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1. Effect of aromatic ring substituents on the ability of catechol to produce brown carbon in iron( iii )-catalyzed reactions

   https://doi.org/10.1039/d0ea00007h  

   Chin, Henry ; Hopstock, Katherine S. ; Fleming, Lauren T. ; Nizkorodov, Sergey A. ; Al-Abadleh, Hind A. ( February 2021 , Environmental Science: Atmospheres)

   Our previous work demonstrated formation of highly insoluble and strongly light-absorbing organic particles in reactions between catechol or guaiacol with Fe( iii ) under pH = 3 conditions characteristic of aerosol liquid water. This work extends these measurements to reactions of Fe( iii ) with 2,4-dinitrophenol, 4-nitrocatechol, 4-methylcatechol, 1,2,4-benzenetriol, 1,2,3-benzenetriol (pyrogallol) and coniferaldehyde to better understand the mechanism of particle formation catalyzed by Fe( iii ). Particles were observed after 2 h of reactions of catechol (43 ± 1% mass yield), 1,2,4-benzenetriol (32 ± 3%), pyrogallol (27 ± 2%) and coniferaldehyde (35 ± 4%), while reactions of 2,4-dinitrophenol and 4-nitrocatechol did not produce any insoluble products. No particles were observed in reaction of 4-methylcatechol after 2 h, however, insoluble products appeared after a 24 h reaction time. Irradiation of a catechol + Fe( iii ) mixture by 405 nm light was found to reduce (but not fully suppress) the particle yield due to a competition between photodegradation and Fe( iii )-catalyzed oligomerization. Particles produced from precursors + Fe( iii ) solutions were dissolved in organic solvents and analyzed with ultra performance liquid chromatography coupled to a photodiode array spectrophotometer and a high resolution mass spectrometer. Major separated chromophores were identifiedmore »as dimeric, trimeric, and tetrameric products of precursor molecules. Purpurogallin was identified as a major reaction product of pyrogallol reaction with Fe( iii ). To test whether this chemistry can occur in more realistic atmospheric aerosols, reactions of biomass burning organic aerosol (BBOA) extracts with Fe( iii ) were also examined. Two BBOA samples collected under flaming conditions produced no particles, whereas a BBOA sample produced under smoldering conditions resulted in particle formation under both dark and 405 nm irradiation conditions. The results suggest that Fe( iii )-catalyzed chemistry can take place in aging BBOA plumes resulting from smoldering fires and make aerosol particles more light-absorbing.« less
2. Reactivity of aminophenols in forming nitrogen-containing brown carbon from iron-catalyzed reactions

   https://doi.org/10.1038/s42004-022-00732-1  

   Al-Abadleh, Hind A. ; Motaghedi, Fatemeh ; Mohammed, Wisam ; Rana, Md Sohel ; Malek, Kotiba A. ; Rastogi, Dewansh ; Asa-Awuku, Akua A. ; Guzman, Marcelo I. ( September 2022 , Communications Chemistry)

   Nitrogen-containing organic carbon (NOC) in atmospheric particles is an important class of brown carbon (BrC). Redox active NOC like aminophenols received little attention in their ability to form BrC. Here we show that iron can catalyze dark oxidative oligomerization ofo- andp-aminophenols under simulated aerosol and cloud conditions (pH 1–7, and ionic strength 0.01–1 M). Homogeneous aqueous phase reactions were conducted using soluble Fe(III), where particle growth/agglomeration were monitored using dynamic light scattering. Mass yield experiments of insoluble soot-like dark brown to black particles were as high as 40%. Hygroscopicity growth factors (κ) of these insoluble products under sub- and super-saturated conditions ranged from 0.4–0.6, higher than that of levoglucosan, a prominent proxy for biomass burning organic aerosol (BBOA). Soluble products analyzed using chromatography and mass spectrometry revealed the formation of ring coupling products ofo- andp-aminophenols and their primary oxidation products. Heterogeneous reactions of aminophenol were also conducted using Arizona Test Dust (AZTD) under simulated aging conditions, and showed clear changes to optical properties, morphology, mixing state, and chemical composition. These results highlight the important role of iron redox chemistry in BrC formation under atmospherically relevant conditions.
3. Molecular composition and photochemical lifetimes of brown carbon chromophores in biomass burning organic aerosol

   https://doi.org/10.5194/acp-20-1105-2020  

   Fleming, Lauren T. ; Lin, Peng ; Roberts, James M. ; Selimovic, Vanessa ; Yokelson, Robert ; Laskin, Julia ; Laskin, Alexander ; Nizkorodov, Sergey A. ( January 2020 , Atmospheric Chemistry and Physics)

   Abstract. To better understand the effects of wildfires on air quality andclimate, it is important to assess the occurrence of chromophoric compoundsin smoke and characterize their optical properties. This study explores themolecular composition of light-absorbing organic aerosol, or brown carbon(BrC), sampled at the Missoula Fire Sciences laboratory as a part of theFIREX Fall 2016 lab intensive. A total of 12 biomass fuels from different planttypes 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 werecollected onto Teflon filters and analyzed offline using high-performanceliquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer(HPLC–PDA–HRMS). Separated BrC chromophores were classified by theirretention times, absorption spectra, integrated absorbance in the near-UVand visible spectral range (300–700 nm), and chemical formulas from theaccurate m∕z measurements. BrC chromophores were grouped into the followingclasses and subclasses: lignin-derived products, which include lignin pyrolysisproducts; distillation products, which include coumarins and flavonoids;nitroaromatics; and polycyclic aromatic hydrocarbons (PAHs). The observedclasses and subclasses were common across most fuel types, although specific BrCchromophores varied based on plant type (gymnosperm or angiosperm) andecosystem component(s) burned. To study the stability of the observed BrCcompounds with respect to photodegradation, BBOA particlemore »samples wereirradiated directly on filters with near UV (300–400 nm) radiation, followedby extraction and HPLC–PDA–HRMS analysis. Lifetimes of individual BrCchromophores depended on the fuel type and the corresponding combustioncondition. Lignin-derived and flavonoid classes of BrC generally hadthe longest lifetimes with respect to UV photodegradation. Moreover,lifetimes for the same type of BrC chromophores varied depending on biomassfuel and combustion conditions. While individual BrC chromophoresdisappeared on a timescale of several days, the overall light absorption bythe sample persisted longer, presumably because the condensed-phasephotochemical processes converted one set of chromophores into anotherwithout complete photobleaching or from undetected BrC chromophores thatphotobleached more slowly. To model the effect of BrC on climate, it isimportant to understand the change in the overall absorption coefficientwith time. We measured the equivalent atmospheric lifetimes of the overallBrC absorption coefficient, which ranged from 10 to 41 d, with subalpinefir having the shortest lifetime and conifer canopies, i.e., juniper, havingthe longest lifetime. BrC emitted from biomass fuel loads encompassingmultiple ecosystem components (litter, shrub, canopy) had absorptionlifetimes on the lower end of the range. These results indicate thatphotobleaching of BBOA by condensed-phase photochemistry isrelatively slow. Competing chemical aging mechanisms, such as heterogeneousoxidation by OH, may be more important for controlling the rate of BrCphotobleaching in BBOA.« less
4. Laboratory studies of fresh and aged biomass burning aerosol emitted from east African biomass fuels – Part 2: Chemical properties and characterization

   https://doi.org/10.5194/acp-20-10169-2020  

   Smith, Damon M. ; Cui, Tianqu ; Fiddler, Marc N. ; Pokhrel, Rudra P. ; Surratt, Jason D. ; Bililign, Solomon ( January 2020 , Atmospheric Chemistry and Physics)

   Abstract. There are many fuels used for domestic purposes in east Africa, producing a significant atmospheric burden of the resulting aerosols, which includes biomass burning particles. However, the aerosol physicochemical properties are poorly understood. Here, the combustion of eucalyptus, acacia, and olive fuels was performed at 500 and 800 ∘C in a tube furnace, followed by immediate filter collection for fresh samples or introduction into a photochemical chamber to simulate atmospheric photochemical aging under the influence of anthropogenic emissions. The aerosol generated in the latter experiment was collected onto filters after 12 h of photochemical aging. 500 and 800 ∘C were selected to simulate smoldering and flaming combustion, respectively, and to cover a range of combustion conditions. Methanol extracts from Teflon filters were analyzed by ultra-performance liquid chromatography interfaced to both a diode array detector and an electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer (UPLC/DAD-ESI-HR-QTOFMS) to determine the light absorption properties of biomass burning organic aerosol constituents chemically characterized at the molecular level. Few chemical or UV–visible (UV: ultraviolet) differences were apparent between samples for the fuels when combusted at 800 ∘C. Differences in single-scattering albedo (SSA) between fresh samples at this temperature were attributed to compounds not captured in this analysis, with eucalyptol being one suspectedmore »missing component. For fresh combustion at 500 ∘C, many species were present; lignin pyrolysis and distillation products are more prevalent in eucalyptus, while pyrolysis products of cellulose and at least one nitro-aromatic species were more prevalent in acacia. SSA trends areconsistent with this, particularly if the absorption of those chromophoresextends to the 500–570 nm region. Upon aging, both show that resorcinolor catechol was removed to the highest degree, and both aerosol types weredominated by loss of pyrolysis and distillation products, though they differed in the specific compounds being consumed by the photochemical aging process.« less
5. A dominant contribution to light absorption by methanol-insoluble brown carbon produced in the combustion of biomass fuels typically consumed in wildland fires in the United States

   https://doi.org/10.1039/d1ea00065a  

   Atwi, Khairallah ; Cheng, Zezhen ; El Hajj, Omar ; Perrie, Charles ; Saleh, Rawad ( March 2022 , Environmental Science: Atmospheres)

   The light-absorption properties of brown carbon (BrC) are often estimated using offline, solvent-extraction methods. However, recent studies have found evidence of insoluble BrC species that are unaccounted for in solvent extraction. In this work, we produced carbonaceous aerosol particles from the combustion of three biomass fuels (pine needles, hickory twigs, and oak foliage). We utilized a combination of online and offline measurements and optical calculations to estimate the mass fractions and contribution to light absorption by methanol-soluble BrC (MSBrC), methanol-insoluble BrC (MIBrC), and elemental carbon (EC). Averaged over all experiments, the majority of the carbonaceous aerosol species were attributed to MSBrC (90% ± 5%), while MIBrC and EC constituted 9% ± 5% and 1% ± 0.5%, respectively. The BrC produced in all experiments was moderately absorbing, with an imaginary component of the refractive index ( k ) at 532 nm ranging between 0.01 and 0.05. However, the k values at 532 nm of the MSBrC (0.004 ± 0.002) and MIBrC (0.211 ± 0.113) fractions were separated by two orders of magnitude, with MSBrC categorized as weakly absorbing BrC and MIBrC as strongly absorbing BrC. Consequently, even though MSBrC constituted the majority of the aerosol mass, MIBrC had a dominant contributionmore »to light absorption at 532 nm (72% ± 11%). The findings presented in this paper provide support for previous reports of the existence of strongly absorbing, methanol-insoluble BrC species and indicate that relying on methanol extraction to characterize BrC in biomass-burning emissions would severely underestimate its absorption.« less