Breakout Summary Report


ARM/ASR User and PI Meeting

Vertical Distribution of Aerosol Properties
26 October 2022
4:15 PM - 6:15 PM
Jennifer Comstock, Swarup China, John Shilling, and Jessie Creamean

Breakout Description

Knowledge of the vertical distributions, specifically chemical composition and cloud nucleating properties, of aerosols in the boundary layer is scarce, yet important for accurately estimating aerosol climatic effects. Moreover, significant uncertainties in these data are encountered when trying to predict the vertical distribution of aerosols in climate models. In this session we will discuss recent measurements and modeling efforts to understand the vertical distribution of aerosol properties. We will also discuss how ground-, tower-, or aerial-based measurements can be used to advance understanding of the vertical distribution of boundary-layer aerosol chemical and physical properties and how DOE’s ARM and EMSL User Facilities can work together as part of the EMSL-ARM FICUS (Facilities Integrating Collaborations for User Science) call and receive community feedback on focused topics for future proposal calls.

Main Discussion

The goal of this session was to highlight recent advancements in measuring the vertical distribution of aerosol properties, including ice nucleating particles (INP), chemical composition, and particle size distribution. Presentations and discussion were designed to identify measurement gaps and help inform topical areas for the next ARM/EMSL joint Facilities Integrating Collaborations for User Science (FICUS) call for proposals. There were three invited presentations and six contributed flash talks. The topical areas covered vertical distribution of aerosol composition, abundance of biological particles, and vertical distribution of cloud condensation nuclei (CCN) and INP. Most of the talks used tethered balloon system (TBS) measurements at one or more ARM sites and several used EMSL capabilities to determine detailed information on the chemical composition, phase (viscosity), and internal mixing state of atmospheric particles.

Key Findings

Vertically resolved measurements at SGP provide important information regarding particle number, size, and composition below cloud base, which are sometimes significantly different from surface-based in situ measurements. Results presented in the session show that measurements indicate new particle formation (NPF) occurs near the surface and particles mix up through the atmosphere via boundary-layer turbulence. NPF also occurs near the top of the boundary layer via a residual layer.
Two presentations covered different aspects of biological particles. First there was a detailed summary of the size and composition of various plant, soil, and pollen particles. Emissions varied with plant life cycle and were typically sized in the accumulation mode though large particles can rupture to form many small particles. Parameterization of these rupture mechanisms are now being tested in WRF-CHEM and show that pollen can be mixed up to 10 km in deep convection.
Measurements using the STAC instrument on board the TBS at SGP were also presented. The STAC is an impactor that collects samples in the field that are analyzed at the EMSL User Facility using nano-spray desorption electrospray ionization (nano-DESI) high-resolution mass spectrometry and electron microscopy. These analyses provide vertically resolved molecular composition of organic aerosols. Notable results were the presence of low-volatility species, such as CHNO, found at the surface and aloft. STAC measurements were also used to examine the aerosol phase state. Results show that phase state varied with altitude, with semi-solid and liquid states dominating at 200 m and solid and semi-solid dominating at 100 and 300 m.
INPs were discussed in several presentations. It was hypothesized that biological particles have characteristics that may make them conducive to ice nucleation. There is uncertainty due to the various phases and numerous types of biological particles measured. INP measurements on the TBS at SAIL showed little variability with height at the SAIL site, whereas SGP showed a high degree of vertical variability in composition between the surface and aloft and significantly varied day to day. Dust was the most common INP on one day, with sulfates and carbonaceous particles dominating on another. Further investigation of particle composition both aloft and at the surface is planned to better understand the relationship between surface and elevated INP measurements.
ARM is now providing two new data products for cloud formation studies. First, Retrieved Number concentration of Cloud Condensation Nuclei (RNCCN) provides vertical distribution of CCN using a combination of Raman lidar data and CCN concentration from the CCN particle counter. RNCCN data are now available at SGP during the HI-SCALE campaign and include CCN concentration at 7 different supersaturations. The second product is filter-based INP measurements at the surface and on the TBS. Routine surface-based measurements are available at SGP, Oliktok, and the SAIL and TRACER field campaigns. TBS-based INP observations at SGP and SAIL will be available soon. Time-resolved INP are collected on a filter and analyzed offline using an ice nucleation spectrometer.


There were some technical issues in the breakout room due to the hybrid format. First, the room was not unlocked until 10 minutes before the session started, making it difficult to get set up in time for the session. Second, the speaker was not working so online presenters (we had 4 virtual speakers) could not be heard. Eventually this got fixed but cut into the session time. Working from a personal laptop was not ideal because there was too much juggling back and forth and it would have been easier if the person in the back could have loaded the presentations as was done in the plenary session.


During the discussion, interest was expressed in the following areas:
● Integrating platforms or deploying several platforms simultaneously (i.e., TBS + ArcticShark or other small UAS).
● Improved measurements of size-resolved composition.
● Need information regarding precursors to better understand mixtures.
● Need strategies for how to better integrate TBS measurements with models.
● Vertical distribution of aerosol mixing state.
● Measurements of VOCs to help understand the boundary layer.
● Better understanding of how recent measurements of aerosol classifications match up with model parameterizations and a need to standardize classification methods.
● Can we measure wet scavenging?
● Qualitative understanding of layering in the atmosphere would be helpful for interpreting model results.
● Measurements of boundary-layer dynamics needed for comparison to LES models
● LASSO needs vertical distribution of number size distribution at cloud base and composition/mixing state.
● There was interest in deploying TBS at NSA and ENA.



Future Plans

● Revisiting and improving existing parameterizations based on new measurements of aerosols and INP classifications.
● Further investigation of particle composition both aloft and at the surface to better understand the relationship between surface and elevated INP measurements.

Action Items