Breakout Summary Report
ARM/ASR User and PI Meeting
13 - 17 March 2017
16 March 2017
1:45 PM - 3:45 PM
40
Xiaohong Liu and Paul DeMott
Breakout Description
The purpose of this breakout session was to review the progress on ice nucleation, from laboratory, in situ, and remote-sensing observations to modeling studies at different scales in the past year, to identify the remaining issues/knowledge gaps and come up with short- and long-term action plans. Ice nucleation is a critical process of ice generation in mixed-phase and cirrus clouds, and it has been shown to affect cloud properties and radiative forcing significantly. Yet, there are still large unknowns with this process and its representation in models. The Atmospheric Radiation Measurement (ARM) Climate Research Facility and Atmospheric System Research (ASR) program have funded a variety of related activities on measuring ice nucleating particles (INPs) and their compositions. In the breakout, we reviewed the progress made in the past year toward improved process understanding and model representation of ice nucleation. A large portion of our breakout was devoted to the discussion of the long-term INP measurements and more focused intensive operational periods at ARM sites to advance ice nucleation science. We discussed the critical needs/gaps and future directions to achieve our goals. We also discussed the science overlaps with other groups on topics such as aerosol mixing state and measurements of coarse mode aerosols.Main Discussion
The session included nine diverse talks covering the recent progress on ice nucleation, from laboratory, in situ, and remote-sensing observations to modeling studies. The discussion phase of the session was centered on ice nucleation measurement status and near-term actions involving multiple principal investigators for a dedicated intensive operational period or long-term studies at U.S. Department of Energy (DOE) sites.
Paul DeMott’s presentation showed that immersion freezing measurement uncertainty is ~1 order of magnitude in comparing different methods in the field conducted at Storm Peak during the Fifth International Ice Nucleation Workshop (FIN) phase 3 (FIN-03). He promoted that while intercomparisons should continue to identify artifacts associated with bulk- and single-particle INP analyses, and studies of time dependence are also needed, much can be gained by establishing long-term studies using multiple methods. These results are encouraging and provide the credibility for future INP measurements with a continuous flow diffusion chamber and offline methods.
However, questions were also raised and discussed regarding whether dedicated intensive operational periods are needed before long-term studies. Such intensive operational periods include an aerosol-INP closure study at the ARM Southern Great Plains atmospheric observatory, where aerosol size-resolved composition and physical properties (full size spectrum, surface area, mixing state, morphology), as well as INP concentrations, are simultaneously measured. Using these measured aerosol information and environmental variables (e.g., temperature and relative humidity), INP concentrations can be calculated from different parameterization schemes (e.g., surface area-based, classical nucleation theory [probability distribution function of contact angles], large particle size-based). The calculated and measured INP concentrations and freezing rates will be compared for the closure. The purpose of this closure is to understand the dominant aerosol properties for ice nucleation: surface area, coarse mode aerosol, composition, mixing state, morphology, time, etc. This closure study can be conducted in the absence of aerosol-cloud interaction studies, which makes the study relatively focused. Another proposed intensive operational period closure study is to simultaneously measure INP and ice number concentrations in relatively stable boundary-layer mixed-phase clouds. The purpose of this study is to examine the ice formation mechanisms in clouds and time dependence. But it was soon determined in the discussion that this INP-ice number closure study should be pursued after the aerosol-INP closure study is conducted.
It was also discussed whether the aerosol-INP closure study is necessary considering the FIN-03 has been conducted with similar objectives. Paul DeMott will provide feedback to the group regarding the comprehensiveness of the FIN-03 data set to meet objectives.
Key Findings
Raymond Shaw found that moving contact lines due to electrowetting enhance ice nucleation rates.
Paul DeMott presented the key results from intercomparisons of INP measurements in the recent few years. From FIN-02, excellent agreement can be obtained between many INP measurement methods under certain circumstances (co-sampling and co-distribution of submicron soil/mineral dusts and IN bacteria in these cases). Daniel Knopf showed the role of anthropogenic (human-caused) and biogenic secondary organic aerosols (SOA) in cold cloud formation. Sulfates reduce the ability of SOA to act as INPs. Biogenic SOA seem not to be efficient INPs under mixed-phase cloud conditions, in contrast to anthropogenic SOA. Xiaohong Liu and Katia Lamer provided examples of climate model evaluations using the ground-based remote-sensing and in situ observations as shown in Arctic site presentations by Gijs de Boer, Damao Zhang, and Jing Yang, which help model developments and improvements.