Abstract
Formation of atmospheric ice plays a crucial role in the microphysical evolution of mixed-phase and cirrus clouds and thus climate. How aerosol particles impact ice crystal formation by acting as ice-nucleating particles (INPs) is a subject of intense research activities. To improve understanding of atmospheric INPs, we examined daytime and nighttime particles collected during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign conducted in summer 2017. Collected particles, representative of a remote marine environment, were investigated for their propensity to serve as INPs in the immersion freezing (IMF) and deposition ice nucleation (DIN) modes. The particle population was characterized by chemical imaging techniques such as computer-controlled scanning electron microscopy with energy dispersive X-ray analysis (CCSEM/EDX) and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). Four major particle-type classes were identified where internally mixed inorganic-organic particles make up the majority of the analyzed particles. Following ice nucleation experiments, individual INPs were identified and characterized by SEM/EDX. The identified INP types belong to the major particle-type classes consisting of fresh sea salt with organics or processed sea salt containing dust and sulfur with organics. Ice nucleation experiments show IMF events at temperatures as low as 231 K including the subsaturated regime. DIN events were observed at lower temperatures of 210 to 231 K. IMF and DIN observations were analyzed with regard to activated INP fraction, ice-nucleation active sites (INAS) densities, and water activity-based immersion freezing model (ABIFM) yielding heterogeneous ice nucleation rate coefficients. Observed IMF and DIN events of ice formation and corresponding derived freezing rates demonstrate the marine boundary layer aerosol particles can serve as INPs under typical mixed-phase and cirrus clouds conditions. The derived IMF and DIN parameterizations allow for implementation in cloud and climate models to evaluate predictive effects of atmospheric ice crystal formation.
Highlights
Understanding how atmospheric aerosol serves as ice-nucleating particles (INPs) is necessary to advance our understanding of cloud microphysical processes that impact precipitation and climate (Boucher et al, 2013;Storelvmo, 2017;Baker and Peter, 2008;Mülmenstädt et al, 2015;Lohmann and Feichter, 2005)
Processed sea salt indicates the loss of chlorine, likely due to chemical reactions with particulate nitric acid, sulfuric acid, and organic acids leading to gaseous HCl (Wang et al, 2015;Laskin et al, 2012a;Angle et al, 2021)
It serves mostly the purpose to assess if identified INPs belong to those 4 major particle-type classes or if they belong to completely different particle types as will discussed below
Summary
Understanding how atmospheric aerosol serves as INPs is necessary to advance our understanding of cloud microphysical processes that impact precipitation and climate (Boucher et al, 2013;Storelvmo, 2017;Baker and Peter, 2008;Mülmenstädt et al, 2015;Lohmann and Feichter, 2005). The ground ENA site was established on Graciosa Island in the Azores, Portugal (39° 5' 30" N, 28° 1' 32" W, 30.48 m above mean sea level, Fig. S1) by the U.S Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility (Wang et al, 2021;Mather and Voyles, 2013). This site experiences a wide range of meteorological and cloud conditions and can be prone to aerosol particles downwind of the North American 65 continent (Wang et al, 2021). We report on composition and ice nucleation analyses of particles collected at the ENA ground site during the first intensive operating period during the summer of 2017
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