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. Primary ice formation in mixed-phase clouds is initiated by aminute subset of the ambient aerosol population, called ice-nucleatingparticles (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–AerosolCharacterization Experiment) field campaign, observations of INPs aswell as of aerosol physical and chemical properties were performed,complemented by source region modeling. This aimed at investigating thenature and sources of INPs. The campaign took place at the High-AltitudeResearch Station Jungfraujoch (JFJ), a location where mixed-phase cloudsfrequently occur. Due to its altitude of 3580 m a.s.l., the station isusually located in the lower free troposphere, but it can also receive airmasses from terrestrial and marine sources via long-range transport. INPconcentrations were quasi-continuously detected with the Horizontal IceNucleation Chamber (HINC) under conditions representing the formation ofmixed-phase clouds at −31 ∘C. The INP measurements were performedin parallel to aerosol measurements from two single-particle massspectrometers, the Aircraft-based Laser ABlation Aerosol MAss Spectrometer(ALABAMA) and the laser ablation aerosol particle time-of-flight massspectrometer (LAAPTOF). The chemical identity of INPs is inferred bycorrelating the time series of ion signals measured by the massspectrometers with the time series of INP measurements. Moreover, ourresults 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 highestcorrelations with the INP time series. Their role as INPs is furthersupported by source emission sensitivity analysis using atmospherictransport modeling, which confirmed that air masses were advected from theSahara and marine environments during times of elevated INPconcentrations and ice-active surface site densities. Indeed, the IPRanalysis showed that, by number, mineral dust particles dominated the IPRcomposition (∼58 %), and biological and metallicparticles are also found to a smaller extent (∼10 % each). Seaspray particles are also found as IPRs (17 %), and their fraction in theIPRs strongly varied according to the increased presence of small IPRs,which is likely due to an impact from secondary ice crystal formation. Thisstudy shows the capability of combining INP concentration measurements withchemical characterization of aerosol particles using single-particle massspectrometry, source region modeling, and analysis of ice residuals in anenvironment directly relevant for mixed-phase cloud formation.

Sources and nature of ice-nucleating particles in the free troposphere at Jungfraujoch in winter 2017