Seasonal variability, sources, and parameterization of ice-nucleating particles in the Rocky Mountain region

Abstract. Atmospheric ice-nucleating particles (INPs) significantly influence cloud microphysics and aerosol-cloud interactions. Given that mountainous regions are vital to water resources, understanding of INPs in these areas 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 ℃ was 2 L⁻¹, with distinct seasonal variation characterized by high summer concentrations and low winter concentrations. INP concentrations were correlated with a coarse dust aerosol type, which dominates PM₁₀ in this region. Converting INP concentrations to IN active surface site densities (n_s) led to reduction in variability, further supporting a relationship between coarse dust and INPs. Reduction of INP concentrations following treatment with H₂O₂ indicated substantial contributions from organic INPs across all activation temperatures, suggesting that organic-containing soil dust dominates the INPs in this region. Heat-labile INPs, likely biological in origin, were identified as dominant at > −15 ℃ through heat treatment of samples and showed significantly lower contributions in winter. Parameterizations based on n_s for the INPs observed in this mountainous region were developed, which effectively reproduced measured INPs 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.