Atmospherically relevant water-soluble organic compounds (WSOC) such as levoglucosan, sucrose, and adipic acid may uptake water and form cloud droplets. WSOC can readily mix with atmospheric inorganic compounds such as ammonium sulfate. Understanding the effect of mixing compositions on water uptake and cloud forming properties will advance the understanding of their indirect radiative effect and contributions to Earth’s radiative forcing. This collaborative research highlights three analytical methods that can be used to measure the sub and supersaturated droplet growth of the aforementioned inorganic and organic mixtures: cavity ring-down spectroscopy (CRDS), hygroscopic tandem differential mobility analysis (H-TDMA), and cloud condensation nuclei counting (CCN). The optical growth factors (f_RH ), geometric growth factors (G_f ), and critical activation diameters (d_p50 ) are retrieved from each method, respectively. Kohler theory is used to convert f_RH, G_f, and d_p50 into the single hygroscopic parameter, κ. The three experimental methods have been shown to be comparable in precision for singular compounds, and in this research, we assess the measurement techniques for binary mixed aerosols. In this work, we show the complexity of internal and external mixtures of AS with organic compounds. The greatest differences in experimental κ value from ideal theory occur above 20% organic volume fraction. Furthermore, the ability to observe differences in binary external and internal mixtures are a function of the differences in hygroscopicity of the initial compounds. This work implies that mixtures of inorganic and organic compounds, can produce non-ideal solutions that modify overall droplet formation.

Miriam Freedman

Penn State

Appearances