Breakout Summary Report
ARM/ASR User and PI Meeting
2 - 6 May 2016
Ice Nucleation
4 May 2016
1:30 PM - 3:30 PM
0
Xiaohong Liu and Paul DeMott
4 May 2016
1:30 PM - 3:30 PM
0
Xiaohong Liu and Paul DeMott
Breakout Description
The purpose of this breakout session is 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 to come up with short-term 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. ARM has co-supported the Fifth International Ice Nucleation (FIN) Workshop, which was conducted in three phases (FIN-1 in November 2014, FIN-2 in March 2015, and FIN-3 in September 2015), and has generated interesting results/findings regarding the inter-comparability of measurements of ice nucleating particles and their compositions. In addition, a variety of related activities are supported by ARM/ASR.Main Discussion
The session included seven 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 centered on ice nucleation measurement status and first results and findings of the DOE ARM/ASR co-sponsored, three-phase Fifth International Ice Nucleation (FIN) Workshop in 2014-2015.Xiaohong Liu’s presentation of the preliminary referee report on the first ever “blind” instrument intercomparisons during FIN-2 (at KIT’s AIDA facility) highlighted findings and issues regarding the use of different ice nucleation measurement methods to sample a common ice nucleating aerosol. The ability of continuous flow diffusion chambers (CFDCs) to access a combination of ice nucleation mechanisms, and their complex response to water supersaturation that has been discussed in recent literature, was made clear again (perhaps for the first time for some attendees) for a group of devices that included commercial SPIN instruments. In the regime below water saturation where deposition nucleation may dominate for relatively pure mineral dust particles, good agreement between measurements of INP number concentration was obtained. At -20°C, deposition nucleation was barely seen at the noisy limits of detection of devices, while at -25°C and colder, all instruments sensed a population of particles capable of initiating freezing below 100% RH. In contrast, strong instrument response to supersaturation, presumably due to immersion freezing, was seen above 100% RH, but the varied INP concentration versus RH response curves showed strong discrepancies for different devices at any particular RH>100%. This is in contrast to the immediate response of ice formation in the AIDA chamber once cloud forms. These results emphasize that these real-time ice nucleation instruments do not operate like CCN instruments to create an accurate step function response to imposition of any RH above 100%. Each instrument design differs in terms of geometry, detection methods, RH accuracy, precision, and value required for full droplet activation. Consequently, a strong recommendation for reporting results from CFDC-style instruments is for device experts to report the maximum activity in the water supersaturated regime instead of response with respect to RH.
Good overall agreement was also obtained in FIN-2 blind studies between multiple “offline” methods that isolate immersion freezing nucleation activity by placing aerosols first into liquid before subsequent cooling. As for the flow chambers, maximum discrepancies could reach up to two orders of magnitude at any temperature. Also in agreement with some previous studies, the immersion freezing data underestimated surface active site density (INP number per surface area) in comparison to values derived from the AIDA chamber. The magnitude of discrepancy depended on INP aerosol type.
In Paul DeMott’s report on FIN-3, a 3-week intercomparison for sampling ambient air at Storm Peak Laboratory, multiple online and offline methods agreed well in terms of response trends of INP concentrations to temperature and changes in INP concentration over time. Variance of results across methods was quantified in dependence on temperature. These were highly encouraging results for measuring ambient INP concentrations.
These summary results spurred discussion and subsequent recommendations regarding two key action items. First, the readiness for sampling using a real-time instrument such as a SPIN (or similar device) at DOE-ARM sites was considered. This instrument could be operated to obtain measurements alternately at below and above 100% RH for comparison to immersion freezing measurements by other instruments. Issues that need to be overcome to support long-term measurements include: 1) demonstration of semi-autonomous operation has not been achieved, so a trained operator is still needed; 2) FIN highlighted that substantial data post-processing and quality control is still required; and 3) even with integrated sampling periods of 10-30 minutes at steady temperature and RH set-points, validated particle concentration methods may be required for statistical sampling at temperatures warmer than -20 °C. Clearly progress has been made, and FIN demonstrated the potential of SPIN in comparison with existing CFDC instruments, but trial IOPs may be favored over long-term deployment at this time.
The readiness of the filter-based collections for immersion freezing post-processing and use for long-term INP measurements at the ARM megasites was also discussed, and recommended. FIN-3 gave high confidence in such measurements for sampling INPs in the ambient atmosphere, and good agreement with real-time sampling instruments. Sample volumes are typically large, such that an entire temperature spectrum of INP number concentration is obtained for each sample, from 0°C to close to -30°C. These methods, practically used worldwide now, are the only realistic present means of obtaining statistically quantifiable INP number concentrations at T> -20°C in the natural atmosphere. In response to a request for a cost estimate for filter-based immersion freezing sampling, DeMott gave his own lab costs for an annual cycle of daily measurements at Oliktok or SGP at between $150-$200 per sample processed (inclusive of shipping, logistics, quality control, archiving, and reporting). ARM technicians can perform filter collections. Additional costs, such as a permanent low-temperature freezer at the collection sites, are modest.
Key Findings
1. From FIN-2, the spread of INP concentrations from different methods/instruments is small at RHw<100%, but larger when RHw>100%. This is due to instrument factors that require further investigation. Lower uncertainty was found at lower temperatures.2. From FIN-3 ambient intercomparison, the maximum spread of INP concentrations from different methods is higher at warmer temperature and decreases with lower temperature: 2.25, 1.85, 1.38, and 1.02 orders of magnitude, at -15°C, -20°C, -25°C, and -30°C, respectively.
3. The University of Frankfurt FRIDGE measurements in FIN-3 at the Storm Peak Laboratory showed that immersion freezing typically dominates by 10 times or more over the deposition at -25 °C and warmer.
4. INP number concentrations from online (real-time) instruments operated in the water supersaturated regime agree with offline (e.g., filter-based) immersion freezing methods for a number of ambient scenarios.