Determining the Abundance, Variability, Sources and Predictability of IceNucleating Particles in the Arctic

Abstract

The frequency and distribution of the ice phase in mixed-phase clouds (MPCs) influence the Earth’s energy budget by determining the clouds’ radiative properties and lifetime. The presence of ice-nucleating particles (INPs), a unique fraction of atmospheric aerosols, is vital to trigger the primary formation of ice crystals in MPCs by lowering the energy barrier during the phase transition. Despite the extraordinary scarcity of atmospheric INPs, their role in cloud-phase feedback to climate is indispensable, particularly in the Arctic, where the climate is experiencing accelerated warming (i.e., Arctic amplification). However, unconstrained representations of complex interactions between clouds and aerosols in climate models induce substantial uncertainties in the cloud-phase feedback. These uncertainties are largely hindered by insufficient knowledge on the abundance, variability, properties, sources and predictability of atmospheric INPs. The principal objective of this dissertation is to narrow this knowledge gap by expanding the observational data coverage of Arctic INPs and associated aerosol properties based on extensive in situ measurements. For this purpose, three field campaigns were conducted at different Arctic sites and during different seasons. During the Ny-Ålesund AeroSol Cloud ExperimeNT (NASCENT) campaign in Svalbard, Norway, in autumn 2019 and spring 2020, INP concentrations and aerosol properties were determined by a set of ground-based measurements. A distinguishable seasonal difference in INP concentrations was not observed during the measurement period. A robust relationship between INP concentrations and the associated physical properties of aerosols could also not be established. In addition, any season-dependent variable (e.g., aerosol burden, fluorescent particle abundance or ambient temperature) would induce a seasonal bias in the prediction of INP concentrations , which is contradictory to our observations of no seasonal difference. We developed a simplistic approach for predicting INP concentrations based on an atmospheric random dilution model, in which a log-normal frequency distribution of INP concentrations is hypothesized. The novel INP parameterization exhibits consistent predictability of atmospheric INP concentrations in Arctic transition seasons (autumn and spring) as a sole function of nucleation temperature without retrieving additional aerosol property parameters. In addition to developing an INP parameterization, we conducted a comprehensive analysis of ambient aerosol properties within selected cases to deduce the nature and origin of INPs. The heat-lability of INPs exhibited in most samples implies the presence of proteinaceous INPs. Significant correlations were found between autumnal INP concentrations at higher ice nucleation temperatures (e.g., T > -15 °C) and fluorescent particle concentration, high wind speeds over the ocean and concentration of methanesulfonic acid (MSA, an oxidation product of dimethylsulfide (DMS), a precursor gas from marine biological productivity). Therefore, marine biogenic aerosols likely serve as important sources of INPs at the Arctic coastal site during the ice-melting seasons. Moreover, for case studies, while high-latitude dust sources from long-range transport (e.g., coastal Greenland) could contribute to INP abundance, diversity in relative aerosol compositions may have a negligible impact. We also investigated the summertime INPs from the ship-based Arctic Century Expedition in 2021 over the previously unexplored Eurasian Arctic Ocean. Immersion-mode INP concentrations varied geographically, with high INP concentrations frequently observed over the ice-free ocean and over the marginal ice zones, in particular in the vicinity of land. INP concentrations were relatively low when the ship was located within the ice pack. Results from heat tests, chemical composition analyses and case studies suggest that marine biogenic INPs sporadically contribute to the Arctic INP populations at warm temperatures (T > -15 °C). INP concentrations-resolved backward trajectories demonstrate that peak INP concentrations occurred in locations close to northern Novaya Zemlya, with air masses originating from the western Siberian coast. Moreover, in situ measurements of aerosolized INPs via a sea spray aerosol (SSA) bubble tank generator revealed a high correlation with ambient INP concentrations but no correlation with INP abundance in the subsurface seawater. Our results suggest the potential importance of the aerosolization mechanism for partitioning INPs between the ocean and the atmosphere. In summary, the present dissertation facilitates a fundamental understanding of primary ice formation relevant for Arctic MPCs by quantifying the abundance, variability and predictability of atmospheric INPs and elucidating the nature and origin of responsible aerosols. Further constraining the role of INPs, particularly their dynamic variations in the warming Arctic, is highly encouraged to reduce the model uncertainties in analyzing the cloud-phase feedback to Arctic amplification.