The limited capacity of bioaerosols to serve as cloud-condensation nuclei may restrict their potential to initiate ice formation in mixed-phase clouds

Bioaerosols are gaining prominence due to their ability to nucleate ice in clouds at high subzero temperatures, thereby impacting cloud characteristics, and longevity. Acting first as cloud condensation nuclei (CCN) and second as ice nucleating particles, bioaerosols induce ice formation through immersion freezing. Nevertheless, insights into bioaerosol ability to act as CCN in situ are currently lacking. We simultaneously collected bioaerosols from the condensed and interstitial phase of clouds using size selective inlets during autumn 2020 from 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 spectral flow cytometers, the PINGUIN cold-stage setup and MiSeq-facilitated amplicon sequencing to assess bacterial and fungal communities, we found that both bioaerosols and biogenic ice-nucleating particles were significantly more abundant in the interstitial phase compared to the condensed phase, suggesting that they were poor CCN, unable to compete with more hygroscopic particles. Most taxa did not exhibit preferential partitioning, suggesting that surface properties generally did not significantly influence their behaviour as CCN. Our results underline the need to systematically investigate the relation between hygroscopicity and ice-nucleating activity of bioaerosols to understand their in-situ effects on cloud formation.