12 Apr 2023
 | 12 Apr 2023

Physicochemical characterization of free troposphere and marine boundary layer ice-nucleating particles collected by aircraft in the eastern North Atlantic

Daniel Alexander Knopf, Peiwen Wang, Benny Wong, Jay M. Tomlin, Daniel P. Veghte, Nurun N. Lata, Swarup China, Alexander Laskin, Ryan C. Moffet, Josephine Y. Aller, Matthew A. Marcus, and Jian Wang

Abstract. Atmospheric ice nucleation impacts the hydrological cycle and climate by modifying the radiative properties of clouds. To improve our predictive understanding of ice formation, ambient ice-nucleating particles (INPs) need to be collected and characterized. Measurements of INPs at lower latitudes in a remote marine region are scarce. The Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign, in the region of the Azores Islands, provided the opportunity to collect particles in the marine boundary layer (MBL) and free troposphere (FT) by aircraft during the campaign’s summer and winter intensive operation period (IOP). The particle population in samples collected was examined by scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy. The identified INPs were analyzed by scanning electron microscopy with energy-dispersive X-ray analysis. We observed differences in the particle population characteristics in terms of particle diversity, mixing state, and organic volume fraction between seasons, mostly due to dry intrusion events during winter, and between the sampling locations of the MBL and FT. These differences are also reflected in the temperature and humidity conditions under which water uptake, immersion freezing (IMF), and deposition ice nucleation (DIN) proceed. Identified INPs reflect typical particle types within the particle population on the samples and include sea salt, sea salt with sulfates, and mineral dust, all associated with organic matter, and carbonaceous particles. IMF and DIN kinetics are analyzed with respect to heterogeneous ice nucleation rate coefficients, Jhet, and ice nucleation active site density, ns, as a function of the water criterion ∆αw. DIN is also analyzed in terms of contact angles following classical nucleation theory. Derived MBL IMF kinetics agree with previous ACE-ENA ground site INP measurements. FT particle samples show greater ice nucleation propensity compared to MBL particle samples. This study emphasizes that the types of INPs can vary seasonally and with altitude depending on sampling location, thereby showing different ice nucleation propensities, crucial information when representing mixed-phase cloud and cirrus cloud microphysics in models.

Daniel Alexander Knopf et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-559', Anonymous Referee #1, 07 May 2023
    • AC1: 'Reply on RC1', Daniel Knopf, 31 May 2023
  • RC2: 'Comment on egusphere-2023-559', Anonymous Referee #2, 13 May 2023
    • AC2: 'Reply on RC2', Daniel Knopf, 31 May 2023

Daniel Alexander Knopf et al.

Daniel Alexander Knopf et al.


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Short summary
Ambient particle populations and associated ice-nucleating particles (INPs) were examined from particle samples collected onboard aircraft in the marine boundary layer and free troposphere in the Eastern North Atlantic during summer- and wintertime. Chemical imaging shows distinct differences in the particle populations seasonally and with sampling altitudes which are reflected in the INP types. Freezing parameterizations are derived for implementation in cloud-resolving and climate models.