The Horizontal Ice Nucleation Chamber (HINC) : INP measurements at conditions relevant for mixed-phase clouds at the High Altitude Research Station Jungfraujoch
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
the accuracy of the thermodynamic conditions of temperature (T)
and relative humidity (RH) in HINC with uncertainties in T
of ±0.4 K and in RH with respect to water
(RHw) of ±1.5 %, which translates
into an uncertainty in RH with respect to ice
(RHi) 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-subsaturated conditions
(RHw = 94 %), relevant for the formation of
ice clouds, and in the water-supersaturated regime
(RHw = 104 %) to represent ice formation
occurring under mixed-phase cloud conditions. In winters 2015 and
2016 the median INP concentrations at RHw = 94 % was below the minimum detectable concentration. At
RHw = 104 %, INP concentrations were an
order of magnitude higher, with median concentrations in winter 2015
of 2.8 per standard liter (std L−1; normalized to
standard T of 273 K and pressure, p, of
1013 hPa) and 4.7 std L−1 in winter 2016. The
measurements are in agreement with previous winter measurements
obtained with the Portable Ice Nucleation Chamber (PINC) of
2.2 std L−1 at the same location. During winter 2015,
two events caused the INP concentrations at RHw = 104 % to significantly increase above the campaign
average. First, an increase to 72.1 std L−1 was measured
during an event influenced by marine air, arriving at the JFJ from
the North Sea and the Norwegian Sea. The contribution from
anthropogenic or other sources can thereby not be ruled out. Second,
INP concentrations up to 146.2 std L−1 were observed
during a Saharan dust event. To our knowledge this is the first time
that a clear enrichment in ambient INP concentration in remote
regions of the atmosphere is observed during a time of marine air
mass influence, suggesting the importance of marine particles on ice
nucleation in the free troposphere
