Bioaerosols and their importance for low-level Arctic clouds

Abstract: Bioaerosols are microorganisms or functional parts of them or other biological matter suspended in air. Examples are bacteria, viruses, pollen, spores, or smaller plant debris. In the atmosphere, bioaerosols can play various functional roles, such as facilitating the spread of genetic material. Moreover, they can play an important role in climate by serving as ice nucleating particles and thus participating in cloud formation. Bioaerosols might play a significant role in a changing Arctic, where aerosol concentrations can be very low, and where natural as well as anthropogenic aerosol sources are subject to drastic changes due to climate change. In the Arctic, aerosols and clouds are prominent actors in climate by mediating short- and long-wave radiation interactions, which are further complicated by the presence of high-albedo surfaces such as sea ice. Thus, constraining the sources of aerosols and their interaction with clouds is key to understanding the Arctic climate and the changes it has been and will undergo.In this work, we used a single-particle instrument to differentiate bioaerosols from other particles on the basis of their fluorescence and light-scattering signal. In the Baltic Sea, we found that bioaerosols are at least 1 in every 104 coarse particles emitted by sea spray. Their temporal emission pattern was not directly correlated with biological tracers, such as chlorophyll; instead, their emission was modulated by the transition between different water masses.The same technique was then applied to a one-year measurement campaign at an Arctic mountain top observatory as part of a greater aerosol-cloud interaction campaign. The recorded seasonal cycle of bioaerosol concentrations peaked in summer and was most likely related to regional terrestrial sources, as its appearance coincided with a decrease in snow cover and an increase in vegetation activity. Moreover, bioaerosols were found to drive the concentration of high-temperature ice nucleating particles, even in winter. In the third study, the importance of bioaerosols serving as cloud seeds was investigated by directly measuring the concentration of bioaerosols within cloud residuals.The presented findings help to elucidate the contribution of bioaerosols to coarse-mode particles for marine and Arctic environments, while also providing a direct link between bioaerosols and clouds. Furthermore, we also provide the first direct observations of bioaerosols involved in cloud formation in the Arctic, along with their possible contribution to the prevalence of mixed-phase clouds in the beginning and end of summer. Thus, these results contribute to a better understanding of atmospheric (bio-)aerosol-cloud-interactions processes in the vulnerable Arctic environment but are also valuable for further developments of Earth system models that include ice nucleating and/or bioaerosol particles.