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Environmental Transformations and Interactions

Ice-nucleating Particle Characteristics Shown to Vary Seasonally and with Altitude

A study of ice-nucleating particles over the Azores islands adds new information about how particles form over lower latitude, remote marine regions  

airplane under a rainbow

The Atmospheric Radiation Measurement user facility’s Gulfstream-159 research aircraft sits on the tarmac on Terceira Island during the Aerosol and Cloud Experiments in the Eastern North Atlantic winter 2018 intensive operational period in the Azores. (Image courtesy of the Atmospheric Radiation Measurement user facility)

The Science  

Ice-nucleating particles consist of tiny specks of material suspended in the atmosphere that act as starting points for the growth of ice crystals in clouds. These rare particles are poorly understood and are not well characterized at lower latitudes in remote marine regions. As part of a published study, a multi-institutional team of researchers collected particles by aircraft in the marine boundary layer and free troposphere over Portugal’s Azores islands as part of the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign. Following the characterization and analyses of particles using a combination of instruments, the team determined that ice-nucleating particles can vary seasonally and with altitude depending on sampling location, thereby resulting in different ice-nucleation tendencies. The information adds important insights regarding the formation of clouds that contain ice crystals crucial for climate models and demonstrates the need for further sampling and study. 

The Impact 

Ice formation by atmospheric particles has an important role in cloud foundation, precipitation, and related atmospheric processes. As a result, these particles have a strong influence on the global radiative budget and water cycle. While scarce, ice-nucleating particles play a central role in cloud microphysical processes. Developing a greater understanding of where these particles reside, how they form, and how they behave and influence different atmospheric processes will help determine their impact on cloud evolution, their role in larger atmospheric models, and, thus, their impact on climate. 


During the ACE-ENA campaign, supported by the Atmospheric Radiation Measurement (ARM) user facility, a multi-institutional team of scientists collected particles in the marine boundary layer and free troposphere over the Portuguese Azores islands. Samples were specifically collected during the campaign’s summer 2017 and winter 2018 intensive operational periods. The team examined particulate matter in the samples using scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy at the Advanced Light Source, a Department of Energy (DOE) Office of Science user facility at Lawrence Berkeley National Laboratory. The team then analyzed those particles and identified the ice-nucleating particles using scanning electron microscopy with energy-dispersive X-ray analysis at the Environmental Molecular Sciences Laboratory (EMSL)—a DOE Office of Science user facility at Pacific Northwest National Laboratory. The team observed differences in a range of particle population characteristics—some of which included particle density, mixing state, and amount of organic matter present between seasons. Additionally, free troposphere particle samples showed greater ice-nucleation tendencies as compared with marine boundary-layer particles. Together, these results show how ice-nucleating particles can vary seasonally and with altitude, amplifying the need to collect measurements that cover seasonal changes and a range of altitudes. 


Daniel Knopf, Stony Brook University, 

Swarup China, EMSL, 

Nurun Nahar Lata, EMSL, 


The study was supported by the Atmospheric System Research program and ARM, both sponsored by the DOE Office of Science, Biological and Environmental Research program. A portion of the research was performed through Large-Scale Research awards from EMSL


D.A. Knopf, et al. “Physicochemical characterization of free troposphere and marine boundary layer ice-nucleating particles collected by aircraft in the eastern North Atlantic.” Atmospheric Chemistry and Physics 23, 8659–8681 (2023). [DOI: 10.5194/acp-23-8659-2023]