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1.
J Phys Chem A ; 121(23): 4475-4485, 2017 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-28521094

RESUMO

We present a critical evaluation of photoionization efficiency (PIE) measurements coupled with aerosol mass spectrometry for the identification of condensed soot-precursor species extracted from a premixed atmospheric-pressure ethylene/oxygen/nitrogen flame. Definitive identification of isomers by any means is complicated by the large number of potential isomers at masses likely to comprise particles at flame temperatures. This problem is compounded using PIE measurements by the similarity in ionization energies and PIE-curve shapes among many of these isomers. Nevertheless, PIE analysis can provide important chemical information. For example, our PIE curves show that neither pyrene nor fluoranthene alone can describe the signal from C16H10 isomers and that coronene alone cannot describe the PIE signal from C24H12 species. A linear combination of the reference PIE curves for pyrene and fluoranthene yields good agreement with flame-PIE curves measured at 202 u, which is consistent with pyrene and fluoranthene being the two major C16H10 isomers in the flame samples, but does not provide definite proof. The suggested ratio between fluoranthene and pyrene depends on the sampling conditions. We calculated the values of the adiabatic-ionization energy (AIE) of 24 C16H10 isomers. Despite the small number of isomers considered, the calculations show that the differences in AIEs between several of the isomers can be smaller than the average thermal energy at room temperature. The calculations also show that PIE analysis can sometimes be used to separate hydrocarbon species into those that contain mainly aromatic rings and those that contain significant aliphatic content for species sizes investigated in this study. Our calculations suggest an inverse relationship between AIE and the number of aromatic rings. We have demonstrated that further characterization of precursors can be facilitated by measurements that test species volatility.

2.
J Phys Chem A ; 121(23): 4447-4454, 2017 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-28521096

RESUMO

We have measured photoionization-efficiency curves for pyrene, fluoranthene, chrysene, perylene, and coronene in the photon energy range of 7.5-10.2 eV and derived their photoionization cross-section curves in this energy range. All measurements were performed using tunable vacuum ultraviolet (VUV) radiation generated at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The VUV radiation was used for photoionization, and detection was performed using a time-of-flight mass spectrometer. We measured the photoionization efficiency of 2,5-dimethylfuran simultaneously with those of pyrene, fluoranthene, chrysene, perylene, and coronene to obtain references of the photon flux during each measurement from the known photoionization cross-section curve of 2,5-dimethylfuran.

3.
Proc Natl Acad Sci U S A ; 113(30): 8374-9, 2016 07 26.
Artigo em Inglês | MEDLINE | ID: mdl-27410045

RESUMO

Many oxygenated hydrocarbon species formed during combustion, such as furans, are highly toxic and detrimental to human health and the environment. These species may also increase the hygroscopicity of soot and strongly influence the effects of soot on regional and global climate. However, large furans and associated oxygenated species have not previously been observed in flames, and their formation mechanism and interplay with polycyclic aromatic hydrocarbons (PAHs) are poorly understood. We report on a synergistic computational and experimental effort that elucidates the formation of oxygen-embedded compounds, such as furans and other oxygenated hydrocarbons, during the combustion of hydrocarbon fuels. We used ab initio and probabilistic computational techniques to identify low-barrier reaction mechanisms for the formation of large furans and other oxygenated hydrocarbons. We used vacuum-UV photoionization aerosol mass spectrometry and X-ray photoelectron spectroscopy to confirm these predictions. We show that furans are produced in the high-temperature regions of hydrocarbon flames, where they remarkably survive and become the main functional group of oxygenates that incorporate into incipient soot. In controlled flame studies, we discovered ∼100 oxygenated species previously unaccounted for. We found that large alcohols and enols act as precursors to furans, leading to incorporation of oxygen into the carbon skeletons of PAHs. Our results depart dramatically from the crude chemistry of carbon- and oxygen-containing molecules previously considered in hydrocarbon formation and oxidation models and spearhead the emerging understanding of the oxidation chemistry that is critical, for example, to control emissions of toxic and carcinogenic combustion by-products, which also greatly affect global warming.


Assuntos
Poluentes Atmosféricos/análise , Fogo , Furanos/química , Hidrocarbonetos/química , Oxigênio/química , Aerossóis/análise , Carbono/análise , Biologia Computacional/métodos , Espectrometria de Massas/métodos , Estrutura Molecular , Espectroscopia Fotoeletrônica/métodos , Hidrocarbonetos Policíclicos Aromáticos/análise , Fumaça/análise , Fuligem/análise
4.
J Vis Exp ; (87)2014 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-24894694

RESUMO

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics Beamline of the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory(1-4). This video demonstrates how the complex chemical structures of laboratory-based model flames are analyzed using flame-sampling mass spectrometry with tunable synchrotron-generated vacuum-ultraviolet (VUV) radiation. This experimental approach combines isomer-resolving capabilities with high sensitivity and a large dynamic range(5,6). The first part of the video describes experiments involving burner-stabilized, reduced-pressure (20-80 mbar) laminar premixed flames. A small hydrocarbon fuel was used for the selected flame to demonstrate the general experimental approach. It is shown how species' profiles are acquired as a function of distance from the burner surface and how the tunability of the VUV photon energy is used advantageously to identify many combustion intermediates based on their ionization energies. For example, this technique has been used to study gas-phase aspects of the soot-formation processes, and the video shows how the resonance-stabilized radicals, such as C3H3, C3H5, and i-C4H5, are identified as important intermediates(7). The work has been focused on soot formation processes, and, from the chemical point of view, this process is very intriguing because chemical structures containing millions of carbon atoms are assembled from a fuel molecule possessing only a few carbon atoms in just milliseconds. The second part of the video highlights a new experiment, in which an opposed-flow diffusion flame and synchrotron-based aerosol mass spectrometry are used to study the chemical composition of the combustion-generated soot particles(4). The experimental results indicate that the widely accepted H-abstraction-C2H2-addition (HACA) mechanism is not the sole molecular growth process responsible for the formation of the observed large polycyclic aromatic hydrocarbons (PAHs).


Assuntos
Fogo , Fuligem/química , Gases/análise , Luz , Espectrometria de Massas/métodos , Fuligem/análise , Síncrotrons
5.
J Air Waste Manag Assoc ; 64(4): 419-35, 2014 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-24843913

RESUMO

UNLABELLED: Motivated by the question of whether and how a state-of-the-art regional chemical transport model (CTM) can facilitate characterization of CO2 spatiotemporal variability and verify CO2 fossil-fuel emissions, we for the first time applied the Community Multiscale Air Quality (CMAQ) model to simulate CO2. This paper presents methods, input data, and initial results for CO2 simulation using CMAQ over the contiguous United States in October 2007. Modeling experiments have been performed to understand the roles of fossil-fuel emissions, biosphere-atmosphere exchange, and meteorology in regulating the spatial distribution of CO2 near the surface over the contiguous United States. Three sets of net ecosystem exchange (NEE) fluxes were used as input to assess the impact of uncertainty of NEE on CO2 concentrations simulated by CMAQ. Observational data from six tall tower sites across the country were used to evaluate model performance. In particular, at the Boulder Atmospheric Observatory (BAO), a tall tower site that receives urban emissions from Denver CO, the CMAQ model using hourly varying, high-resolution CO2 fossil-fuel emissions from the Vulcan inventory and Carbon Tracker optimized NEE reproduced the observed diurnal profile of CO2 reasonably well but with a low bias in the early morning. The spatial distribution of CO2 was found to correlate with NO(x), SO2, and CO, because of their similar fossil-fuel emission sources and common transport processes. These initial results from CMAQ demonstrate the potential of using a regional CTM to help interpret CO2 observations and understand CO2 variability in space and time. The ability to simulate a full suite of air pollutants in CMAQ will also facilitate investigations of their use as tracers for CO2 source attribution. This work serves as a proof of concept and the foundation for more comprehensive examinations of CO2 spatiotemporal variability and various uncertainties in the future. IMPLICATIONS: Atmospheric CO2 has long been modeled and studied on continental to global scales to understand the global carbon cycle. This work demonstrates the potential of modeling and studying CO2 variability at fine spatiotemporal scales with CMAQ, which has been applied extensively, to study traditionally regulated air pollutants. The abundant observational records of these air pollutants and successful experience in studying and reducing their emissions may be useful for verifying CO2 emissions. Although there remains much more to further investigate, this work opens up a discussion on whether and how to study CO2 as an air pollutant.


Assuntos
Poluentes Atmosféricos/análise , Dióxido de Carbono/análise , Monóxido de Carbono/análise , Monitoramento Ambiental/métodos , Combustíveis Fósseis , Modelos Teóricos , Dióxido de Enxofre/análise , Atmosfera/química , Clima , Óxido Nitroso/análise , Material Particulado/análise , Análise Espaço-Temporal , Estados Unidos
6.
Appl Opt ; 46(33): 8095-103, 2007 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-18026549

RESUMO

Temporal behavior of pulses from a Q-switched Nd:YAG laser with an unstable resonator can vary significantly with radial position in the beam. Our laser provides pulses with position-dependent durations spanning 8-11.5 ns at 1064 nm and 7-10 ns at 532 nm. Pulses emerge first and have the longest duration at the center of the beam; they are shorter (by up to 4 ns) and increasingly delayed (by up to 10 ns) with increasing radial distance from the center. This behavior can have a dramatic effect on time-sensitive experiments, such as laser-induced incandescence of soot, if not taken into account.

7.
Appl Opt ; 46(19): 4032-40, 2007 Jul 01.
Artigo em Inglês | MEDLINE | ID: mdl-17571143

RESUMO

Spontaneous emission measurements are discussed for the Swings transitions of the C(3) radical in laser-generated graphite plasma, and the spectroscopy of the C(3) radical in carbon vapor and plasma is summarized. A review is given of some theoretical calculations and emission spectroscopic investigations are presented. Time-averaged, laser-induced optical breakdown spectra are reported from Nd:YAG laser generated graphite microplasma. In 200-300 Torr of argon and helium, and depending on the specific experimental configuration, a weak emission continuum is observed centered at 400 nm when using a laser fluence of typically 1 J/cm(2). Such continua were not detected in our previous experiments using focused laser radiation. The possibilities for the origin of this continuum are considered.


Assuntos
Carbono/química , Lasers , Nanotecnologia/métodos , Argônio/análise , Desenho de Equipamento , Radicais Livres , Gases , Grafite , Hélio/análise , Nanotecnologia/instrumentação , Espectrofotometria/métodos , Fatores de Tempo
8.
Appl Opt ; 46(6): 959-77, 2007 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-17279144

RESUMO

We investigated the physical and chemical changes induced in soot aggregates exposed to laser radiation using a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope to perform near-edge x-ray absorption fine structure spectroscopy. Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm(2) at 532 nm and 0.22 J/cm(2) at 1064 nm. Our results indicate that new particle formation proceeds via (1) vaporization of small carbon clusters by thermal or photolytic mechanisms, followed by homogeneous nucleation, (2) heterogeneous nucleation of vaporized carbon clusters onto material ablated from primary particles, or (3) both processes.

9.
Appl Opt ; 42(27): 5577-90, 2003 Sep 20.
Artigo em Inglês | MEDLINE | ID: mdl-14526849

RESUMO

We present a data set for testing models of time-resolved laser-induced incandescence of soot. Measurements were made in a laminar ethene diffusion flame over a wide range of laser fluences at 532 nm. The laser was seeded to provide a smooth temporal profile, and the beam was spatially filtered and imaged into the flame to provide a homogeneous spatial profile. The particle incandescence was imaged onto a fast photodiode. The measurements are compared with the standard Melton model [Appl. Opt. 23,2201 (1984)] and with a new model that incorporates physical mechanisms not included in the Melton model.

10.
Appl Opt ; 41(33): 6968-79, 2002 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-12463241

RESUMO

Version 3 of the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment data set for some 30 trace and minor gas profiles is available. From the IR solar-absorption spectra measured during four Space Shuttle missions (in 1985, 1992, 1993, and 1994), profiles from more than 350 occultations were retrieved from the upper troposphere to the lower mesosphere. Previous results were unreliable for tropospheric retrievals, but with a new global-fitting algorithm profiles are reliably returned down to altitudes as low as 6.5 km (clouds permitting) and include notably improved retrievals of H2O, CO, and other species. Results for stratospheric water are more consistent across the ATMOS spectral filters and do not indicate a net consumption of H2 in the upper stratosphere. A new sulfuric-acid aerosol product is described. An overview of ATMOS Version 3 processing is presented with a discussion of estimated uncertainties. Differences between these Version 3 and previously reported Version 2 ATMOS results are discussed. Retrievals are available at http://atmos.jpl.nasa.gov/atmos.

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