The Puy de Dôme Ice Nucleation Intercomparison Campaign

The formation and impact of ice crystals in clouds remain a challenge to understand and thus to be predicted in models. In recent years, measurements of ice-nucleating particles (INPs) in ambient air have become more frequent, using well-established and novel techniques such as mobile cloud chambers and filter-based freezing assays. To assure that these techniques are working as intended, validation and intercomparison measurements are required. This is especially relevant due to ongoing efforts for the establishment of INP monitoring networks at the European level (ACTRIS; Aerosol, Clouds and Trace Gases Research Infrastructure) and in the United States (ARM; Atmospheric Radiation Measurement).Here we present results from PICNIC (The Puy de Dôme ICe Nucleation Intercomparison Campaign), conducted at a mountain site in Central France (1465 m a.s.l.) in October 2018. INP concentrations relevant in the mixed-phase cloud regime were determined using three online INP techniques (Colorado State University-Continuous Flow Diffusion Chamber, CSU-CFDC; Spectrometer for Ice Nuclei, SPIN; Portable Ice Nucleation Experiments, PINE) and seven filter-based offline freezing devices (FRankfurt Ice Nuclei Deposition FreezinG Experiment, FRIDGE; Ice Nucleation Droplet Array INDA; Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology, INSEKT; CSU Ice Spectrometer, IS; Leipzig Ice Nucleation Array, LINA; LED based Ice Nucleation Detection Apparatus LINDA; Micro-Orifice Uniform Deposit Impactor–Droplet Freezing Technique, MOUDI-DFT). The campaign focused on INP concentration measurements performed at the same time and at comparable nucleation temperatures, which is why filter sampling for offline techniques was started and stopped simultaneously, and online INP measurements were conducted at similar thermodynamic conditions. While the ice chambers yielded reasonable agreements within factors of 2 to 5, with lower concentrations found by SPIN, a systematic deviation between filter samples collected directly outside on the station’s rooftop and those sampled downstream from a whole air inlet is observed. A potential loss of larger aerosol particles via the inlet and an impact of the disaggregation of larger aerosol particles in solution might cause these differences, which needs to be investigated in future studies.