Breakout Summary Report
ARM/ASR User and PI Meeting
19 - 23 March 2018
Developing particle standards having known size, shape, and composition to improve measurement and model performance
21 March 2018
1:30 PM - 3:30 PM
35
James Smith and Peter McMurry
21 March 2018
1:30 PM - 3:30 PM
35
James Smith and Peter McMurry
Breakout Description
The purpose of this breakout was to bring together theorists, modelers, and experimentalists from the broad ASR community to articulate needs for new types of aerosol standards that will both aid in interpreting lab and field measurements and assure that those measurements address the needs of models. Speakers were asked to address methods that could be employed in the near term with established technologies, if appropriate. However, presenters were particularly asked to bring up improvements needed to address current needs in their respective research areas, that is, to focus on “unsolved challenges.” Experts were invited from a broad range of disciplines within the umbrella of ASR science, covering the measurement of aerosol composition, gas-phase precursors, cloud condensation nuclei (CCN), and ice nuclei (IN). Following the presentations, the group discussed possible “next steps” towards identifying and addressing the challenges.Main Discussion
The discussion opened with Peter McMurry laying out the motivation for the session. He discussed two important inventions during the 1970s, the Vibrating Orifice Aerosol Generator (VOAG) and the Differential Mobility Analyzer (DMA), and the impact that these instruments have had on aerosol measurement. Their success lies in the fact that both are easy to use, are easily replicated for use by laboratories worldwide, and when implemented reduce uncertainties in measurements of important aerosol properties. At last year’s ARM/ASR meeting, Nicole Riemer and Matt West co-chaired a session, “Aerosol mixing state: Metrics, modeling, measurements.” In that session they hypothesized that discrepancies between their model predictions for mixing state may well be due to measurement errors, and argued that there is a need to quantify and, ultimately, reduce measurement uncertainties. This led Pete to ask: “What types of new aerosol standards are needed to reduce uncertainties of measurements with modern aerosol instrumentation, especially mass spectrometers, and how can we respond to those needs?” In his view the purpose of this breakout was to articulate those needs.Following Pete’s introduction, Matt West spoke about “Challenges from the aerosol modeling community.” Matt spoke in greater detail about the experiments and analyses that motivated this breakout. In those experiments, the difficulty of reconciling model and measurements for a very simple mixing state test (mixing ammonium sulfate particles with carbon black particles) tells Matt that more work needs to be done in understanding how measurements map onto mixing state, and vice versa. It also suggested that we need to pay more attention to uncertainties in both models and measurements. The discussion that followed Matt’s talk addressed specific ways of reducing uncertainties in the measurements described in his example. The take-away message is that, for such a simple system as a collection of black carbon particles and salt particles mixed in a barrel and then sampled by state-of-the-art instruments, clear mappings between measurements and the targeted property (i.e., “mixing state”) were elusive.
Following Matt’s talk, there were a series of talks by invited experts with the theme “Unsolved challenges for aerosol standards.” Jose Jimenez presented a talk entitled “Some thoughts on Aerosol Standards for
Chemical Composition.” Jose defined two types of needs: one he called “Operational” and the other “Researchy.” Operational refers to calibration methods that currently exist but are often expensive and difficult to employ. He suggested that a “simpler push-button system to generate particles of known mass and composition seems like the greatest need.” Jose then showed an eye-opening plot of the total number concentration from five scanning mobility particle sizers and the large variability that is often characteristic of this “simple” measurement. Another similar plot showed particulate sulfate from four Aerosol Mass Spectrometers. Jose was a proponent of having intercomparisons as part of every lab or field campaign. Jose ended his talk by describing what he felt are current “Researchy needs.” Those included generating particles of known chemical morphology (X coated by Y) reproducibly, and generating particles of known shape factor, reproducibly.
Joel Thornton presented thoughts on measurements of “Vapors that grow particles.” As calibration needs, he identified a “reproducible, independently quantified, and transportable source of several multi-functional organics and some inorganics” for use in constraining instrument response as a function of molecular properties. He felt that there does not need to be many (some even very simple) such compounds to allow quantification of a broad suite of components. The important aspects are they span a range of binding energies (to reagent ion), molecular weight, and functional groups.
Paul DeMott spoke next on “Aerosol standards for ice-nucleating particle measurement methods – a preliminary set of thoughts.” He showed that, currently, there are no IN standards that have worked well. All systems are problematic and complex, with activation temperatures that are rarely distinct. He expressed the need for standards that reflect basic level ice-nucleation units that have sizes from below 20 nm (e.g., pollen washing water) to as large as 10 microns. Those are the components, but as atmospheric particles they will almost always have sizes larger than 100 nm and will usually be dominated by particles at sizes larger than 500 nm. The standard can be quite complex, like mixed salt-organic particles and mixed mineral particles with varied feldspar amounts, with particle sizes in upper accumulation through coarse modes. However, the use of feldspar alone has been shown to be affected by interactions subsequent to its being put into water. He ended by mentioning biological particles, for example Snomax, which contains cellular units sized 1-2 microns and proteins at smaller sizes. However, this is only a good calibrant at temperatures below -10 C because its activity at warmer temperatures depends on different protein aggregates that are unstable in dependence on storage and how quickly they are used after thawing, etc. Paul’s ending thought is that there are many, diverse standards that are being used for the ~45 different IN instruments being used around the world currently. He felt that there is a need to learn a lot more about molecular controls on ice nucleation simply to specify appropriate standards.
The final speaker of the session was Markus Petters, who presented on “Unsolved challenges for aerosol standards: CCN.” An important issue facing that community is that there is currently no closure between modeled instrument supersaturation and predictions from Köhler theory. Two issues that he brought up regarding test particles were that supersaturation calculations requires particle mass; however, calibration is usually done based on mobility diameter. Also, particle sphericity and void space are complicating issues. In addition, the purity of particle composition (both bulk and surfaces) appears to be very important, but difficult to establish. Addressing the current state of the science in CCN measurement, Markus pointed out that atomized, dried, and mobility-selected ammonium sulfate is the de facto aerosol standard for calibrating CCN instruments. However, methods for generation and models to compute supersaturation are not standardized and in most cases the quality of test aerosol is unclear. More work needs to be done.
Following these talks, some brief presentations by Matthew Fraund (student with Ryan Moffet) and Alla Zelynuk were made on current calibration methods for STXM and aerosol mass spectrometers, respectively.
Finally, the floor was opened up for discussion, focusing on “Where to go from here?” It was decided that a smaller committee would form to discuss the development of a white paper, which would synthesize many of the ideas presented in this session and include other measurements that were not specifically discussed.
Future Plans
Following the breakout, Jim, Pete, and Nicole met to come up with a specific plan for a white paper. The goal will be for this paper to be completed prior to AAAR. Overall, there was strong interest in our session and expectations that we will move forward. There is a strong culture of “white papers” in DOE, and the committee felt that this might be the appropriate first step. The most straightforward approach might be to invite experts to write sections pertinent to their areas of expertise. This would be a good first step in allowing us to think through the issues. It might (or might not) be a step towards writing a more substantive journal article.Moving forward, the committee developed a structure for each write-up to ensure some uniformity in text. A possibility might be (with approximate page numbers):
• Description of measurement method that section addresses (1/2 page)
• Data objectives for use by modelers (1 page)
• Summary of calibration methods that are currently used, including different approaches used by different research groups (2 pages)
• Concise discussion of measurement errors or inconsistencies that are not being addressed by existing calibration techniques, illustrated with figures where possible (2 pages)
• Intercomparisons, standards, best practices, etc., that promise to reduce uncertainties (1 page).
The proposed sections were (with suggestions for contributors and a short status report):
• Aerosol mixing state (Nicole Riemer; Nicole said “yes” on 4/2/18).
• Particle hygroscopicity: Water uptake and CCN measurements (Markus Petters; “yes”, 3/29/18).
• Aerosol size distributions and number concentrations (Chongai Kuang; Chongai said “yes” on 3/27/18).
• Sub-50 nm particle composition calibration (Jim Smith and Murray Johnston; confirmed)
• Species-resolved mass concentrations (TBD, Jose will suggest candidates)
• Quantifying precursor vapor species (Joel Thornton; Joel said “yes” on 3/29/18; Lindsay Yee has agreed to contribute).
• BC/ABS (Tim Onash, Art Sedlacek; Tim: “yes” with collaborations by Art, 3/28/18).
• INP measurement (Paul DeMott; on 3/27 Paul responded: “I am not sure what your timeline is, but I will do it, Peter. I have a few coauthors in mind to give it a balanced perspective.” In a subsequent message he indicated August 1 would be OK).