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: January 2020

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.