The Puy de Dôme Ice Nucleation Intercomparison Campaign (PICNIC): Airmass Impact on the Comparison between Online and Offline Freezing Techniques

Aerosol particles which trigger the first formation of ice crystals in clouds (ice nucleating particles; INP) are a key parameter for mixed-phase cloud properties. However, the understanding of the ambient INP concentration remains challenging due to their naturally very low concentration (DeMott et al., 2010; Kanji et al., 2017) and limitations in measurement techniques (DeMott et al., 2011; Cziczo et al., 2017). In recent years, several detection methods for INPs have been established. Offline methods using filter collections and subsequent freezing profit from a high sensitivity to detect low INP numbers, but have a low time resolution of ~hours. The latter limits the interpretation in terms of INP sources. Online INP instruments are able to measure INPs with higher time resolutions of ~minutes, but are typically blind to concentrations below ~0.1 L^-1. Thus, it is beneficial to combine offline and online methods. In order to create consistency between different measurement devices, they need to be validated and compared. Next to laboratory studies, using well characterized aerosol samples (Burkert-Kohn et al., 2017; DeMott et al., 2018), INP methods should be tested under ambient sampling conditions. Since ambient aerosols show a large variability in their physical and chemical identity, it might be possible that specific features of the aerosol sampling and INP analysis might impact the results, as e.g. the presence of coarse mode INPs. Moreover, the performance of the online instruments when measuring low INP concentrations needs to be tested.

During the Puy de Dôme ICe Nucleation Intercomparison Campaign (PICNIC), a total of eight INP methods were compared over a time span of ~2 weeks in October 2018. The site, being located at 1465 m a.s.l., offered a natural sampling environment of aerosols which are relevant for mixed-phase cloud formation. In this work, we present the comparison between the online INP counters CSU-CFDC (Colorado State University Continuous Flow Diffusion Chamber), PINE (Portable Ice Nucleation Experiment), and SPIN (Spectrometer for Ice Nuclei). Moreover, during times of overlapping sampling time and temperature, INP concentrations from PINE are compared to the freezing spectra of the offline-freezing techniques FRIDGE, IS (Ice Spectrometer), INDA (Ice Nucleation Droplet Array) which used samples from the newly built aerosol sampler HERA (High and Low Volume Flow Aerosol Particle Filter Sampler), INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology), LINDA (LED based Ice Nucleation Detection Aparatus) and MOUDI-DFT (Micro-Orifice Uniform Deposit Impactor-Droplet Freezing Technique). While the online instruments measure similar INP concentrations within a measurement temperature +/- 1K for most of the time, some discrepancies occur which need explanation. Moreover, while some of the offline samples collected during an 8-hour sampling time interval compare well to simultaneous PINE measurements, more than an order of magnitude difference was observed during some measurement periods. To investigate potential causes leading to those mismatches, we will investigate the impact of the present air mass by using information on its chemistry, as well as information on the aerosol size distribution.