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1. Examination of brown carbon absorption from wildfires in the western US during the WE-CAN study

   https://doi.org/10.5194/acp-22-13389-2022  

   Sullivan, Amy P.; Pokhrel, Rudra P.; Shen, Yingjie; Murphy, Shane M.; Toohey, Darin W.; Campos, Teresa; Lindaas, Jakob; Fischer, Emily V.; Collett Jr., Jeffrey L. (January 2022, Atmospheric Chemistry and Physics)

   Abstract. Light absorbing organic carbon, or brown carbon (BrC), can be a significantcontributor to the visible light absorption budget. However, the sources ofBrC and the contributions of BrC to light absorption are not wellunderstood. Biomass burning is thought to be a major source of BrC.Therefore, as part of the WE-CAN (Western Wildfire Experiment for CloudChemistry, Aerosol Absorption and Nitrogen) study, BrC absorption data werecollected on board the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C-130 aircraft as it intercepted smoke fromwildfires in the western US in July–August 2018. BrC absorptionmeasurements were obtained in near real-time using two techniques. The firstcoupled a particle-into-liquid sampler (PILS) with a liquid waveguidecapillary cell and a total organic carbon analyzer for measurements ofwater-soluble BrC absorption and WSOC (water-soluble organic carbon). Thesecond employed a custom-built photoacoustic aerosol absorption spectrometer(PAS) to measure total absorption at 405 and 660 nm. The PAS BrC absorption at 405 nm (PAS total Abs 405 BrC) was calculated by assuming the absorption determined by the PAS at 660 nm was equivalent to the black carbon (BC) absorption and the BC aerosol absorption Ångström exponent was 1. Data from the PILS and PAS were combined to investigate the water-soluble vs. total BrC absorption at 405 nm in the various wildfire plumes sampled during WE-CAN. WSOC, PILS water-soluble Abs 405, and PAS total Abs 405 tracked each other in and out of the smoke plumes. BrC absorption was correlated with WSOC (R2 value for PAS =0.42 and PILS =0.60) and CO (carbon monoxide) (R2 value for PAS =0.76 and PILS =0.55) for all wildfires sampled. The PILS water-soluble Abs 405 was corrected for thenon-water-soluble fraction of the aerosol using the calculated UHSAS(ultra-high-sensitivity aerosol spectrometer) aerosol mass. The correctedPILS water-soluble Abs 405 showed good closure with the PAS total Abs 405BrC with a factor of ∼1.5 to 2 difference. This differencewas explained by particle vs. bulk solution absorption measured by the PASvs. PILS, respectively, and confirmed by Mie theory calculations. DuringWE-CAN, ∼ 45 % (ranging from 31 % to 65 %) of the BrCabsorption was observed to be due to water-soluble species. The ratio of BrC absorption to WSOC or ΔCO showed no clear dependence on firedynamics or the time since emission over 9 h. 

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2. Study of microstructure of iron species in plants using X ray absorption spectroscopy

   https://doi.org/10.18260/1-2-530.540.660.1139-54978  

   Lee, David; Rahman, Mashtura; Sharma, Shivansh; Ventourat, Michael; Torreno, Julianne; Guha, Nealesh; Cheung, Tak_Choi David; An, Guozheng; Dehipawala, Sunil; Robinson, Lexi; et al (March 2025, ASEE Conferences)

   The X-ray absorption spectroscopy including Extended X ray Absorption Fine Structure (EXAFS) , X ray Absorption Near Edge Structure (XANES) and X-ray absorption near edge and pre edge feature is used to analyze the nature and concentration of iron in plants, spinach and parsley. Primary data was collected at Cornell High Energy Synchrotron Source. Data analyses were done by community college students. EXAFS data analysis process include edge normalization, background subtraction, transformation to k space from energy, Fourier transformation, back transform of selected peaks, and curve fitting to identify type and number of near neighbor atoms. X-ray absorption pre=edge feature, which appears as very small absorption peak before the main peak, is used to study ironoxygen bonding and asymmetry of iron compounds. Pre edge data analysis include extraction of the pre-edge from main absorption spectrum, normalization to main edge height, and Lorentzian fit to back ground subtracted data. X-ray absorption data from spinach and parsley, as well as several iron supplements and the soil of which plants were grown was analyzed. The preliminary data analysis pre edge feature yields two forms of iron in both plants. One form has oxygen as near neighbor atoms and the other without oxygen. 

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3. Nonlinear absorption in phthalocyanine thin films and solutions using a continuous wave laser

   https://doi.org/10.1142/S0218863522500187  

   Morrison, Nathaniel; Gredig, Thomas (December 2022, Journal of Nonlinear Optical Physics & Materials)

   The nonlinear absorption of nanoscale thin films of iron phthalocyanine, zinc phthalocyanine, and metal-free phthalocyanine fabricated via thermal evaporation at various deposition temperatures was studied using the open-aperture z-scan method with a continuous wave laser. A solution of tetra-tert-butyl-phthalocyanine in chloroform was also studied via z-scan. The thin films exhibit pronounced saturable absorption, with the degree of nonlinearity highly dependent on the type of metal complex. No significant correlation was found between deposition temperature and nonlinear absorption. The liquid sample exhibits a transition from saturable absorption to reverse saturable absorption at high irradiance. 

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4. The absorption indicatrix as an empirical model to describe anisotropy in X-ray absorption spectra of pyroxenes

   https://doi.org/10.2138/am-2021-7950  

   Steven, Cody J.; Dyar, M Darby; McCanta, Molly; Newville, Matthew; Lanzirotti, Antonio (April 2022, American Mineralogist)

   Abstract Anisotropic absorption in crystals is routinely observed in many spectroscopic methods and is recognized in visible light optics as pleochroism in crystalline materials. As with other spectrosco-pies, anisotropy in Fe K-edge X-ray absorption spectroscopy (XAS) can serve both as an indicator of the general structural arrangement and as a conundrum in quantifying the proportions of absorbers in crystals. In materials containing multiple absorbers, observed anisotropies can typically be represented by a linear relationship between measured spectroscopic peak intensities and relative absorber concentrations. In this study, oriented XAS analysis of pyroxenes demonstrates that the macroscopic theory that describes visible light absorption anisotropy of triaxially anisotropic materials can also be applied to X-ray absorption in pyroxenes, as long as the orientation and magnitude of the characteristic absorption vectors are known for each energy. Oriented single-crystal XAS analysis of pyroxenes also shows that the measured magnitude of characteristic absorption axes at a given orientation is energy-dependent and cannot be reproduced by linear combination of intermediate spectra. Although the macroscopic model describes a majority of the anisotropy, there is distinct discordance between the observed and interpolated spectra in the pre-edge between 7109 and 7115 eV, which is marked by spikes in RMSE/mean intensity ratio. Absorption indicatrices for samples analyzed in the visible and at X-ray wavelengths are modeled with a three-dimensional (3D) pedal surface, which functions as an empirical way of interpolating between the observed absorption data. This surface only requires a maximum of three coefficients, and results from the summation of 3D lemniscates. An absorption indicatrix model can be used to characterize anisotropic absorption in crystals and provides a way of comparing XAS spectra from randomly oriented crystals, such as those from polished sections, to a database of characterized crystals. 

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5. Observationally constrained representation of brown carbon emissions from wildfires in a chemical transport model

   https://doi.org/10.1039/D1EA00059D  

   Neyestani, Soroush E.; Saleh, Rawad (March 2022, Environmental Science: Atmospheres)

   The month of August 2015 featured extensive wildfires in the Northwestern U.S. and no significant fires in Alaska and Canada. With the majority of carbonaceous aerosols (CA), including black carbon (BC) and brown carbon (BrC), over the U.S. dominated by emissions from Northwestern wildfires, this month presented a unique opportunity for testing wildfire BrC representation in the Weather Research and Forecasting model with chemistry (WRF-Chem). We performed parallel simulations that (1) did not account for BrC absorption, (2) accounted for BrC absorption, and (3) accounted for BrC absorption as well as its decay due to photobleaching. We used a comprehensive set of extensive and pseudo-intensive optical properties, namely the aerosol optical depth (AOD), aerosol absorption optical depth (AAOD), absorption Ångström exponent (AAE), and single scattering albedo (SSA) to constrain the model output against observations from the Aerosol Robotic Network (AERONET). We found that accounting for BrC absorption and photobleaching resulted in the best agreement with observations in terms of aerosol absorption (AAOD and AAE). However, the model severely underestimated AOD and SSA compared to observations. We attributed this discrepancy to missing scattering due to missing secondary organic aerosol (SOA) formation from wildfire emissions in the model. To test this hypothesis, we applied a zeroth-order representation of wildfire SOA, which significantly improved the AOD and SSA model-observation comparison. Our findings indicate that BrC absorption, the decay of its absorption due to photobleaching, as well as SOA formation should be accounted for in chemical transport models in order to accurately represent CA emissions from wildfires. 

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