Aerosol Composition, Mixing State, and Phase State of Free Tropospheric Particles and Their Role in Ice Cloud Formation

The prediction of ice cloud formation in the atmosphere remains challenging. Free tropospheric aerosols can act as ice nucleating particles, affecting cloud properties and precipitation. The physicochemical properties of free tropospheric particles are modified upon long-range transport by different atmospheric processes. These modifications affect the ice formation potential of individual particles. We investigated the physicochemical properties of free tropospheric particles collected at the remote Pico Mountain Observatory at 2225 m a.s.l. in the North Atlantic Ocean using multimodal micro-spectroscopy and chemical imaging techniques. We probed their ice nucleation (IN) activity using an IN stage interfaced with an environmental scanning electron microscope. Retroplume analysis, chemical imaging, and micro-spectroscopy analysis indicated that the size-resolved chemical composition, mixing state, and phase state of the particles with similar aging times but different transport patterns were substantially different. Relative humidity-dependent glass-transition temperatures estimated from meteorological conditions were consistent with the observed organic component of the particles' phase. More viscous (solid and semi-solid-like) particles are more ice active in the deposition mode at temperatures ranging from 205 to 220 K than less viscous particles. This study provides a better understanding of the phase and mixing state of long-range transported free tropospheric aerosols and their role in ice cloud formation.

Automated summary

This study investigated the physicochemical properties and ice nucleation activity of free tropospheric particles collected at a remote observatory in the North Atlantic Ocean. It was found that particles with different transport patterns had significant differences in size-resolved chemical composition, mixing state, and phase state. Particles with higher viscosity were found to be more ice active in the deposition mode at specific temperatures. This research provides a better understanding of the role of long-range transported free tropospheric aerosols in ice cloud formation.