. In general, the model that comprehensively accounts for the surfactant properties including bulk-to-surface partitioning, surface tension reduction, micellization and the effect of RH on hygroscopicity parameter, ĸ (for SDS-AS particles) provides the best predictions for the experimental hygroscopicity of SDS and SDS-AS particles at subsaturation conditions. Besides, factors affecting the accuracy of theoretical model are discussed in detail. The ĸ derived from subsaturated measurement is strongly dependent on RH and dry diameter.