EXINPENA

Examining the Ice-Nucleating Particles from ENA

28 September 2020 - 11 March 2021

Lead Scientist: Naruki (seonggi) Hiranuma

Observatory: ENA (Eastern North Atlantic)

A specific subset of atmospheric particles can act as ice-nucleating particles (INPs) in mixed-phase clouds and, ultimately, influence precipitation and Earth’s radiative energy balance. Despite the importance of INPs, current ambient INP data derived from field measurements are not well interpreted with detailed aerosol and cloud properties. ExINP fills this gap by using measurements from the Atmospheric Radiation Measurement (ARM) user facility’s atmospheric observatories. In particular, detailed ARM observational data of aerosol chemical composition speciation, abundance, CCN activity, and hygroscopicity are of the utmost importance for a better understanding of INPs’ mixing state as well as their implications in cloud, precipitation, and regional weather.

To complement the current ARM capabilities, ExINP will experimentally characterize abundance and physicochemical properties of ambient INPs at the ARM’s Eastern North Atlantic (ENA) observatory during May-June 2020. Different INP episodes that are unique at ENA will be assessed to help understand convective and mixed-phase cloud systems that are typically observed in this region. This field campaign and subsequent laboratory analyses will generate the data to understand how particle chemical composition and mixing state influence ambient ice nucleation propensity at the ENA observatory. Such a data set has long been a missing piece in the study area of cloud microphysics and atmospheric chemistry and is of importance to improve atmospheric models of cloud feedbacks and determine their impact on the global radiative energy budget.

Key scientific questions to be addressed by the proposed campaign include:

• What are the sources, abundance, chemical composition, and ice nucleation processes of INPs at the ENA site?
• What is a predominant ice nucleation mechanism at ENA — how dominant is condensation freezing, requiring a water saturation condition, or CCN activation prior to ice nucleation?
• What ice nucleation pathway is the most sensitive to the chemical mixing state of ambient aerosol at the ENA observatory?

With a new in situ Portable Ice Nucleation Expansion chamber (PINE; Bilfinger Noell, Würzburg, Germany) developed based on the design of the renowned AIDA cloud chamber, ExINP-ENA will carry out autonomous monitoring of atmospheric INP abundance over a wide subzero temperature range and temporal distributions at the ENA observatory. A unique aspect of the PINE chamber is its plug-and-play feature (so it runs on a standard power outlet), autonomous cryo-cooler-based temperature-ramping operation, capability of quantifying INPs in different IN modes (e.g., immersion freezing and deposition mode at >-60 °C), small particle loss through the system (~5% for <5 mm diameter particles), and sensitive optical particle detection of INP concentration (≤0.1 L-1 at T > -15 °C), promising stand-alone operation at remote locations. To date, the PINE instrument has been tested in field campaigns in the Southern Great Plains. With a turnover time of ~6 minutes, PINE ran continuously and scanned at different temperature intervals to assess different INP episodes. Our first field data at ENA compared to our measurements at other locations will be looked into.

Additionally, ExINP will collect airborne particle samples using a polycarbonate filter impactor and a swirling liquid particle impinger in the side-by-side position next to PINE. These particulate samples will be further characterized by conducting offline droplet-freezing assay studies. An offline droplet-freezing assay instrument will enable us to measure fine-temperature-resolved INPs concentrations using the particulate samples collected at the ENA observatory under different atmospheric states and meteorological conditions. This offline INP propensity data will be a good complement of online PINE data.

INP measurements will help us understand the nature of aerosols at the site and complement current ARM missions. Ice formation processes are very poorly represented in climate models, including E3SM, and our study supports this U.S. Department of Energy (DOE) mission by providing INP parameterizations representative of the ARM sites. To constrain E3SM, ExINP will provide a variety of INP parameterizations, such as ice nucleation active surface site density, cumulative number concentration of INPs per volume of air, and water activity-based freezing description.