Breakout Summary Report

The Southeast USA is characterized by forests emitting biogenic VOCs and prescribed burning and wildfires emissions that undergo chemistry in gas-phase, aqueous particles and clouds forming secondary organic aerosols (SOA). SOA formation drives the growth of particles to cloud condensation nuclei (CCN) sizes that impact clouds and precipitation. Planned 5-year measurements have begun at the ARM Bankhead National Forest (BNF) site to understand aerosol-cloud-radiation interactions. This breakout session will focus on addressing the following science questions:

• What are the key knowledge gaps in our understanding of biogenic and biomass burning driven particle formation and growth to CCN at BNF?

• How does aqueous and cloud chemical processing of VOCs at BNF impact CCN and cloud droplets?

• How can we use various ARM AMF3 observations to determine the impacts of SOA on local convective storms, clouds and precipitation at BNF?

• What is the role of clouds and precipitation in modulating new particle formation and growth driven by SOA formation at BNF?

• How can we better coordinate detailed cloud convection, atmospheric chemistry and SOA measurements as a part of various ASR/ARM/EMSL proposals, and integrate measurements with detailed regional modeling activities planned at BNF to address the above science questions related to SOA-cloud interactions?

The Southeast (SE) USA is characterized by forests, emitting biogenic VOCs, and prescribed burning and wildfires emissions that undergo chemistry in gas-phase, aqueous particles and clouds forming secondary organic aerosols (SOA). SOA are often the major components of submicron aerosol mass, especially in forested locations. SOA formation drives the growth of particles to cloud condensation nuclei (CCN) sizes that impact clouds and precipitation. This session discussed how the 5-year ARM measurements at the Bankhead National Forest (BNF) site at Alabama in SE USA could be used to address key science gaps related to SOA-cloud-radiation interactions. Discussions also focused on how baseline BNF measurements could be supplemented with more detailed gas- and particle-phase chemistry measurements needed to understand SOA formation, and how models can used as tools to understand processes such as new particle formation and growth to CCN sizes, the dynamically varying properties of SOA like volatility and hygroscopicity that govern their lifetimes and CCN ability, the role of leaf and soil chemistry in emissions of biogenic SOA precursors, and interactions of cloud SOA chemistry with turbulence.

Discussions touched on the following key aspects:

• Biogenic VOC (BVOC) Measurements: These measurements are critical as BVOCs are precursors to SOA formation and change the oxidizing capacity of the atmosphere (including oxidants like ozone and OH radicals). It was recognized that long-term BVOC measurements are missing at the BNF site. There is a need to measure variabilities in BVOC emissions at a range of spatial and temporal scales: interannual to capture drought and plant stressor variabilities, seasonal and diurnal to capture variabilities in emissions and chemistry, and spatial to capture horizontal and vertical variations of SOA within and above canopy at the BNF site. Several groups indicated the critical need for measuring BVOCs and their oxidation products: Extremely low volatility (ELVOCs), semi-volatile and Intermediate volatility (SVOC/IVOCs) in both gas- and particle phases to understand their role in SOA formation and CCN. Participants discussed whether currently planned TRAVIS/FICUS measurements of BVOCs using collected cartridges from TBS systems are quantitative with the needed detection limits.

• Impact of Stressors: Heat waves, wildfires and insect infestation like the southern pine beetle outbreak are stressors that might impact BVOC emissions, and BVOC composition. For example, acyclic terpenes might be elevated compared to cyclic terpenes in stressed plant emissions compared to healthy plants. Understanding the effects of ecosystem stressors on BVOC and SOA needs detailed measurements including isomeric composition of BVOCs.

• LIDAR Mapping: Lidars could help connect the vertical profile of SOA and aerosols with cloud convective transport and cloud chemistry. Discussions focused on leveraging lidar measurements from ARM at BNF site and recommended working with NEON AOP to supplement lidar mapping over the site.

• Sulfuric acid (H2SO4) & NPF Relationship: There was a lot of interest in H2SO4 measurements, particularly with respect to their role in the formation of initial molecular clusters relevant to new particle formation (NPF). The effects of meteorology (like clear skies and high downwelling solar radiation) versus the role of emissions on NPF was also discussed.

• Application and Improvement of Models: Models at 1D (canopy scale) and 3D regional scales (like WRF-Chem, E3SM) could greatly benefit from detailed BVOC emissions and composition profile measurements, aerosol volatility and NPF measurements. These models can also guide critical measurement needs to understand the role of SOA in CCN. For example, detailed regional WRF-Chem simulations have identified the need to make highly sensitive and low detection limit measurements of H2SO4, NOx, BVOCs and their oxidation products (ELVOCs) upto pptv levels to accurately predict the size distribution of aerosols and their growth to CCN.

Participants recognized the critical need for detailed measurements of gas- and particle phase BVOCs (e.g. using GC-MS, CIMS, PTR-MS) and their oxidation products to understand SOA formation and CCN at the BNF site. This is a critical knowledge gap that needs to be filled through measurements and collaborations among different guest instrument PIs. A consensus emerged on measuring the size distribution of aerosols down to 1 nm sizes to understand NPF. There was widespread recognition of the need for unified scheduling (IOPs and guest instruments) and deployment timelines to support community coordination. Such coordination will increase the scientific impact through deployment of detailed chemistry instruments that characterize the composition and volatilities of SOA precursors at the BNF site and supplemental sites around BNF to capture the above mentioned spatial and temporal heterogeneities.

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Action Items: The following are some recommended action items mostly focused on deploying detailed SOA chemistry instruments, coordinating IOPs among ASR and EMSL FICUS PIs, and displaying data from ARM measurements for rapid outreach and scientific collaborations:

• Instrument Upgrades & Deployments:
  • Rollout of HTDMA as indicated.
  • Consideration of adopting an ARM GCMS (preferred over PTRMS due to cost considerations). Long-term GCMS measurements of speciated BVOCs are critical for process-based understanding of SOA chemistry and CCN at various spatial and temporal scales.

• Enhanced Data Displays:
  • Development of a campaign timeline-type display to capture biogenic aerosol sources and new particle formation and growth.

• Scheduling Improvements:
  • Organize a unified calendar that consolidates all planned IOP schedules and instrument calibration schedules.
  • Establish a fixed annual IOP schedule to capture different seasonal processes, e.g., winter IOP, “leaf-in”/March IOP, leaf-out/August IOP, biomass burning IOP.