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

Abstract

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.

Micro-spectroscopic and freezing characterization of ice-nucleating particles collected in the marine boundary layer in the Eastern North Atlantic

“…Both samples show the greatest physicochemical similarity among the four particle samples as discussed above. Also, the particle loading is similar ( IMF and DIN proceed at similar conditions as observed in our previous studies applying particles samples collected at the Pico Mountain Observatory (PMO) in the Azores Islands (China et al, 2017;Lata et al, 2021). Since the particle samples are dominated by OCIN particles and display organic coatings (Fig.…”

“…All but one out of 21 identified INPs, do not display different morphology or composition than the particle-type classes making up the sampled particle population. In other words, application of nanoscale analytical tools cannot resolve the difference between the major particle-type classes and the INPs, similar to findings of previous studies (Knopf et al, 2018;China et al, 2017;Knopf et al, 2014;Lata et al, 2021). This is difficult to reconcile in a singular hypothesis/deterministic concept of ice nucleation but may be resolved assuming CNT.…”

“…Particle size information provides an estimate of the total particle number and surface area present during an ice nucleation experiment. SEM/EDX was employed to characterize individual INPs spotted in optical microscopy (OM) ice nucleation experiments similar to our previous work (China et al, 2017;Knopf et al, 2014;Lata et al, 2021). Ice crystal formation by INPs was first recorded by OM at different magnifications as shown in Fig.…”

“…Understanding how atmospheric aerosol serves as INPs is necessary to advance our understanding of cloud microphysical processes that impact precipitation and climate (Boucher et al, 2013;Storelvmo, 2017;Baker and Peter, 2008;Mülmenstädt et al, 2015;Lohmann and Feichter, 2005). To improve our predictive capabilities of atmospheric ice formation, laboratory and field-based research addressing the efficacy and physicochemical nature of INPs has intensified over recent years (Cantrell and Heymsfield, 2005;Murray et al, 2012;Kanji et al, 2017;Cziczo et al, 2017;DeMott et al, 2011;Knopf et al, 2018;Knopf et al, 2021).…”

“…Discussion started: 28 January 2022 c Author(s) 2022. CC BY 4.0 License.the OC particle fractions is given by the Tg and its full deliquescence RH (FDRH)(Knopf et al, 2018;Charnawskas et al, 2017) Lata et al (2021). have shown the presence of highly viscous particles with Tg values greater than room temperature in samples collected at PMO.…”