Dieter Bauer

University of Miami, كورال غيبلز، فلوريدا, Florida, United States

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Publications (16)39.2 Total impact

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    ABSTRACT: During two intensive studies in summer 2010 and spring 2011, measurements of mercury species including gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM), trace chemical species including O-3, SO2, CO, NO, NOY, and black carbon, and meteorological parameters were made at an Atmospheric Mercury Network (AMNet) site at the Grand Bay National Estuarine Research Reserve (NERR) in Moss Point, Mississippi. Surface measurements indicate that the mean mercury concentrations were 1.42 +/- 0.12 ng.m(-3) for GEM, 5.4 +/- 10.2 pg.m(-3) for GOM, and 3.1 +/- 1.9 pg.m(-3) for PBM during the summer 2010 intensive and 1.53 +/- 0.11 ng.m(-3) for GEM, 5.3 +/- 10.2 pg.m(-3) for GOM, and 5.7 +/- 6.2 pg.m(-3) for PBM during the spring 2011 intensive. Elevated daytime GOM levels (>20 pg.m(-3)) were observed on a few days in each study and were usually associated with either elevated O-3 (>50 ppbv), BrO, and solar radiation or elevated SO2 (>a few ppbv) but lower O-3 (similar to 20-40 ppbv). This behavior suggests two potential sources of GOM: photochemical oxidation of GEM and direct emissions of GOM from nearby local sources. Lack of correlation between GOM and Beryllium-7 (Be-7) suggests little influence on surface GOM from downward mixing of GOM from the upper troposphere. These data were analyzed using the HYSPLIT back trajectory model and principal component analysis in order to develop source-receptor relationships for mercury species in this coastal environment. Trajectory frequency analysis shows that high GOM events were generally associated with high frequencies of the trajectories passing through the areas with high mercury emissions, while low GOM levels were largely associated the trajectories passing through relatively clean areas. Principal component analysis also reveals two main factors: direct emission and photochemical processes that were clustered with high GOM and PBM. This study indicates that the receptor site, which is located in a coastal environment of the Gulf of Mexico, experienced impacts from mercury sources that are both local and regional in nature.
    Atmosphere 06/2014; 5(2-2):230-251. DOI:10.3390/atmos5020230 · 1.05 Impact Factor
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    ABSTRACT: The use of programmable thermal dissociation (PTD) as an approach to investigating the chemical speciation of reactive gaseous mercury (RGM, Hg2+) has been explored in a field study. In this approach RGM is collected on a denuder and analyzed using PTD. The denuder is placed in an oven and the dissociation of the RGM is measured, as a function of temperature, by monitoring the evolution of elemental mercury (GEM, Hg0) in real time using laser-induced fluorescence (LIF). The technique was tested in a field campaign at a coal-fired power plant in Pensacola, Florida. Uncoated tubular denuders were used to obtain samples from the plant's stack exhaust gases and from the stack plume, downwind of the stack using an airship. The PTD profiles from these samples were compared with PTD profiles of HgCl2.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2013; 12(3):33291-33322. DOI:10.5194/acpd-12-33291-2012 · 5.30 Impact Factor
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    ABSTRACT: The relative quantum yield for the production of radical products, H + HCO, from the UV photolysis of formaldehyde (HCHO) has been measured using a pulsed laser photolysis–pulsed laser induced fluorescence (PLP–PLIF) technique across the 30,400–32,890 cm(–1) (304–329 nm) spectral region of the Ã(1)A2–X̃(1)A1 electronic transition. The photolysis laser had a bandwidth of 0.09 cm(–1), which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (0.07 cm(–1)), and the yield spectrum shows detailed rotational structure. The H and HCO photofragments were monitored using LIF of the OH radical as a spectroscopic marker. The OH radicals were produced by rapid reaction of the H and HCO photofragments with NO2. This technique produced an “action” spectrum that at any photolysis wavelength is the product of the H + HCO radical quantum yield and HCHO absorption cross section at the photolysis wavelength and is a relative measurement. Using the HCHO absorption cross section previously obtained in this laboratory, the relative quantum yield was determined two different ways. One produced band specific yields, and the other produced yields averaged over each 100 cm(–1). Yields were normalized to a value of 0.69 at 31,750 cm(–1) based on the current recommendation of Sander et al. (Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17; Jet Propulsion Laboratory: Pasadena, CA, USA, 2011). The resulting radical quantum yields agree well with previous experimental studies and the current JPL recommendation but show greater wavelength dependent structure. A significant decrease in the quantum yield was observed for the 5(0)(1) + 1(0)(1)4(0)(1) combination band centered at 31,125 cm(–1). This band has a low absorption cross section and has little impact on the calculated atmospheric photodissociation rate but is a further indication of the complexity of HCHO photodissociation dynamics.
    The Journal of Physical Chemistry A 05/2012; 116(26):6983-95. DOI:10.1021/jp2117399 · 2.78 Impact Factor
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    ABSTRACT: Absolute room temperature (294 ± 2 K) absorption cross sections for the Ã(1)A(2)-X̃(1)A(1) electronic transition of formaldehyde have been measured over the spectral range 30,285-32,890 cm(-1) (304-330 nm) using ultraviolet (UV) laser absorption spectroscopy. Accurate high-resolution absorption cross sections are essential for atmospheric monitoring and understanding the photochemistry of this important atmospheric compound. Absorption cross sections were obtained at an instrumental resolution better than 0.09 cm(-1), which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (∼0.07 cm(-1)) and so we were able to resolve all but the most closely spaced lines. Comparisons with previous data as well as with computer simulations have been made. Pressure broadening was studied for the collision partners He, O(2), N(2), and H(2)O and the resulting broadening parameters have been measured and increase with the strength of intermolecular interaction between formaldehyde and the collision partner. The pressure broadening coefficient for H(2)O is an order of magnitude larger than the coefficients for O(2) and N(2) and will contribute significantly to spectral line broadening in the lower atmosphere. Spectral data are made available as Supporting Information.
    The Journal of Physical Chemistry A 02/2012; 116(24):5910-22. DOI:10.1021/jp210008g · 2.78 Impact Factor
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    ABSTRACT: A detailed understanding of the role of mercury (Hg) in ecosystems is a critical issue from a human health perspective, particularly regarding the ingestion of methyl-mercury from contaminated fish. The atmospheric Hg burden has increased by a factor of three in the past one hundred years primarily due to anthropogenic input. Atmospheric Hg exists principally in its elemental form, Hg(0), which, until recently, was thought to be unreactive in the gas phase. Hg(0) is insoluble in water and has a low deposition rate. In contrast, oxidized Hg, typically referred to as RGM (reactive gaseous mercury), is soluble and appears to be efficiently removed from the atmosphere by both wet and dry deposition, therefore increasing Hg contamination. Although the CVAFS (cold vapour atomic fluorescence spectroscopy) approach is well established for measurement of Hg0, no measurement technique has shown the combination of sensitivity, temporal resolution and precision necessary for direct measurement of Hg0 fluxes. We are developing experimental approaches using both sequential two photon and single photon laser induced fluorescence (LIF) that are designed to address some of these issues. During our recent flight experiment, we used both KCl coated annular denuders and uncoated etched tubular denuders along with a Tekran 2537B mercury analyzer over the S.E. United States collecting and analyzing air samples. Analyses are still ongoing, but consistent structures at multiple variables are being initially observed giving us the preliminary impression that Hg levels are being identified. Following this project, more calibrations will be performed to fully analyze our data.
    2011 Society for Advancement of Hispanics/Chicanos and Native Americans in Science National Conference; 10/2011
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    ABSTRACT: Formaldehyde (HCHO) is the most abundant and most important organic carbonyl compound in the atmosphere. The sources of formaldehyde are the oxidation of methane, isoprene, acetone, and other volatile organic compounds (VOCs); fossil fuel combustion; and biomass burning. The dominant loss mechanism for formaldehyde is photolysis which occurs via two pathways: (R1) HCHO + hv → HCO + H (R2) HCHO + hv → H2 + CO The first pathway (R1) is referred to as the radical channel, while the second pathway (R2) is referred to as the molecular channel. The products of both pathways play a significant role in atmospheric chemistry. The CO that is produced in the molecular channel undergoes further oxidation to produce CO2. Under atmospheric conditions, the H atom and formyl radical that are produced in the radical channel undergo rapid reactions with O2 to produce the hydroperoxyl radical (HO2) via (R3) and (R4). (R3) HCO + O2 → HO2 + CO (R4) H + O2 → HO2 Thus, for every photon absorbed, the photolysis of formaldehyde can contribute one CO2 molecule to the global greenhouse budget or two HO2 radicals to the tropospheric HOx (OH + HO2) cycle. The HO2 radicals produced during formaldehyde photolysis have also been implicated in the formation of photochemical smog. The HO2 radicals act as radical chain carriers and convert NO to NO2, which ultimately results in the catalytic production of O3. Constraining the yield of HO2 produced via HCHO photolysis is essential for improving tropospheric chemistry models. In this study, both the absorption cross section and the quantum yield of the radical channel (R1) were measured at high resolution over the tropospherically relevant wavelength range 304-330 nm. For the cross section measurements a narrow linewidth Nd:YAG pumped dye laser was used with a multi-pass cell. Partial pressures of HCHO were kept below 0.3 torr. Simultaneous measurement of OH LIF in a flame allowed absolute calibration of the wavelength scale. Pressure broadening in He, O2, N2, and H2O bath gas was also examined. Measurements of the radical yield of HCHO photolysis were conducted by converting the H atom to OH through reaction with NO2 via (R5) and then detecting OH LIF using a Pulsed Laser Photolysis-Pulsed Laser Induced Fluorescence (PLP-PLIF) technique. (R5) H + NO2 → NO + OH The resulting relative quantum yield was converted to an absolute yield by using Cl2 photolysis (and the subsequent reaction of the Cl atom with HCHO) coupled with a photofragment-LIF variation of the PLP-PLIF technique.
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    ABSTRACT: Most current models of the atmospheric chemistry of gas phase elemental mercury, Hg(0), assume that oxidation takes place via reaction with the hydroxyl radical, OH, and ozone, O3. Recent high level ab-initio calculations on the thermochemistry of HgO suggest that this molecule is weakly bound and, as a consequence, reasonable oxidation pathways involving OH or O3 are calculated to be strongly endothermic and unlikely to play a role in the homogeneous oxidation of Hg(0) in the atmosphere. Chlorine and bromine atoms both react with Hg(0) to produce relatively stable monohalides via a pressure dependent three body recombination, however the limited number of measurements of the rate coefficients for these reactions show large discrepancies. We will report measurements of the three-body recombination of Hg(0) with Br at higher temperatures which allow us to measure the recombination rate coefficient, the thermal dissociation rate of HgBr and the rate coefficient for the recombination of HgBr with Br atoms to form HgBr2. In addition we will describe measurements of reactive gaseous mercury (RGM) in the stack gas and exhaust plume of a power plant and the use of programmable thermal desorption as a route to RGM speciation.
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    ABSTRACT: A pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to study the recombination of mercury and bromine atoms, Hg + Br + M --> HgBr + M (1) and the self-reaction of bromine atoms, Br + Br + M --> Br2 + M (2). Rate coefficients were determined as a function of pressure (200-600 Torr) and temperature (243-293 K) in nitrogen buffer gas and as a function of pressure (200-600 Torr) in helium buffer gas at room temperature. For reaction 1, kinetic measurements were performed under conditions in which bromine atoms were the reactant in excess concentration while simultaneously monitoring the concentration of both mercury and bromine. A temperature dependent expression of (1.46 +/- 0.34) x 10(-32) x (T/298)(-(1.86+/-1.49)) cm6 molecule(-2) s(-1) was determined for the third-order recombination rate coefficient in nitrogen buffer gas. The effective second-order rate coefficient for reaction 1 under atmospheric conditions is a factor of 9 smaller than previously determined in a recently published relative rate study. For reaction 2 we obtain a temperature dependent expression of (4.31 +/- 0.21) x 10(-33) x (T/298)(-(2.77+/-0.30)) cm6 molecule(-2) s(-1) for the third-order recombination rate coefficient in nitrogen buffer gas. The rate coefficients are reported with a 2sigma error of precision only; however, due to the uncertainty in the determination of absolute bromine atom concentrations and other unidentified systematic errors we conservatively estimate an uncertainty of +/-50% in the rate coefficients. For both reactions the observed pressure, temperature and buffer gas dependencies are consistent with the expected behavior for three-body recombination.
    The Journal of Physical Chemistry A 06/2006; 110(21):6623-32. DOI:10.1021/jp054688j · 2.78 Impact Factor
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    ABSTRACT: A pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to study the recombination of mercury and chlorine atoms, Hg + Cl + M --> HgCl + M (1), and the self-reaction of chlorine atoms, Cl + Cl + M --> Cl(2) + M (2). Rate coefficients were determined as a function of pressure (200-600 Torr) and temperature (243-293 K) in N(2) buffer gas and as a function of pressure (200-600 Torr) in He buffer gas at room temperature. For reaction (1) kinetic measurements were obtained under conditions in which either mercury or chlorine atoms were the reactant in excess concentration while simultaneously monitoring the concentration of both reactants. An Arrhenius expression of (2.2 +/- 0.5) x 10(-32) exp{(680 +/- 400)((1)/(T) - (1)/(298))} cm(6) molecule(-2) s(-1) was determined for the third-order recombination rate coefficient in nitrogen buffer gas. The effective second-order rate coefficient for reaction 1 under atmospheric conditions is much smaller than prior determinations using relative rate techniques. For reaction (2) we obtain an Arrhenius expression of (8.4 +/- 2.3) x 10(-33) exp{(850 +/- 470)((1)/(T) - (1)/(298))} cm(6) molecule(-2) s(-1) for the third-order recombination rate coefficient in nitrogen buffer gas. The rate coefficients are reported with a 2sigma error of precision only; however, due to the uncertainty in the determination of absolute chlorine atom concentrations we conservatively estimate an uncertainty of +/-50% in the rate coefficients. For both reactions the observed pressure, temperature, and buffer gas dependencies are consistent with the expected behavior for three-body recombination.
    The Journal of Physical Chemistry A 09/2005; 109(34):7732-41. DOI:10.1021/jp051354l · 2.78 Impact Factor
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    ABSTRACT: The kinetics and mechanism of the three-body recombination of OH with NO2 were studied using a pulsed laser photolysis pulsed laser induced fluorescence technique. The rate coefficients for deactivation of vibrationally excited OH (v = 1-5) by NO2 were found to be independent of vibrational level with a value of (6.4 +/- 0.3) x 10(-11) cm3 molecule s (-1) at 298 K. The rate coefficient for reaction of 18OH with NO2 was measured and found to be much faster than for unlabeled OH with a "zero pressure" rate of 1 x 10(-11) cm3 molecule(-1) s(-1) at 298 K and 273 K. Observation of temporal profiles of 16OH and 18OH suggest that isotopic scrambling in the initially formed [H18ON16O2] complex is complete on the microsecond time scale of our experiments. The rate coefficient for reaction of unlabeled OH with NO2 was measured at 413 K in 400 Torr of He. Biexponential temporal profiles were obtained and are consistent with a 10 +/- 3% yield of the weakly bound HOONO isomer.
    Faraday Discussions 02/2005; 130:111-23; discussion 125-51, 519-24. DOI:10.1039/B417458P · 4.19 Impact Factor
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    ABSTRACT: The deactivation kinetics of vibrationally excited OH X 2Π (v = 1–5) were studied using a pulsed laser photolysis-pulsed laser induced fluorescence technique. Temporal profiles of OH (v) were obtained by exciting off-diagonal (Δv = −1,−3) transitions in the A–X band of OH and monitoring the diagonal, blue shifted fluorescence. Photolysis of O3 at 266 nm was used to produce O1D which reacted rapidly with H2, CH4 and H2O to produce OH (v). Deactivation rate coefficients for OH (v = 1–5) with O2 and N2 and for OH (v = 2,1) with O3 were obtained. The deactivation rate coefficients show an exponential dependence on vibrational level for both O2 and N2, however O2 is much more efficient.
    Physical Chemistry Chemical Physics 01/2004; 6(17). DOI:10.1039/b407297a · 4.20 Impact Factor
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    ABSTRACT: We have examined the sensitivity of single and sequential two-photon laser-induced fluorescence (LIF) techniques for the detection of elemental mercury, Hg(0), in the gas phase. Single photon LIF involves excitation of the 63P1–61S0 transition at 253.7nm, followed by observation of resonance fluorescence. Sequential two-photon techniques follow the initial 63P1–61S0 excitation with a second excitation step to either the 71S0 or 73S1 levels followed by observation of blue or red shifted fluorescence. We have examined four variants of these approaches which all exceed the sensitivity of single photon LIF. The most sensitive detection approach involves the initial 253.7nm excitation followed by excitation of the 71S0–63P1 transition at 407.8nm. Fluorescence is observed on the 61P1–61S0 transition at 184.9nm. Using this approach, our limits of detection are 0.1ngm−3 with a 10s integration time in air. We have also examined the effects of saturation, quenching and line-width on detection sensitivity. We have used the pulsed laser photolysis–pulsed laser-induced fluorescence (PLP-PLIF) technique to study the kinetics of the reaction of elemental mercury with the hydroxyl radical under atmospheric conditions at 298K. We see no evidence for reaction and obtain an upper limit of 1.2×10−13cm3molecule−1s−1 for the rate coefficient.
    Journal of Photochemistry and Photobiology A Chemistry 05/2003; 157(2):247-256. DOI:10.1016/S1010-6030(03)00065-0 · 2.29 Impact Factor
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    ABSTRACT: We have examined the sensitivity of sequential two photon laser induced fluorescence (LIF) detection of elemental mercury, Hg(0) in the gas phase. The most sensitive approach involves an initial laser excitation of the 6(3)P1-6(1)S0 transition at 253.7 nm, followed by excitation with a second laser to the 7(1)S0 level. Blue shifted fluorescence is observed on the 6(1)P1-6(1)S0 transition at 184.9 nm. The excitation scheme, involving sequential excitation of two atomic transitions, followed by detection of the emission from a third is extremely specific and precludes detection of anything other than atomic mercury. Using our 10 Hz laser system we have achieved a detection sensitivity of 0.1 ng m(-3) at a sampling rate of 0.1 Hz, i.e. averaging 100 laser shots at a pressure of one atmosphere in air. At low concentrations we sampled simultaneously with an automated mercury analyzer (Tekran 2537A), to ensure accuracy. We have examined the linearity of the technique, generating flows containing mercury concentrations between 1 and 10,000 ng m(-3) using a permeation tube and dynamic dilution, but relying on the concentrations given by the Tekran at low levels and the concentration calculated from dilution at high levels. We find that the detection is linear over the five orders of magnitude that we were able to vary the concentration. Our measured detection limits in He and Ar are much lower as these gases are inefficient fluorescence quenchers.
    Journal of Environmental Monitoring 07/2002; 4(3):339-43. DOI:10.1039/B111688F · 2.11 Impact Factor
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    ABSTRACT: The kinetics of the recombination reaction, OH+NO2+(M)→HNO3 +(M) have been investigated by the pulsed laser photolysis−pulsed laser induced fluorescence (PLP−PLIF) technique in air, nitrogen, oxygen, and helium buffer gases at room temperature and as a function of pressure (30−700 Torr). Additional measurements in nitrogen at 273 K (100−700 Torr) are reported. The third-body efficiency of water vapor has also been investigated. Our values for the absolute rate coefficient in nitrogen at room temperature and at 273 K are in excellent agreement with the JPL 1994 recommendation but lie substantially above the current JPL 2000 recommendation. Our rate coefficients in helium agree with previous literature studies, suggesting that systematic errors are small. Oxygen is found to be about 20% less efficient than nitrogen, and we see no significant enhancement in recombination in the presence of water vapor. Our results suggest that formation of the pernitrous acid isomer cannot explain the discrepancies in the current experimental database.
    The Journal of Physical Chemistry A 10/2001; 105(46). DOI:10.1021/jp012250n · 2.78 Impact Factor
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    ABSTRACT: The pulsed laser photolysis–pulsed laser induced fluorescence (PLP–PLIF) technique has been used to study the reactions of OH with DMS and DMS-d6. The effective rate coefficient for the reaction of OH with DMS-d6 has been determined as a function of O2 partial pressure at 600 Torr total pressure in N2/O2 mixtures at 298 and 261 K and for both DMS and DMS-d6 at 240 K. Currently recommended rates are based on an empirical fit to a two-channel mechanism. This work shows that at low temperatures the currently recommended expression underestimates both the effective rate coefficient for reaction together with the branching ratio between addition and abstraction.
    Chemical Physics Letters 08/2001; 344(1):61-67. DOI:10.1016/S0009-2614(01)00764-3 · 1.99 Impact Factor
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    ABSTRACT: The formation of O 1D from the photolysis of ozone has been observed at photolysis wavelengths between 305 and 375 nm and relative quantum yields for O 1D production have been determined. Ozone was photolyzed using a frequency doubled, optical parametric oscillator. O 1D was monitored indirectly, using laser induced fluorescence (LIF) detection of vibrationally excited OH as a ‘‘spectroscopic marker’’. Relative O 1D yields were measured from 305 to 375 nm at 295 K and from 305 to 340 nm at 273 K. The quantum yield decreases between 305 and 325 nm with a temperature dependence in the range between 310 and 325 nm. This is consistent with O 1D production via both spin-allowed and spin-forbidden processes. Between 325 and 375 nm a constant quantum yield of 0.064±0.006 was obtained suggesting production via an exclusively spin-forbidden process and indicating that absorption takes place to a single excited state which either predissociates, yielding O3P and O23Σ−, or crosses to another surface before dissociating to produce O 1D and O23Σ−.
    Physical Chemistry Chemical Physics 01/2000; 2(7):1421-1424. DOI:10.1039/b000159g · 4.20 Impact Factor