Bioaerosols outcompete dust as dominant immersion-mode-INPs in central Europe and redefine INP parameterizations

Abstract. Knowledge gaps in the source and parameterization of ice-nucleating particles (INPs) remain a major uncertainty in quantifying the properties and climate impacts of mixed-phase clouds (MPCs). Bioaerosols are increasingly recognized as important INPs for MPCs, yet it is unclear whether their contribution is critical through modelling studies. We investigate this using field observations at a semi-rural site in the central Europe, combining INP and aerosol measurements, remote sensing, and air-mass source analysis. We synergically use the results of in-situ and offline measurements to identify INPs originated from different sources, including biological aerosols, dust, and biomass burning aerosols, and to quantify their abundance and relative contributions to total INPs. More than 85% of immersion-mode INPs (>−24°C) are heat-labile and significantly correlated with fluorescent biological aerosols particles and pollen, while heat-resistant INPs (<~10%) are likely mineral dust, while biomass burning is an insignificant source. The proposed bioaerosol-ware INP parameterization reproduces observations across multi-regional datasets, showing that neglecting bioaerosol-INPs results in an average ~50% (~32%) bias in predictions of immersion-mode INPs active warmer than −15°C (−24°C), with frequent deviations of up to an order of magnitude from observations.