In this work we describe the Horizontal Ice Nucleation Chamber, HINC as a new instrument to measure ambient ice nucleating particle (INP) concentrations for conditions relevant to mixed-phase clouds. Laboratory verification and validation experiments confirm accuracy of the thermodynamic conditions of temperature (T) and relative humidity (RH) in HINC with uncertainties in temperature of ±0.4 K and in RH with respect to water (RH<sub>w</sub>) of ±1.5 %, which translates to an uncertainty in RH with respect to ice (RH<sub>i</sub>) of ±3.0 % at T > 235 K. For further validation of HINC as a field instrument, two measurement campaigns were conducted in winters 2015 and 2016 at the High Altitude Research Station Jungfraujoch (JFJ; Switzerland, 3580 m a.s.l.) to sample ambient INPs. During winters 2015 and 2016 the site encountered free tropospheric conditions 92 % and 79 % of the time respectively. We measured INP concentrations at 242 K at water sub-saturated conditions (RH<sub>w</sub> = 94 %), relevant for the formation of ice clouds, and in the water supersaturated regime (RH<sub>w</sub> = 103–104 %) to represent ice formation occurring under mixed-phase cloud conditions. In winter 2015 and 2016 the median INP concentrations at RH<sub>w</sub> = 94 % was below the minimum detectable concentration. At RH<sub>w</sub> = 104 %, INP concentrations were an order of magnitude higher, with median concentrations in winter 2015 of 2.8 per standard liter (stdL<sup>−1</sup>; normalized to standard temperature T = 273 K and pressure p = 1013 hPa) and 4.7 stdL<sup>−1</sup> in winter 2016. The measurements are in agreement with previous winter measurements obtained with the Portable Ice Nucleation Chamber, PINC, of 2.2 stdL<sup>−1</sup> at the same location. During winter 2015, two events caused the INP concentrations at RH<sub>w</sub> = 103–104 % to significantly increase above the campaign average. First, an increase to 72.1 stdL<sup>−1</sup> was measured during an event influenced by marine air, coming from the Northern Sea and the Norwegian Sea. Second, INP concentrations up to 146.2 stdL<sup>−1</sup> were observed during a Saharan dust event. To our knowledge this is the first time that a clear enrichment in ambient INP concentration is observed during a time of marine air mass influence, indicating the importance of marine particles on ice nucleation in the free troposphere.