Fungal liquid jets as a source of sub- and supermicron particles

Primary biological aerosols such as pollen, fungal spores, bacteria and plants debris have traditionally been associated with the coarse particle mode. In contrast, small organic particles in the submicron range have largely been attributed to secondary formation processes, as few primary biogenic sources were known [1-3]. Due to their hygroscopic properties, bioaerosols may act as cloud condensation nuclei (CCN) and ice nuclei (IN), potentially influencing cloud formation and precipitation [4]. In the Amazon rainforest, coarse particles are typically present at lower number concentrations, whereas fine organic particles are more abundant and thus are known to contribute significantly to cloud microphysics under certain conditions [1,5].

In this study, we investigate a previously overlooked primary biogenic source of organic aerosol droplets linked to spore release by many fungi and lichens, with measurements conducted at the Amazon Tall Tower Observatory (ATTO) site in Brazil [6]. Many lichenized and non-lichenized Ascomycota release spores actively, building pressure in their reproductive cells through osmolyte-driven water influx until the spores are suddenly expelled.

We combined controlled laboratory experiments with ambient field measurements to characterize particles emitted during this process. Particle size distributions were measured in isolated chamber experiments using two complementary particle sizers covering a broad size range, providing information on both particle size and emission strength. Field experiments gave insights into emission patterns and triggers under natural tropical forest conditions. Droplets were additionally collected by impaction for further microscopic and chemical analyses. The chemical composition was determined using scanning transmission X-ray microscopy with near-edge X-ray absorption and fine structure (STXM-NEXAFS) spectroscopy, as well as high performance liquid chromatography (HPLC) with electrospray ionisation ultra-high resolution orbitrap mass spectrometry (ESI-UHR-Orbitrap-MS).

This integrated approach allows us to assess the size, chemical composition, and emission strength of fungal aerosol emissions. The findings provide new insights into the contribution of sub- and supermicron fungal emissions to organic aerosol populations and their potential implications for atmospheric processes.

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[2] Barbosa, C. G. G., et al. (2022). Amazon rainforest aerosols: Characterization and implications for climate. npj Climate and Atmospheric Science, 5, 73. https://doi.org/10.1038/s41612-022-00294-y
[3] Graham, B., et al. (2003). Source attribution and seasonality of Amazon aerosol: Implications for cloud formation. Journal of Geophysical Research: Atmospheres, 108. https://doi.org/10.1029/2003JD004049
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[5] Moran-Zuloaga, D., et al. (2018). Long-term study on coarse mode aerosols in the Amazon rainforest with frequent intrusion of Saharan dust plumes. Atmospheric Chemistry and Physics, 18(13), 10055–10088. https://doi.org/10.5194/acp-18-10055-2018
[6] Andreae, M. O., et al. (2015). The Amazon Tall Tower Observatory (ATTO): Overview of pilot measurements on ecosystem ecology, meteorology, trace gases, and aerosols. Atmospheric Chemistry and Physics, 15(18), 10723–10776. https://doi.org/10.5194/acp-15-10723-2015