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1. 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
2. Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation

   https://doi.org/10.3390/atmos11010062  

   Sarpong, Emmanuel ; Smith, Damon ; Pokhrel, Rudra ; Fiddler, Marc N. ; Bililign, Solomon ( January 2020 , Atmosphere)

   Biomass burning (BB) aerosols contribute to climate forcing, but much is still unknown about the extent of this forcing, owing partially to the high level of uncertainty regarding BB aerosol optical properties. A key optical parameter is the refractive index (RI), which influences the absorbing and scattering properties of aerosols. This quantity is not measured directly, but it is obtained by fitting the measured scattering cross section and extinction cross section to a theoretical model using the RI as a fitting parameter. We used the Rayleigh–Debye–Gans (RDG) approximation to retrieve the complex RI of freshly emitted BB aerosol from two fuels (eucalyptus and olive) from Africa in the spectral range of 500–580 nm. Experimental measurements were carried out using cavity ring-down spectroscopy to measure extinction over the range of wavelengths of 500–580 nm and nephelometry to measure scattering at three wavelengths of 450, 550, and 700 nm for size-selected BB aerosol particles. The fuels were combusted in a tube furnace at a temperature of 800 °C, which is representative of the flaming stage of burning. Filter samples were collected and imaged using tunneling electron microscopy to obtain information on the morphology and size of the particles, which was used inmore »the RDG calculations. The mean radii of the monomers were 27.8 and 31.5 nm for the eucalyptus and the olive fuels, respectively. The components of the retrieved complex RI were in the range of 1.31 ≤ n ≤ 1.56 and 0.045 ≤ k ≤ 0.468. The real and complex parts of the RI increase with increasing particle mobility diameter. The real part of the RI is lower, and the imaginary part is higher than what was recommended in literature for black carbon generated by propane or field measurements from fires of mixed wood samples. Fuel dependent results from controlled laboratory experiments can be used in climate modeling efforts and to constrain field measurements from biomass burning.« less
3. Determination of Emission Factors of Pollutants From Biomass Burning of African Fuels in Laboratory Measurements

   https://doi.org/10.1029/2021JD034731  

   Pokhrel, Rudra P. ; Gordon, Janica ; Fiddler, Marc N. ; Bililign, Solomon ( October 2021 , Journal of Geophysical Research: Atmospheres)

   Biomass burning (BB) is a major source of pollutants that impact local, regional, and global climate, air quality, and public health. However, the influence of burning conditions and fuel type on the emission factors of pollutants is still not well understood. Here, we present the results from a laboratory study of emission factors (EFs) of pollutants from six different sub‐Saharan African biomass fuels combusted under a wide range of burning conditions, ranging from smoldering to flaming. We found that particulate matter (PM) and carbon monoxide (CO) EFs (g (kg wood)⁻¹) are highly sensitive to the burning conditions, with an order of magnitude variation between flaming and smoldering burning conditions. Nitric oxide (NO) EF shows a fuel type dependence, with higher NO EFs for fuels with larger nitrogen content. While CO is not generally a proxy for PM_2.5emissions, in this work a correlation was found between CO and PM emissions generated by combustion of seven wood fuels with moisture content (dry basis) <10% in a tube furnace and measured from a laboratory smog chamber with a temperature of ∼21–24°C and an RH below 5%. Unlike total PM, EFs of inorganic particle‐phase species do not show dependence on burning conditions. Finally, wemore »showed that burning biomass fuels in a tube furnace would be a useful experimental approach to study BB emission under controlled burning conditions.

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4. Production of Secondary Organic Aerosol During Aging of Biomass Burning Smoke From Fresh Fuels and Its Relationship to VOC Precursors

   https://doi.org/10.1029/2018JD029068  

   Ahern, A. T. ; Robinson, E. S. ; Tkacik, D. S. ; Saleh, R. ; Hatch, L. E. ; Barsanti, K. C. ; Stockwell, C. E. ; Yokelson, R. J. ; Presto, A. A. ; Robinson, A. L. ; et al ( March 2019 , Journal of Geophysical Research: Atmospheres)

   After smoke from burning biomass is emitted into the atmosphere, chemical and physical processes change the composition and amount of organic aerosol present in the aged, diluted plume. During the fourth Fire Lab at Missoula Experiment, we performed smog‐chamber experiments to investigate formation of secondary organic aerosol (SOA) and multiphase oxidation of primary organic aerosol (POA). We simulated atmospheric aging of diluted smoke from a variety of biomass fuels while measuring particle composition using high‐resolution aerosol mass spectrometry. We quantified SOA formation using a tracer ion for low‐volatility POA as a reference standard (akin to a naturally occurring internal standard). These smoke aging experiments revealed variable organic aerosol (OA) enhancements, even for smoke from similar fuels and aging mechanisms. This variable OA enhancement correlated well with measured differences in the amounts of emitted volatile organic compounds (VOCs) that could subsequently be oxidized to form SOA. For some aging experiments, we were able to predict the SOA production to within a factor of 2 using a fuel‐specific VOC emission inventory that was scaled by burn‐specific toluene measurements. For fires of coniferous fuels that were dominated by needle burning, volatile biogenic compounds were the dominant precursor class. For wiregrass fires, furansmore »were the dominant SOA precursors. We used a POA tracer ion to calculate the amount of mass lost due to gas‐phase oxidation and subsequent volatilization of semivolatile POA. Less than 5% of the POA mass was lost via multiphase oxidation‐driven evaporation during up to 2 hr of equivalent atmospheric oxidation.

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5. 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