Measurements of ice-nucleating particle concentration and size at Storm Peak Laboratory

The presence of ice in mixed-phase clouds has a vital impact on their radiative properties, lifetime, and ability to precipitate. Ice crystal formation is initially induced by a rare subset of ambient aerosol particles called ice-nucleating particles (INPs). Despite the importance of INPs on aerosol-cloud interactions and century-long research efforts, the knowledge about their nature and atmospheric abundance still needs improvement. Recent instrument developments allow more automated and continuous INP measurements with a high time resolution, to gain a better understanding of the natural variability of INPs in different locations, and to investigate the identity and source regions of them. Here we present long-term observations of INPs at Storm Peak Laboratory (SPL) located in the Rocky Mountains of Colorado. SPL is at an altitude of 3200 m a.s.l. within the lower free troposphere, and in winter it is a location where mixed-phase clouds frequently occur. Therefore, the present aerosol particles are directly relevant for ice formation in such clouds. The ongoing INP measurements started in October 2021, and are conducted with the Portable Ice Nucleation Experiment (PINE) at conditions resembling the formation of mixed-phase clouds at temperatures between -22°C and -32°C. Results on the short-term and inter-seasonal variability will be presented, with a focus on parallel measurements of aerosol particle properties and meteorology. During the winter months of January and February 2022, we characterized the size of the INPs by selecting ice crystal residuals downstream of PINE using a pumped-counterflow virtual impactor and a novel optical particle counter. As this setup operates continuously, it is capable to investigate INP properties for longer time periods and to improve sampling statistics. In the future, it will be used with other diagnostic instruments, such as a single particle mass spectrometer, giving insights into the size and chemical composition of INPs, and thus allows to have a direct measure of the nature of INPs in ambient air.