Effect of Particle Morphology on Cloud Condensation Nuclei Activity

Cloud condensation nuclei (CCN) activation is sensitive to the size, composition, and morphology of aerosol particles of <200 nm. By controlling the particle morphology of internally mixed samples (i.e., homogeneous versus phase separated), we have probed the effect of morphology on CCN activity using model organic aerosol systems, where ammonium sulfate was mixed with either pimelic acid or succinic acid in a 50:50 mixture by weight. Surprisingly, for systems of the same composition but distinct morphology, we observe a noticeable impact on CCN activity. Specifically, a phase-separated morphology results in activation diameters close to that of ammonium sulfate, while a homogeneous morphology yields an activation diameter in between the pure inorganic and organic components. Our results suggest that morphology-resolved CCN data may be an important parameter to consider in cloud microphysics models to improve predictions of CCN activity of complex organic aerosols. For laboratory CCN studies, it is important to control or account for atomized solution drying rates, which have been shown to affect morphology and ultimately CCN activity.