Limited Capacity of Bioaerosols to Serve as Cloud-Condensation Nuclei May Restrict Their Potential to Initiate Immersion Freezing in Mixed-Phase Clouds.

Bioaerosols are gaining prominence because of their ability to nucleate ice in clouds at high subzero temperatures, thereby impacting cloud characteristics and longevity. A major pathway, through which cloud ice is formed, is immersion freezing, where bioaerosols first act as cloud-condensation nuclei (CCN) and second as ice-nucleating particles. Nevertheless, insights into the ability of bioaerosols to act as CCN in situ are currently lacking. We simultaneously collected bioaerosols from the condensed and interstitial phases of clouds using size-selective inlets during autumn 2020 at the Otlica observatory (Slovenia). With a polarization Raman lidar, we confirmed the coexistence of ice particles and liquid droplets during one of the three sampling campaigns. Using flow cytometers, droplet-freezing assays, and amplicon sequencing to assess bacterial and fungal communities, we found that both bioaerosols and biological ice-nucleating particles were significantly more abundant in the interstitial phase compared to the condensed phase, suggesting that they did not act as CCN, unable to compete with more hygroscopic particles. Most taxa did not exhibit preferential partitioning, suggesting that specific surface properties associated with different taxa did not seem to enhance their behavior as CCN. Our results underline the need to systematically investigate the relationship between hygroscopicity and ice-nucleation activity of bioaerosols to understand their in situ effects on cloud formation.