Special issue  

Ice nucleation is a particularly hot topic at the moment in atmospheric sciences, with profound implications for the hydrological cycle and climate. However, the detailed links between the atmospheric aerosol chemical and physical character, ice-nucleating particles (INPs), and the ice nucleation processes in different temperature regimes remain open (e.g. Hoose and Möhler, 2012, Knopf et al., 2018). We performed a comprehensive field study in Hyytiälä, Finland, during 2018 (HyICE campaign) with a clear focus on characterizing the INP concentrations in different temperature ranges with a suite of ice nucleation counters (online and offline). The work was complemented with thorough measurement of physical (size distribution 1 nm–10 um, bioaerosol concentrations) and chemical characteristics (high-resolution AMS) of the atmospheric aerosols from nanometre scale to super-micron sizes done at the Station for Measuring Ecosystem – Atmosphere Relations II (SMEAR II) site. The location provides an excellent facility to study INPs as we can compare our data to the 20-year data set already available at the site. The observation period started in February, with the most intensive INP measurements in March–May 2018. We operated a suite of online ice nucleation counters (SPIN, PINE, PINCII) and performed filter sampling at the ground level, above the canopy, and onboard a research aircraft. We continue the filter sampling providing understanding of the seasonality of INP concentrations in the boreal environment. In SMEAR II we have also the weather radar and lidar capacity to address the cloud microphysics within the clouds. These activities enable us 1) to perform an intercomparison between the ice nucleation chambers, 2) to assess the vertical variability of INPs inside the boundary layer and in the free atmosphere, 3) to connect the observed INP concentrations to physical and chemical parameters and aerosol source apportionment and 4) seasonal variability of the INPs in the boreal context, 5) to connect the ground-based INP measurements to the cloud processes aloft, and 6) to develop parameterizations based on our observations and utilize them in models of various scales.