Aerosol–Ice Formation Closure: A Southern Great Plains Field Campaign

Abstract

Prediction of ice formation in clouds presents one of the grand challenges in the atmospheric sciences. Immersion freezing initiated by ice-nucleating particles (INPs) is the dominant pathway of primary ice crystal formation in mixed-phase clouds, where supercooled water droplets and ice crystals coexist, with important implications for the hydrological cycle and climate. However, derivation of INP number concentrations from an ambient aerosol population in cloud-resolving and climate models remains highly uncertain. We conducted an aerosol-ice formation closure pilot study using a field-observational approach to evaluate the predictive capability of immersion freezing INPs. The closure study relies on co-located measurements of the ambient size-resolved and single-particle composition and INP number concentrations. The acquired particle data serve as input in several immersion freezing parameterizations, that are employed in cloud-resolving and climate models, for prediction of INP number concentrations. We discuss in detail one closure case study in which a front passed through the measurement site, resulting in a change of ambient particle and INP populations. We achieved closure in some circumstances within uncertainties, but we emphasize the need for freezing parameterization of potentially missing INP types and evaluation of the choice of parameterization to be employed. Overall, this closure pilot study aims to assess the level of parameter details and measurement strategies needed to achieve aerosol-ice formation closure. The closure approach is designed to accurately guide immersion freezing schemes in models, and ultimately identify the leading causes for climate model bias in INP predictions.

Annual cycle observations of aerosols capable of ice formation in central Arctic clouds

“…Samples were processed using the CSU Ice Spectrometer (IS) 29 , 31 , 108 – 113 , a technique which is comparable to the cold plate method 114 . Filters were placed in sterile centrifuge tubes, 8 ml of 0.1 µm-filtered deionized water added, and particles re-suspended by tumbling end-over-end on a rotator for 20 min.…”

Aerosol Composition, Mixing State, and Phase State of Free Tropospheric Particles and Their Role in Ice Cloud Formation

“…The aerosol’s cold cloud formation potential is driven by multiple factors, including aerosol size distribution, aerosol composition, mixing state, phase state, and morphology. A recent aerosol–ice formation closure study highlighted that size-resolved chemical composition of individual particles is crucial to achieving closure between measured and predicted INP number concentrations . Though the previous OMP study also measured the IN propensity and characterized the INP population, it did not provide any detailed characterization of the ambient aerosol population to predict the IN activity .…”

Ice‐Nucleating Particles That Impact Clouds and Climate: Observational and Modeling Research Needs

“…While field studies correlating INP concentrations with particle types have led to insights about the dominant sources of INP at specific locations and times (e.g., Huffman et al., 2013; Mason et al., 2015; Si et al., 2019), there is also a need for more experiments that provide independent, quantitative tests for our understanding of the processes driving INP variability in the atmosphere. A stronger and more complete test of predictability is an experiment that directly evaluates whether INP parameterizations produce accurate simulations of INP numbers in real‐world ambient measurements, termed a “closure” experiment (Knopf et al., 2021).…”

“…Given the impressive ice nucleation behavior of agricultural aerosol organic‐rich soils, further studies are needed that quantify their atmospheric concentrations and test our ability to predict their contribution to atmospheric INP concentrations through closure experiments (Knopf et al., 2021). Additionally, more work is needed to link INP properties of agricultural dusts to the detailed particle composition, since particles with different sizes and different organic and biological components may potentially nucleate ice differently.…”