Challenges in measuring sticky biogenic ice-nucleating macromolecules

Abstract. Ice-nucleating particles (INPs) are aerosol particles that influence mixed-phase clouds and in doing so impact weather and climate worldwide. To improve our understanding of ice production in mixed-phase clouds, we need techniques capable of accurately measuring atmospheric INP concentration spectra. However, there are sometimes discrepancies between the techniques commonly used to measure ambient INP concentrations, particularly when used in environments with abundant biogenic ice-nucleating material. Proteins and macromolecules adsorb to surfaces, such as filters, but the impact of this interaction on INP measurements is unknown. Here, we compare a widely used technique that involves washing collected aerosol particles off polycarbonate filters into aqueous suspension for subsequent INP droplet freezing assay (wash-off), with a technique where droplets are placed directly atop PTFE filters on a cold stage (drop-on) and also an online technique using an expansion chamber. Our results show that the wash-off technique underestimates the INP activity when free ice-nucleating proteins are present due to the poor recovery of the proteins from the filter into the wash-off suspension. However, there is much better agreement between techniques for INPs associated with coarse-mode mineral dust particles or cell fragments and for polysaccharide INPs from pollen. These findings indicate that some field-based INP measurements that use a wash-off technique may produce atmospheric INP concentrations that are biased low, particularly in regions with abundant proteinaceous INPs.