Abstract
Abstract. To better understand the effects of wildfires on air quality and climate, it is important to assess the occurrence of chromophoric compounds in smoke and characterize their optical properties. This study explores the molecular composition of light-absorbing organic aerosol, or brown carbon (BrC), sampled at the Missoula Fire Sciences laboratory as a part of the FIREX Fall 2016 lab intensive. A total of 12 biomass fuels from different plant types were tested, including gymnosperm (coniferous) and angiosperm (flowering) plants and different ecosystem components such as duff, litter, and canopy. Emitted biomass burning organic aerosol (BBOA) particles were collected onto Teflon filters and analyzed offline using high-performance liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer (HPLC–PDA–HRMS). Separated BrC chromophores were classified by their retention times, absorption spectra, integrated absorbance in the near-UV and visible spectral range (300–700 nm), and chemical formulas from the accurate m∕z measurements. BrC chromophores were grouped into the following classes and subclasses: lignin-derived products, which include lignin pyrolysis products; distillation products, which include coumarins and flavonoids; nitroaromatics; and polycyclic aromatic hydrocarbons (PAHs). The observed classes and subclasses were common across most fuel types, although specific BrC chromophores varied based on plant type (gymnosperm or angiosperm) and ecosystem component(s) burned. To study the stability of the observed BrC compounds with respect to photodegradation, BBOA particle samples were irradiated directly on filters with near UV (300–400 nm) radiation, followed by extraction and HPLC–PDA–HRMS analysis. Lifetimes of individual BrC chromophores depended on the fuel type and the corresponding combustion condition. Lignin-derived and flavonoid classes of BrC generally had the longest lifetimes with respect to UV photodegradation. Moreover, lifetimes for the same type of BrC chromophores varied depending on biomass fuel and combustion conditions. While individual BrC chromophores disappeared on a timescale of several days, the overall light absorption by the sample persisted longer, presumably because the condensed-phase photochemical processes converted one set of chromophores into another without complete photobleaching or from undetected BrC chromophores that photobleached more slowly. To model the effect of BrC on climate, it is important to understand the change in the overall absorption coefficient with time. We measured the equivalent atmospheric lifetimes of the overall BrC absorption coefficient, which ranged from 10 to 41 d, with subalpine fir having the shortest lifetime and conifer canopies, i.e., juniper, having the longest lifetime. BrC emitted from biomass fuel loads encompassing multiple ecosystem components (litter, shrub, canopy) had absorption lifetimes on the lower end of the range. These results indicate that photobleaching of BBOA by condensed-phase photochemistry is relatively slow. Competing chemical aging mechanisms, such as heterogeneous oxidation by OH, may be more important for controlling the rate of BrC photobleaching in BBOA.
Highlights
Forests have naturally occurring wildfire cycles that maintain the forest ecosystem, but global climate change is altering the cycles with unpredictable consequences (Shvidenko and Schepaschenko, 2013; Weber and Stocks, 1998)
One of the first studies incorporating brown carbon (BrC) into models was by Feng et al (2013), who found that in areas where primary BrC emissions are high the absorbing component of biomass burning organic aerosol (BBOA) can dominate over the scattering component, switching net radiative forcing by organic aerosols from negative to positive at the top of the atmosphere
This study explored the diversity in the molecular composition of BrC chromophores found in BBOA samples generated by burning forest fire fuels, and examined how the chemical composition and optical properties change during UV irradiation of BrC materials in the absence of gas-phase oxidants
Summary
Forests have naturally occurring wildfire cycles that maintain the forest ecosystem, but global climate change is altering the cycles with unpredictable consequences (Shvidenko and Schepaschenko, 2013; Weber and Stocks, 1998). In addition to the impacts of wildfires on ecosystems, biomass burning plumes have pronounced effects on atmospheric chemistry and climate (Boulanger et al, 2014; Moriondo et al, 2006; Shvidenko and Schepaschenko, 2013; Wotton et al, 2010; Wotton and Flannigan, 1993). One of the first studies incorporating BrC into models was by Feng et al (2013), who found that in areas where primary BrC emissions are high the absorbing component of BBOA can dominate over the scattering component, switching net radiative forcing by organic aerosols from negative (cooling) to positive (warming) at the top of the atmosphere. For a comprehensive understanding, we should consider the diversity of BrC, spanning nonpolar to polar molecules, and BBOA from a range of sources
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call