On the abundance and sources of biological aerosol serving as ice nuclei in the high Arctic

Aerosols significantly influence Arctic cloud properties, affecting radiative balance and climate dynamics. Among them, ice-nucleating particles (INPs) are particularly impactful, as they nucleate ice at higher temperatures than homogeneous freezing, altering cloud radiative properties and lifetimes. Primary biological aerosol particles (PBAPs), a subset of aerosols of biological origin, have garnered attention due to their abundance and widespread presence. PBAPs may influence clouds more than previously recognized, particularly in the Arctic, where aerosol-cloud interactions are crucial for regional climate regulation. Understanding the sources, seasonality, and mechanisms of PBAP-induced cloud microphysics is critical, especially as Arctic environmental changes potentially amplify or mitigate these interactions.

Here, we present an overview of recent observational and experimental evidence linking PBAPs to INPs and their subsequent impact on cloud phase and radiative properties. Observations across diverse Arctic regions from Ny-Ålesund (Svalbard) to the central Arctic Ocean over the pack ice will be presented.  Through a synthesis of multi-year and expedition-based studies using a wide range of experimental and modelling approaches, we provide evidence linking fluorescent PBAPs to INPs, and their yearly dominance in the high-temperature regime. Our results show that PBAPs are closely associated with heat-labile high-temperature INPs, especially during the biologically active summer and early fall seasons. Concentrations of fluorescent PBAPs range from 10^-3 to 10^-1 L^-1, peaking in summer when biological activity and terrestrial vegetation are at their height (Freitas et al., 2023). PBAP were also for the first time directly observed in-situ within cloud residuals (dried cloud particles) using single-particle fluorescence spectroscopy and electron microscopy coupled to a ground-based counterflow virtual impactor inlet. Seasonal cloud observations linked their presence to a possible influence on the prevalence of mixed-phase clouds during warm months (Freitas et al., 2024).

While the sources of fluorescent PBAP over Svalbard are mainly suggested to be of regional and terrestrial origin, over the Arctic Ocean, marine sources emerge as significant contributors to fluorescent PBAP emissions, particularly during ice-free periods in biologically productive areas. At the North pole, air parcel trajectory analysis, combined with ocean productivity reanalysis, links episodic fluorescent PBAP and high temperature INP events to biologically active regions (Kojoj et al., 2024). 

Our findings underscore the critical role of PBAPs acting as INP and in determining the phase and radiative properties of low-level Arctic MPCs. This work has important implications for improving the representation of Arctic aerosol sources - especially of biological origin - and their interactions in climate models. As the Arctic undergoes profound transformations in its hydrological and biogeochemical cycles, it is essential to understand the sources and characteristics of PBAPs and their links to INPs in order to better predict future cloud and climate dynamics in this sensitive region.

References:

Freitas et al. Nature Communications 14.1 (2023): 5997. https://doi.org/10.1038/s41467-023-41696-7

Freitas et al. Atmospheric Chemistry and Physics 24.9 (2024): 5479-5494. https://doi.org/10.5194/acp-24-5479-2024

Kojoj et al. Tellus. Series B, 76.1 (2024): 47-70. https://doi.org/10.16993/tellusb.1880