First Aerosol Filter—Based Ice Nucleating Particle Measurements in Hyytiälä during the Transition Period from Winter to Summer 2018

Ice nucleating particles (INPs), a very minor fraction of the atmospheric aerosol, initiate primary ice formation in tropospheric clouds and thereby frequently initiate the formation of precipitation. For instance, the ice phase can only be caused by INPs in shallow clouds with cloud top temperatures above -35°C, the homogeneous freezing temperature of supercooled cloud droplets. In such clouds, INPs with a strongly temperature dependent concentration in the range of about 0.001 L^-1 close to 0°C to about 10 L^-1 at -30°C, let only a minor fraction of the supercooled cloud droplets freeze. The resulting ice crystals grow in expense of the remaining liquid droplets by a cascade of secondary gas, ice and liquid phase processes, leading to the formation of snow fall or rain.

The basic ice nucleation properties have been investigated for a number of INP types in laboratory experiments, and many cloud observation and modelling studies have addressed the formation of ice and precipitation in tropospheric clouds. However, the sources, types and concentrations of INPs are still almost unknown, in particular in Northern Europe, where only a few INP measurements have been performed up-to-date. Here we report a first long-term record of INP concentration measurements over 6 months from March to August 2018 at the SMEAR II station located at the Hyytiälä Forestry Research Station in Juupajoki, Finland. The measurements were part of the HyICE campaign, which was also supported through Trans-National Access (TNA) of the ACTRIS 2 project.

The INP measurements reported and discussed here are based on aerosol filter samples taken on a daily basis at two station locations, the so-called tower container located above canopy and the aerosol cottage located inside the forest. The filters were rinsed with ultraclean water, and the resulting particle suspensions were analyzed on site for the INP concentration in the temperature range from about -5°C to -25°C. Two methods were used, the Ice Nucleation Spectrometer Experiment of the Karlsruhe Institute of Technology (INSEKT), which was adapted from the Ice Spectrometer of the Colorado State University, and the Nucleation by Immersed Particle Instrument (NIPI) of the University of Leeds.

Heat treatment of the suspension before analysis provides information on the relative contribution of heat-sensitive biological components to the total INP concentration of the untreated suspension. The day-by-day variability will be presented for both untreated and heat-treated samples and discussed in the context of meteorological conditions as well as aerosol concentration and size distribution measurements available from the SMEAR II station. The results will contribute to a better understanding of INP sources, abundance and variability in northern Europe, and will provide input to cloud models in order to improve their formulations and parameterizations of primary ice formation as a basis for precipitation initiation.