Seasonal variability, sources, and parameterization of ice-nucleating particles in the Rocky Mountain region: importance of soil dust and biological contributions

Abstract. Atmospheric ice-nucleating particles (INPs) significantly influence cloud microphysics and aerosol-cloud interactions. Understanding INPs in mountain regions is important for predicting impacts on regional clouds and precipitation. In this study, we conducted comprehensive measurements of immersion-freezing INPs at Mt. Crested Butte in the Rocky Mountains from September 2021 to June 2023 as part of the Surface Atmosphere Integrated Field Laboratory (SAIL) campaign. The average number concentration of INPs active at −20 °C was 2 L−1, with distinct seasonal variation characterized by high summer concentrations and low winter concentrations. Aerosol sources were resolved, and INP concentrations were correlated with a coarse dust aerosol type, which dominates PM10 in this region. Calculating IN active surface site densities (ns) further supporting the primary contribution from coarse dust to INPs. Treatment with H2O2 indicated substantial contributions (91 % on average) from organic INPs across all activation temperatures, suggesting that supermicron organic-containing soil dust dominates the INPs in this region. Heat-labile INPs, likely biological in origin, were identified as dominant at >-15 °C through heat treatment of samples and showed significantly lower contributions in winter (∼ 96 % reduction). Parameterizations based on ns for the INPs observed in this mountainous region were developed, which effectively reproduced measured INP concentrations, particularly when accounting for seasonal differences. This study provides the first long-term, comprehensive characterization of INPs for the Upper Colorado River Basin region and offers a parameterization potentially useful for predicting INPs in other remote continental regions.