Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol

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.

Biomass-burning-derived particles from a wide variety of fuels: Part 2: Effects of photochemical aging on particle optical and chemical properties

“…It is possible that solvent extraction alters the BrC absorptivity in some OA-composition-dependent manner, leading to an apparent time-dependent decay in the BrC absorption. Consistent with this suggestion, Fleming et al (2020) reported that (i) the total absorptivity lifetime of BB particles photolytically aged on a filter greatly exceeded those of individual chromophores (>10 days versus <2 days), and (ii) the apparent changes in total absorption depended on whether the absorption was measured for particles on the filter versus for solution extracts. Nonetheless, this suggestion remains speculative as direct, quantitative comparisons between BrC absorption measured for suspended particles https://doi.org/10.5194/acp-2020-137 Preprint.…”

“…) Lee et al (2014). photolytically aged water-soluble extracts of secondary BrC produced from photooxidation of naphthalene and observed a continuous decrease in the absorptivity, but again with a time constant that was likely too long to have influenced our experiments Fleming et al (2020). photolytically aged particles from 12 of the FIREX burns that were collected on Teflon filters.…”

Molecular Characterization of Water‐Soluble Brown Carbon Chromophores in Beijing, China

“…In these studies, the importance of nitrated and aromatic aerosol components for near UV light absorption has been highlighted. Several classes and subclasses of BrC chromophores have been identified, including nitroaromatics, polycyclic aromatic hydrocarbon (PAHs) and their derivatives, and lignin-derived products (Fleming et al, 2018(Fleming et al, , 2020Lin et al, 2018). Nitroaromatic compounds especially nitrophenols and nitrocatechols have been recognized as the prominent classes of BrC compounds, especially in biomass burning source samples and biomass burning influenced ambient aerosol and cloud water samples.…”

Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke

“…Moreover, Saturno et al (2018b) have shown that the brown carbon (BrC) contribution to total absorption becomes increasingly important towards the end of the dry season. This is a further indication of the predominance of regional fires towards the later BB season, given that BrC is quickly photodegraded in the atmosphere after emission, with a typical lifetime of a few days to weeks (Fleming et al, 2020;Wong et al, 2019), comparable to the transport times of African BB emissions across the Atlantic.…”

“…During their typical atmospheric lifetime of several days, BC particles undergo photochemical aging, creating internally mixed BC aerosols via the condensation of low and semi-volatile compounds, coagulation, and cloud processing (Bond et al, 2013;Cubison et al, 2011;Konovalov et al, 2017Konovalov et al, , 2019Schwarz et al, 2008;Willis et al, 2016). The formation of non-absorbing or semi-transparent coatings on the BC cores changes the particles' optical properties (Fuller et al, 1999;Moffet and Prather, 2009;Pokhrel et al, 2017;Schnaiter, 2005;Zhang et al, 2015) as well as their ability to act as cloud condensation nuclei (CCN) (Laborde et al, 2013;Liu et al, 2017;Tritscher et al, 2011), which influences their atmospheric transport and lifetime.…”