Breakout Summary Report

ARM/ASR User and PI Meeting

13 - 17 March 2017

Session Title: Biomass Burning Aerosol Breakout
Session Date: 13 March 2017
Session Time: 1:30 PM - 3:30 PM
Number of Attendees: 100
Summary Authors: Tim Onasch and Art Sedlacek

Breakout Description

Aerosols from biomass burning are recognized to perturb Earth’s climate through direct effects (e.g., scattering and absorption of incoming shortwave radiation, surface albedo/snow modifications), semi-direct effects (e.g., cloud modifications due to absorption), and indirect effects (e.g., influencing cloud formation, extent, and precipitation). Biomass and biofuel burning are projected to increase due to droughts accompanying increasing global temperatures and increased energy requirements and therefore continue to command our attention since their radiative forcing contribution will likely continue to increase in the foreseeable future. The goals of this breakout session are to identify gaps in our understanding of the role of biomass burning (BB) in climate change and to continue to foster collaborative research within the DOE ASR/ARM community (and beyond), tying together ARM infrastructure applications (e.g., G-1, AOS), ASR field and laboratory projects, and ARM/ASR regional and global modeling. Targeted presentation topics include characterizing the chemical, physical, and optical properties of biomass burning (wildfires, prescribed burns, and biofuels) emissions in the near-to-far fields (see related follow-on breakout session: Bringing Remote Sensing and In Situ Measurements Together for Constraining Aerosol Radiative Forcing), aerosol-cloud interactions and effects, and modelers focused on quantifying biomass burning radiative impacts and climate forcing.
Participants: The planned agenda includes short PI presentations with time for questions and discussions. We solicit PI presentations for inclusion, with emphasis on helping to identifying gaps in knowledge and on proposing forward-thinking research directions.

Main Discussion

The presentations in this breakout session focused on the science of biomass burning emissions, including their (a) chemical and physical characteristics, (b) evolution during transport through the atmosphere, and (c) effects on clouds and climate. We hosted nine presentations from DOE-funded principal investigators, ranging from field studies to modeling results. It is apparent that DOE is a leader in biomass burning research with the (1) 2013 Biomass Burning Observation Project (BBOP) successfully characterizing the near-field emissions from wildland fires and agricultural burns in unprecedented detail, (2) the concurrent Layered Atlantic Smoke Interactions with Clouds (LASIC) study providing preliminary data on the down-mixing of biomass burning plumes into the stratus clouds off the west coast of Africa, and (3) the ARM-supported modeling work on the effects of absorbing biomass burning particles on atmospheric radiation and on the effects of biomass burning plumes on clouds.
Specific science focus areas in biomass burning-related research that were mentioned during this breakout session included the need to better: (1) Quantify BB emissions (wildfire and agricultural burn) including bottom‐up and top‐down approaches; (2) Quantify BB emissions during nighttime; (3) Represent the highly variable (spatially and temporally) components of emissions and understand fresh plume dynamics; (4) Characterize chemistry of emissions and understand chemical evolution and SOA in downwind plume, and; (5) Understand smoke–cloud interactions.

Key Findings

1. Biomass burning emissions from forest fires (wild, prescribed), agricultural burning, and residential heating and cooking strongly influence ambient particulate mass loading and composition and are important sources of gas and particle-phase pollutants.
• They are ubiquitous and increasing globally.
• They impact air quality, health, ecosystems, and climate.
2. Gas and particulate components in biomass burning emissions exhibit large variability and vary dynamically based on:
• Combustion conditions (i.e., modified combustion efficiency) and fuel type, which vary dramatically even within one fire and across spatially different fires, and;
• Atmospheric processing during transport, including SOA formation, transforms biomass burning aerosols from a positive radiative forcing agent (warming) to a negative forcing agent (cooling).
3. Biomass burning emissions include absorbing particles (i.e., brown carbon) and tar balls:
• Tar balls are observed to be weakly absorbing secondary aerosol, and;
• Brown carbon, when included in global models, can significantly impact the atmospheric radiation balance and help close the gap between models and AAOD measurements.
4. Biomass burning plumes may have significant effects on cloud properties:
• LASIC is the DOE field study focused on the specific intersection of African biomass burning plumes and offshore stratus clouds, and;
• ORACLES is the NASA field study coordinating with LASIC with the objective of furthering the development of models of cloud-biomass burning emissions interactions.

Needs

1. Instrument development support from DOE to enable better measurements to be collected in the field to further our scientific understanding of biomass burning emissions. Some specific instrument-related issues include:
• Flight restrictions when sampling near-field burns;
• Instrument time responses on aircraft passing in and out of small plumes;
• Particle probe coincidence limits that can constrain the ability to sample both background and in-plume conditions accurately;
• Filter-based light absorption measurements, which have filter-based biases that cannot be fully removed from the measurements, and;
• SP-AMS measurements of BC coating, which are extremely important to measure but were compromised during BBOP by the need to operate dual vaporizers.
2. Further field observations.
• If tar balls are secondary, what is the role of photochemistry in their production? Is there a pronounced diurnal cycle in the production of TBs? How variable is the tar ball mass fraction in wildland fires? No tar balls are observed in agricultural burns, but they are observed in most wildland fires studied during BBOP. Is there a fuel source dependence on the production of tar balls?
• Characterization of the emission at the source and sample regional and source-specific differences.
• Capture dynamic processes – internal, external mixing, photochemistry, etc.
• Closure studies. Need for ambient aerosol in situ measurements – ground (statistics), air (vertical profiles), and improving the link between them.
• Better constraint on absorption (including brown carbon) and overall optical properties.
3. Laboratory experiments to augment field observations.
• How representative are laboratory‐generated TBs of those observed in the field?
• What is the origin of the imaginary component of the TB refractive index?
• What is the refractory character of TBs? What is its mpact on detection by other techniques?
• Controlled study of field observations, e.g,. SAAS, BC, FLAME, etc.
• Collaborative studies hosted at DOE facilities, e.g., PNNL environmental chamber
4. Model development and testing
• What is the best-estimate SSA? Can we explain it? Does the SSA evolve with time?
• How well can we describe aerosol-cloud vertical structure, and how do models compare?
• How do we best set up process model studies using the ARM data?
• Evaluation of the primary BrC parametrization with recent field data sets -- link MCE‐based optical properties with BC‐to‐OA ratio, compare near‐source versus downwind data, tar balls, etc.
• Secondary BrC formation from low‐yield SOA precursors.
• Background conditions are very important to assessing BBA effects. What are BL aerosol and cloud properties prior to direct BBA plume contact? Use near‐coast in situ measurements.
• Different LES models give different results re BBA effects on the SCT, especially CF. What is the evolution of CF along BL trajectories? Use airborne imagery.
• Aerosol indirect and semi‐direct effects on SW and LW fluxes are individually substantial, but also commonly offsetting. Expect complex interplay of radiative effects from ΔNc, CF, BL depth, drizzle. Diurnal cycle a further complication. ORACLES observations will be foundation for next round of modeling.