Micro-Spectroscopic Analysis of Ice-Nucleating Particles in Relation to Ambient Aerosol in a Remote Marine Environment

Abstract:

During the Aerosol and Cloud Experiment in the Eastern North Atlantic (ACE-ENA) field study located on the Azores islands, atmospheric particles were sampled at ground site and onboard the G-1 aircraft for examination of ice-nucleating particles (INPs). At ground, particle collection proceeded during day- and nighttime over multiple days. Airborne particle collection was performed at different altitudes in the marine boundary layer (BL) and free troposphere (FT). Immersion freezing and deposition ice nucleation as a function of temperature and humidity were assessed using a controlled vapor cooling-stage microscope system. The physicochemical characteristics of aerosol particles and INPs were determined by means of Computer Controlled Scanning Electron Microscopy with Energy Dispersed X-ray Spectroscopy (CCSEM/EDX) and Scanning Transmission X-ray Microscopy with Near Edge Fine Structure Spectroscopy (STXM/NEXAFS). The acquired particle population characteristics are set in context with the properties of the INPs. The ground-collected particles can be differentiated into 4 distinct particle type classes where night particle population displays a different compositional make-up. This is also reflected in the characteristics of the INPs where the INPs reflect the composition of the day or nighttime particle population with differences in their freezing propensity. The aircraft collected samples analyzed by STXM/NEXAFS show that the BL collected samples mainly consist of fresh organic-coated marine particles, similar to the daytime samples at the ACE-ENA ground site. In contrast, FT collected samples show more versatile particle types, including long-range transported aged organic particles, dusts and soot particles. FT aerosol shows relatively higher heterogeneous ice nucleation efficiency, and exerts about one order of magnitude greater ice nucleation rates, compared to those from the BL. The identification of FT INPs by SEM/EDX indicated that INPs reflect the present aerosol population including purely organic aerosols, and aged sea salt particles and mineral dusts both associated with organic material. Immersion freezing and deposition ice nucleation parameterizations for BL and FT aerosol have been derived.