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

Abstract

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, their composition, and source in cloud-relevant environments is still limited. During the joint INUIT/CLACE (Ice Nuclei research UnIT/ CLoud–Aerosol Characterization Experiment) 2017 field campaign, observations of INPs, as well as of aerosol physical and chemical properties were performed, complemented by source region modelling. 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 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 INP is further supported by source emission sensitivity analysis using atmospheric transport modelling, which confirmed that air masses were advected from the Saharan desert 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 also biological and metallic particles are found to a smaller extent (~10 %). 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 modelling, and analysis of ice-residuals in an environment directly relevant for mixed-phase cloud formation.