Breakout Summary Report


ARM/ASR User and PI Meeting

Coarse Mode Atmospheric Aerosols: Size Distributions and Composition Measurements and Modeling
24 October 2022
4:00 PM - 6:00 PM
Maria Zawadowicz, Chongai Kuang, Yan Feng

Breakout Description

Measurement and modeling of atmospheric super-micron aerosols, or the coarse aerosol mode, are challenging due to limits in instrumentation, lack of observations, and complex processes involved. Nevertheless, super-micron aerosols have significant effects on aerosol-radiation interactions and formation of both warm (“giant” CCN) and ice clouds. Common coarse- mode aerosols--dust, sea salt, and bioaerosols-- participate in important atmospheric processes, such as CCN formation and ice nucleation, and they tend to be released into the atmosphere from surface sources, making their characterization close to the ground especially important. This session will review the ongoing modeling, measurement, and experimental activities within ARM/ASR related to coarse aerosol mode and identify the critical gaps in capabilities. While ARM is now providing coarse-mode size distributions in addition to the sub-micrometer sizing instruments such as the SMPS, there are questions about the detection limit of upper size and closure of aerosol mass in merging different measurements. The aerosol speciation measurements currently cover only the sub-micrometer range with the ACSM. Coarse-mode aerosol speciation is challenging with existing ARM instruments, because non-refractory compositions that dominate in the coarse mode do not thermally desorb at 600°C and are therefore inaccessible to the ACSM even with an extended-range PM2.5 inlet. The main goal of this session is to discuss the needs and strategy for developing observational constraints of coarse-mode aerosols for improving the model representation of aerosol-related processes in climate simulations. The first part of the session will focus on ongoing ARM- and ASR-sponsored work in atmospheric bioaerosol characterization. The second part will focus on modeling and experimental studies of mineral dust and sea spray aerosols.

Main Discussion

The Coarse-Mode Atmospheric Aerosols session was aimed at showcasing ongoing ASR research on coarse-mode aerosols and discussing measurement and modeling needs to advance coarse-mode aerosol science. The session included four presentations: (1) Current ARM coarse-mode capabilities presented by Chongai Kuang from BNL, (2) Probing bioaerosol to improve climate models presented by Swarup China from PNNL, (3) Inferring and simulating coarse aerosols from primary biological aerosol particles presented by Allison Steiner from University of Michigan, and (4) Dust modeling in the E3SM presented by Yan Feng from ANL. Each presentation was followed by a discussion period. Main discussion points brought up by the participants included: (1) (co)-deployment of APS and OPC in the AOS, including advantages and disadvantages to both; (2) best practices and analytical methods in measuring coarse-mode composition, including bioaerosol and dust; (3) measurement needs to advance modeling of bioaerosol and dust. Overall, the participants agreed that measurements of bioaerosols and dust (ideally including both concentrations and speciation) are important for understanding the impact of coarse-mode aerosols on climate.

In the first part of the session, an overview presented coarse-mode measurement techniques (e.g., aerodynamic vs. optical scattering sizing), measurement challenges (e.g., instrument calibration protocols, inlet sampling configuration, drying requirements, and transmission efficiencies), operational challenges (e.g., instrument stability and durability), and ongoing studies based on a synthesis of laboratory experiments and long-term field measurement campaigns and fixed sites (e.g., TRACER and SGP) to inter-compare the TSI aerodynamic particle sizer (APS, aerodynamic sizing) and the GRIMM optical particle counter (OPC, optical scattering sizing) measurement platforms. Key findings included: a preliminary AOS inlet transmission efficiency analysis with a lower and upper Dp50 of ~0.016 and ~6 microns; comparable sizing and counting of size-resolved calibration aerosol (700 nm PSL) between the APS and OPC; and better OPC-SMPS overlap vs. APS-SMPS overlap of number size distributions. While the OPC has several advantages compared to the APS (e.g., lower cost, lower operational complexity, greater apparent stability/durability), there are known coarse-mode sizing challenges associated with the OPC due to optical sizing uncertainties regarding ambient particle shape and refractive index.  

The session also included a presentation of EMSL resources for quantifying and characterizing atmospheric bioaerosol. Advantages and disadvantages of both online and offline methods were discussed. The online methods included the wideband integrated bioaerosol sensor (WIBS), and offline methods included single-particle micro-spectroscopy analysis and bulk high-resolution mass spectrometry. Examples of bioaerosol speciation and quantification, including the use of novel machine learning methods, were presented for all three.

Finally, the modeling talks gave insight on ongoing efforts and challenges in representation of complex bioaerosol and dust sources and compositions in global models, including E3SM.

Key Findings

  • Update on ongoing work by mentors on instrument evaluation and comparison of APS vs. OPC for coarse-mode size distribution measurements in the AOS. Currently the trade-offs of both systems suggest that their co-deployment informed by science drivers would be optimal.

  • Analytical challenges in differentiating between bioaerosol/dust/other carbonaceous material. Combining various analytical techniques is essential for adequately characterizing coarse-mode aerosol composition.

  • Measurements of dust and bioaerosol concentration and composition are necessary to better constrain the models, but are not currently performed by ARM.







Future Plans

Due to considerable interest in this session, we are hoping to have it again at upcoming ARM/ASR joint meetings.

Action Items