Sources and nature of ice-nucleating particles in the free troposphere at Jungfraujoch in winter 2017
Authors: Lacher Larissa , Clemen Hans-Christian , Shen Xiaoli , Mertes Stephan , Gysel-Beer Martin , Moallemi Alireza , Steinbacher Martin , Henne Stephan , Saathoff Harald , Möhler Ottmar , Höhler Kristina , Schiebel Thea , Weber Daniel , Schrod Jann , Schneider Johannes , Kanji Zamin A. 
Published: November 2021

Abstract. Primary ice formation in mixed-phase clouds is initiated by a
minute subset of the ambient aerosol population, called ice-nucleating
particles (INPs). The knowledge about their atmospheric concentration,
composition, and source in cloud-relevant environments is still limited.
During the 2017 joint INUIT/CLACE (Ice Nuclei research UnIT/CLoud–Aerosol
Characterization Experiment) field campaign, observations of INPs as
well as of aerosol physical and chemical properties were performed,
complemented by source region modeling. This aimed at investigating the
nature and sources of INPs. The campaign took place at the High-Altitude
Research Station Jungfraujoch (JFJ), a location where mixed-phase clouds
frequently occur. Due to its altitude of 3580 m a.s.l., the station is
usually located in the lower free troposphere, but it can also receive air
masses from terrestrial and marine sources via long-range transport. INP
concentrations were quasi-continuously detected with the Horizontal Ice
Nucleation Chamber (HINC) under conditions representing the formation of
mixed-phase clouds at −31 ∘C. The INP measurements were performed
in parallel to aerosol measurements from two single-particle mass
spectrometers, the Aircraft-based Laser ABlation Aerosol MAss Spectrometer
(ALABAMA) and the laser ablation aerosol particle time-of-flight mass
spectrometer (LAAPTOF). The chemical identity of INPs is inferred by
correlating the time series of ion signals measured by the mass
spectrometers with the time series of INP measurements. Moreover, our
results are complemented by the direct analysis of ice particle residuals
(IPRs) by using an ice-selective inlet (Ice-CVI) coupled with the ALABAMA.
Mineral dust particles and aged sea spray particles showed the highest
correlations with the INP time series. Their role as INPs is further
supported by source emission sensitivity analysis using atmospheric
transport modeling, which confirmed that air masses were advected from the
Sahara and marine environments during times of elevated INP
concentrations and ice-active surface site densities. Indeed, the IPR
analysis showed that, by number, mineral dust particles dominated the IPR
composition (∼58 %), and biological and metallic
particles are also found to a smaller extent (∼10 % each). Sea
spray particles are also found as IPRs (17 %), and their fraction in the
IPRs strongly varied according to the increased presence of small IPRs,
which is likely due to an impact from secondary ice crystal formation. This
study shows the capability of combining INP concentration measurements with
chemical characterization of aerosol particles using single-particle mass
spectrometry, source region modeling, and analysis of ice residuals in an
environment directly relevant for mixed-phase cloud formation.