Breakout Summary Report

ARM/ASR User and PI Meeting

10 - 13 June 2019

Breakout Description

The uncertainty in estimating aerosol light absorption remains unacceptably large for climate applications. It is crucial using the combined observational and modeling approaches to characterize the degree of aerosol absorption for global and regional constraints. Of particular interest are light-absorbing organic aerosols, also known as brown carbon (BrC). BrC characterization is proving to be uniquely challenging because it exhibits a wide range of absorption efficiencies that depend on the source types and vary during aging. Additionally, when coated by BrC, the enhanced absorption of BC can be quite different from that with transparent coatings.

In this breakout session, we intend to review recent progress in laboratory and field observations regarding the fundamental chemical, microphysical, hygroscopic, and optical properties of the light-absorbing aerosols. We also invite discussions on model representations of BrC formation and photochemical aging at different scales, source attribution of aerosol optical properties (i.e., using AERONET), and validation of modeling against observations. The goal of this session is to develop a road map toward improved constraints on BrC and BC light absorption from observations and modeling. This session is coordinated with a session focusing on absorbing aerosols and their interactions with clouds led by Paquita Zuidema, Allison Aiken, and Art Sedlacek.

Agenda:

Short presentations (10:30am-11:40am)

• Composition and Chemical Identity of BrC — Alex Laskin


• Tar Balls: An Important Class of BrC — A. Sedlacek


• Mie Analysis of BB Aerosol Optical Observations and Towards Measurements of RH effect on Absorption — M. Dubey


• Should MAC be the Only Item on the menu? — E. Lewis


• Temporal and special variability and properties of BB aerosols sampled during HI-SCALE — Alla Zelenyuk-Imre


• Long-range transported North American wildfire aerosols observed in marine boundary layer of eastern North Atlantic — J. Wang


• Atmospheric processing of wildfire plumes – aging of BBOA and change of aerosol light absorption — Qi Zhang


• Characterizing Biomass Burning Plumes in the Western US: Mount Bachelor Observatory Measurements — A. Sedlacek


• SP2 laser-induced Charring of BrC — T. Onasch


• Challenges in modeling BrC and aerosol absorption — Y. Feng

Discussions (11:40am-12:30pm)

• Where do we stand now in laboratory studies and field observations?


• What are the major gaps between observed and modeled aerosol absorption?

Main Discussion

We opened the session with short presentations, reviewing recent progress from laboratory and field observations focusing on dynamic absorption of BrC and BC, i.e., due to coating and aging. The presentations were followed by about an hour discussion. We discussed four main topics:

1. First several talks focused on BrC composition and chemical identity. Alex Laskin’s presentation showed that about 50% of the OA absorption could be attributed to individual BrC chromophores identified in lab experiments. Consistent with this finding, Qi Zhang reported that composition measurements conducted at MBO found similar chromophores such as nitro-phenols, although only by a small fraction, as MBO observes aged biomass burning aerosols after 6hrs up to 2 days. There was a lengthy discussion about tar balls following Art Sedlacek’s talk where less diversity in BrC absorption properties was suggested since some wildfires contain a large mass fraction of tar balls. Finally, Tim Onasch presented evidence that the SP2 can bring about charring of some types of BrC that can lead to misclassification of BC (from BrC) and that this observation suggests the possibility of measuring some BrC types with the SP2.


2. A second topic of discussion was about the water uptake by absorbing carbon aerosols and the enhancement in aerosol absorption. From the experimental perspective, Dubey showed how this humidity effect could be measured and uncertainty associated. Ernie Lewis discussed about theoretical calculations of humidity dependency of the mass absorption coefficient by carbon aerosols. Interestingly, both research efforts drew opposite conclusions, with Dubey’s work suggesting a pronounced enhancement and Lewis suggesting a more muted effect (until the RH approached ~ 95%). More work in this area is needed.


3. Another set of presentation topics focused on the variability in absorption properties of aged biomass burning aerosols observed far away from the sources. Presentations given by Jian Wang, Alla Zelenyuk-Imre and Art Sedlacek (for Dan Jaffe’s group) on field observations from ENA and SGP sites and MBO all suggested significant changes in BB aerosol properties.


4. Finally, Yan Feng discussed the main challenges for modelers to connect the experimental findings to model improvement in large-scale models especially with E3SM, and her recent global model study of BrC bleaching effect. That stimulates discussions on what observations are needed to evaluate and constrain the models, and how we should prioritize them.

Key Findings

• Recent measurements suggest that absorption properties of tar balls are not as diverse as in the previous studies. This could pave the way for examining the importance of tar balls in large-scale models and their radiative forcing. It may be worth investigating first if tar balls should be treated similarly as other BrC types or externally mixed class.


• Aged biomass burning aerosols are not as absorbing as observed in the previous near-field measurements. This highlights the importance of including the BrC bleaching in the models. Since we have shown in a recent global model study that BrC radiative forcings are very sensitive to the bleaching effect considered, it is critical to evaluate and improve the model parametrization constrained by observations.


• In addition to radiative heating effects, absorbing biomass burning aerosols are also important cloud condensation nuclei in remote marine boundary layer and contribute to total particle number/mass concentrations in the free troposphere. We need combined measurements of aerosol microphysical, size, mixing state, and optical properties of absorbing aerosols.

Issues

Needs

Decisions

Future Plans

• A compilation of recent BrC measurements will be developed and reviewed. They will be used to constrain and improve the current BrC parameterization in global climate models.


• Cloud processing of aerosols and impact on aerosol absorption will be investigated.


• Humidity effect on aerosol absorption will be conducted in laboratory studies and field observations.

Action Items