Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol
Authors: Fleming Lauren T. , Lin Peng , Roberts James M. , Selimovic Vanessa , Yokelson Robert , Laskin Julia , Laskin Alexander , Nizkorodov Sergey A. 
Published: June 2019

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. Twelve 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 particles were collected onto Teflon filters and analyzed offline using high performance liquid chromatography/ photodiode array/high resolution mass spectrometry (HPLC/PDA/HRMS). Separated BrC chromophores were classified by their retention times, absorption spectra, integrated PDA 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, which includes lignin pyrolysis products; distillation products, which include coumarins and flavonoids; nitroaromatics; and polycyclic aromatic hydrocarbons (PAHs). The observed classes/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, biomass burning organic aerosol (BBOA) particle samples were irradiated directly on filters with near UV (300–400 nm) radiation, followed by extraction and the HPLC/PDA/HRMS analysis. Lifetimes of individual BrC chromophores depended on the fuel type and the corresponding combustion conditions, but 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 photolysis 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 absorption coefficient with time. We measured the equivalent atmospheric lifetimes of the overall BrC absorption coefficient which ranged from 10 to 41 days, with subalpine fir having the shortest lifetime, and conifer canopies having the longest. 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 by atmospheric photolysis is relatively slow. Other chemical aging mechanisms, such as heterogeneous oxidation by OH, may be more important for BrC degradation than photolysis for predicting the decay of BBOA BrC absorption in models.