10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Characterizing Chemical Composition and Evolution of Brown Carbon Organic Aerosol from Primary and Photochemically-Aged Biomass Burning Emissions during 2016 FIREX Campaign
TIANQU CUI, Sophie Tomaz, Zhexi Zeng, Yuzhi Chen, Shiva Tarun, Kenneth Sexton, Shantanu Jathar, Jason Surratt, Barbara Turpin, University of North Carolina at Chapel Hill
Abstract Number: 972 Working Group: Aerosol Chemistry
Abstract Light-absorbing carbonaceous constituents of organic aerosol (OA), referred to as brown carbon (BrC), can efficiently absorb near-UV and visible radiation, potentially altering Earth’s radiative forcing and cloud formation. BrC can be emitted, formed and transformed in the atmosphere, and can contribute substantially both to OA mass and to aerosol light absorption. BrC emitted from biomass burning (BB), which is usually caused by wild and prescribed forest fires, burning of crop residues, and domestic cooking and heating, is poorly characterized. Improved molecular-level characterization of biomass burning organic aerosol (BBOA) may aid understanding of the chemical mechanisms of formation and atmospheric evolution, and help determine the impacts of BrC species on air quality and climate.
The identities, quantities, and evolution of BrC directly emitted and evolved were determined using over a hundred combustion experiments systematically performed in an exclusive indoor facility at the Fire Science Laboratory in Missoula, Montana, during the six-week Fire Influence on Regional and Global Environments Experiment (FIREX) 2016 campaign. Combusted fuels were characteristic of western North America, including Ponderosa pine, Douglas fir, Engelmann spruce, Lodgepole pine, manzanita, and chamise. Specific components of the tree/bush (e.g., canopy, litter, and duff) and mixtures of these were combusted under a variety of conditions. Two general types of combustion experiments were conducted during the campaign. For the first set, fire emissions were directly delivered through a tall exhaust stack (stack burn). In the second set, emissions were allowed to fill and mix in the large (roughly 3500 m3) combustion room for up to several hours. In addition, primary BB emissions from certain fuel types were aged in the Colorado State University (CSU) 10-m3 portable photochemical smog chamber to determine how photochemical reactions may alter primary BrC and/or produce secondary BrC in BBOA. Teflon filter and particle-into-liquid sampler (PILS) samples of PM2.5 were collected from all the experiments, and then characterized at the molecular level by: (1) ultra-performance liquid chromatography (UPLC) coupled to both diode array detection (DAD) and electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (ESI-HR-QTOFMS) operated in both positive and negative ion modes, in order to chromatographically resolve BBOA constituents, with a focus on BrC components; and (2) gas chromatography interfaced with electron ionization mass spectrometry (GC/MS) with prior trimethylsilylation, for known BBOA tracers such as levoglucosan.
Several well-established BBOA tracers and BrC species from primary and aged emissions were identified, with authentic or surrogate standards that agreed with retention times and MS/MS fragments. Currently, 37 individual BrC species such as nitro-aromatics and guaiacol/styrene derivatives, were quantified from the primary emissions from six fuels. These compounds contributed up to 12% of the PM2.5 mass. Notably, this mass fraction of BrC species varied substantially by fuel type. From the photochemical experiments, 46 BBOA species were identified that were newly formed or enhanced during photochemistry of the primary (gaseous and particle) emission mixtures initiated by hydroxyl radicals in the presence of NOx. Most of these were BrC constituents such as nitro-aromatics. Our measurements provide insights into the role of photochemical aging on the evolution of the BBOA, including BrC, and enable us to further investigate their composition, quantities, absorption, evolution and other keys properties associated with varying fuel types and burn conditions.