Examining the Ice-Nucleating Particles from SGP

1 October 2019 - 14 November 2019

Lead Scientist: Naruki (seonggi) Hiranuma

Observatory: SGP

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) atmospheric mega sites. 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 Southern Great Plains (SGP) during October-November 2019. Different INP episodes (e.g., agricultural) will be assessed to help understand convective and mixed-phase cloud systems that are typically observed in this region. This field campaign and laboratory analyses followed will generate the data to understand how particle chemical composition and mixing state influence ambient ice nucleation propensity at the SGP mega site. Such dataset 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 global radiative energy budget.

Key scientific questions of the proposed campaign include:

• What are sources, abundance, chemical composition and ice nucleation processes of INPs at the SGP site? • What is a predominant ice nucleation mechanism at SGP – how dominant is the 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 SGP mega site?

With a new in situ Portable Ice Nucleation Expansion chamber (PINE, Bilfinger Noell, Würzburg, Germany) developed in collaboration with Karlsruhe Institute of Technology and University of Leeds, ExINP will carry out autonomous monitoring of atmospheric INPs abundance over a wide subzero temperature range and temporal distributions at the SGP observatory. A unique aspect of PINE includes its fully automated operation, minimum background particle concentration and its compact size, promising stand-alone operation even at remote locations with minimum on-site supervisions (ambient preliminary data available upon request). With a minimum detection of INP concentration <0.1 L-1 and high-temporal-resolution, this instrument can be used as a natural extension in variety of current ongoing SGP field measurements and long-term atmospheric measurement networks, and will add unprecedented values to achieve a scientific breakthrough. In addition, 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 to measure fine-temperature-resolved INPs concentrations using the particulate samples collected at the SGP mega site under different atmospheric states and meteorological conditions encountered. This offline INP propensity data will be a good complement of online PINE data.

INP measurements will help understand the nature of aerosols at the site and complement current ARM missions. Ice formation processes are very poorly represented in the climate models, including E3SM, and our study supports this 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.