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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. 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. In situ measurements of trace gases, PM, and aerosol optical properties during the 2017 NW US wildfire smoke event

   https://doi.org/10.5194/acp-19-3905-2019  

   Selimovic, Vanessa ; Yokelson, Robert J. ; McMeeking, Gavin R. ; Coefield, Sarah ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. In mid-August through mid-September of 2017 a major wildfire smoke and hazeepisode strongly impacted most of the NW US and SW Canada. During this periodour ground-based site in Missoula, Montana, experienced heavy smoke impactsfor ∼ 500 h (up to 471 µg m−3 hourly averagePM2.5). We measured wildfire trace gases, PM2.5 (particulate matter≤2.5 µm in diameter), and black carbon and submicron aerosolscattering and absorption at 870 and 401 nm. This may be the most extensivereal-time data for these wildfire smoke properties to date. Our range oftrace gas ratios for ΔNH3∕ΔCO and ΔC2H4∕ΔCO confirmed that the smoke from mixed, multiple sourcesvaried in age from ∼ 2–3 h to ∼ 1–2 days. Our study-averageΔCH4∕ΔCO ratio (0.166±0.088) indicated a largecontribution to the regional burden from inefficient smoldering combustion.Our ΔBC∕ΔCO ratio (0.0012±0.0005) for our groundsite was moderately lower than observed in aircraft studies (∼ 0.0015)to date, also consistent with a relatively larger contribution fromsmoldering combustion. Our ΔBC∕ΔPM2.5 ratio (0.0095±0.0003) was consistent with the overwhelmingly non-BC (black carbon),mostly organic nature of the smoke observed in airborne studies of wildfiresmoke to date. Smoldering combustion is usually associated with enhanced PMemissions, but our ΔPM2.5∕ΔCO ratio (0.126±0.002)was about half the ΔPM1.0∕ΔCO measured in freshwildfire smoke from aircraft (∼ 0.266). Assuming PM2.5 isdominated by PM1, this suggests that aerosol evaporation, atmore »least nearthe surface, can often reduce PM loading and its atmospheric/air-qualityimpacts on the timescale of several days. Much of the smoke was emitted latein the day, suggesting that nighttime processing would be important in theearly evolution of smoke. The diurnal trends show brown carbon (BrC),PM2.5, and CO peaking in the early morning and BC peaking in the earlyevening. Over the course of 1 month, the average single scattering albedo forindividual smoke peaks at 870 nm increased from ∼ 0.9 to ∼ 0.96.Bscat401∕Bscat870 was used as a proxy for the size and“photochemical age” of the smoke particles, with this interpretation beingsupported by the simultaneously observed ratios of reactive trace gases toCO. The size and age proxy implied that the Ångström absorptionexponent decreased significantly after about 10 h of daytime smoke aging,consistent with the only airborne measurement of the BrC lifetime in anisolated plume. However, our results clearly show that non-BC absorption canbe important in “typical” regional haze and moderately aged smoke, with BrCostensibly accounting for about half the absorption at 401 nm on average forour entire data set.« less
5. Formation of insoluble brown carbon through iron-catalyzed reaction of biomass burning organics

   https://doi.org/10.1039/d2ea00141a  

   Hopstock, Katherine S. ; Carpenter, Brooke P. ; Patterson, Joseph P. ; Al-Abadleh, Hind A. ; Nizkorodov, Sergey A. ( January 2023 , Environmental Science: Atmospheres)

   Biomass burning organic aerosol (BBOA) is one of the largest sources of organics in the atmosphere. Mineral dust and biomass burning smoke frequently co-exist in the same atmospheric environment. Common biomass burning compounds, such as dihydroxybenzenes and their derivatives, are known to produce light-absorbing, water-insoluble polymeric particles upon reaction with soluble Fe( iii ) under conditions characteristic of aerosol liquid water. However, such reactions have not been tested in realistic mixtures of BBOA compounds. In this study, model organic aerosol (OA), meant to replicate BBOA from smoldering fires, was generated through the pyrolysis of Canary Island pine needles in a tube furnace at 300, 400, 500, 600, 700, and 800 °C in nitrogen gas, and the water-soluble fractions were reacted with iron chloride under dark, acidic conditions. We utilized spectrophotometry to monitor the reaction progress. For OA samples produced at lower temperatures (300 and 400 °C), particles (P300 and P400) formed in solution, were syringe filtered, and extracted in organic solvents. Analysis was conducted with ultrahigh pressure liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer (UHPLC-PDA-HRMS). For OA samples formed at higher pyrolysis temperatures (500–800 °C), water-insoluble, black particles (P500–800) formed in solution. In contrastmore »to P300 and P400, P500–800 were not soluble in common solvents. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and transmission electron microscopy (TEM) were used to image P600 and determine bulk elemental composition. Electron microscopy revealed that P600 had fractal morphology, reminiscent of soot particles, and contained no detectable iron. These results suggest that light-absorbing aerosol particles can be produced from Fe( iii )-catalyzed reactions in aging BBOA plumes produced from smoldering combustion in the absence of any photochemistry. This result has important implications for understanding the direct and indirect effects of aged BBOA on climate.« less