The Puy de Dôme ICe Nucleation Intercomparison Campaign (PICNIC): Comparison between online and offline methods in ambient air

Only a tiny fraction of all aerosol particles nucleate ice (ice nucleating particles; INPs) and their concentration over the relevant temperature range for mixed-phase clouds covers up to ten orders of magnitude, providing a challenge for contemporary INP measurement techniques. INP concentrations can be detected online with high-time resolutions of minutes, or offline, where aerosols are collected on filters for hours to days. Here we present measurements of INP concentrations in ambient air under conditions relevant to mixed-phase clouds from a total of ten INP methods over two weeks in October 2018 at the Puy de Dôme observatory in central France. INP concentrations were detected online in the immersion freezing mode, between ~ -5 °C and -30 °C. Two continuous flow diffusion chambers (CFDC; Colorado State University-Continuous Flow Diffusion Chamber, CSU-CFDC; Spectrometer for Ice Nuclei, SPIN) and an expansion chamber (Portable Ice Nucleation Experiment, PINE) measured the INP concentration with a time resolution of several minutes and at temperatures below -20 °C. Seven offline freezing techniques determined the temperature-dependent INP concentration above ~ -30 °C using water suspensions of filter-collected particles sampled over 8 hours (FRankfurt Ice Nuclei Deposition FreezinG Experiment, FRIDGE; Ice Nucleation Droplet Array INDA; Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology, INSEKT; 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). A special focus in this intercomparison campaign was placed on having overlapping sampling periods for the methods: INP concentrations measured with the online instruments were compared within 10 minutes and at the same temperature (±1 °C), while the filter collections for offline methods were started and stopped simultaneously and aerosol freezing spectra were compared at 1 °C steps. The majority of INP concentrations measured with PINE agreed well with the CSU-CFDC within a factor of two and five (71 % and 100 % of the data, respectively). There was a consistent observation of lower INP concentration with SPIN, and only 35 % of the data are within a factor of two from the CSU-CFDC, but 80 % of the data are still within a factor of five. This might have been caused by an incomplete exposure of all aerosol particles to water-supersaturated conditions within the instrument – a feature inherent to CFDC-style instruments – demonstrating the need to account for aerosol lamina spreading when interpreting INP concentration data from online instrument’s data. The comparison of the offline methods, which deposited aerosol particles on filters in the laboratory via a whole air inlet, revealed that more than 45 % of the data fall within a factor of two from the results obtained with INSEKT. Measurements using different filter materials and filter holders revealed no difference in the temperature-dependent INP concentration at overlapping temperatures. However, consistently higher INP concentrations were observed from aerosol filters collected on the rooftop at the Puy de Dôme station without the use of an inlet, compared to measurements performed behind the whole air inlet system.