Joseph M. Conny

The isotopic characterization of methane, non-methane hydrocarbons and formaldehyde in the troposphere

Interest in the isotopic composition of trace species in the troposphere continues to increase because of the utility of isotopic information for interpreting timely concerns such as the effects of climate change, O3 production and OH depletion. With the exception of CO2, isotopic information has served, to a large extent, only an ancillary role in determining global cycles of trace species, including their chemistries. Often, isotopic information is only used to interpret isolated parts of atmospheric cycles. For example, hydrogen and carbon isotopic ratios of CH4 source emissions have been used to help quantify source contributions to the global CH4 budget. Less emphasis has been placed on isotopic information for interpreting the atmospheric cycling of CH4 and non-methane hydrocarbons (NMHCs) to formaldehyde and CO. Here, we review compositions of the stable isotopes (13C and 2H) and radioactive isotopes (l4C and 3H) for the various hydrocarbon components and formaldehyde in the tropospheric hydrocarbonOHCO system. Included are isotopic ratios of CH4 and NMHCs at sources and in ambient air as well as isotope effects during chemical transformation. Variations in the data are discussed with respect to measurement uncertainty and natural variation. Emphasis is on multi-isotopic signatures, for example, 13C12C and DH ratios and 14C activities for individual CH4 sources.

Optimizing Thermal-Optical Methods for Measuring Atmospheric Elemental (Black) Carbon: A Response Surface Study

The chemical, physical, and morphological complexity of atmospheric aerosol elemental carbon (EC) presents major problems in assuring measurement accuracy. Since EC and black carbon are often considered equivalent, methods based on thermal-optical analysis (TOA) are widely used for EC in ambient air samples because no prior knowledge of the aerosols absorption coefficient is required. Nevertheless, different TOA thermal desorption protocols result in wide EC-to-total-carbon (TC) variation. We created three response surfaces with the following response variables: EC/TC, maximum laser attenuation in the He phase ( L max ), and laser attenuation at the end of the He phase ( L He4 ). A two-level central-composite factorial design comprised of four factors considered the temperatures and durations of all desorption steps in TOAs inert (He) phase and the initial step in TOAs oxidizing (O 2 -He) phase. L max was used to assess the positive bias caused by nonvolatile unpyrolized organic carbon (OC char) being measured as native EC. A negative bias that the attenuated laser response does not detect is caused by the loss of native EC in the He phase. L He4 was used as a surrogate indicator for the loss of native EC in the He phase. The intersection between the L max and L He4 surfaces revealed TOA conditions where both the production of OC char in the He phase was maximized and the loss of native EC in the He phase was minimized, therefore leading to an optimized thermal desorption protocol. Based on the sample types used in this study, the following are generalized optimal conditions when TOA is operated in the fixed-step-durations, laser-transmission mode (i.e., TOT): step 1 in He, 190°C for 60 s; step 2 in He, 365°C for 60 s; step 3 in He, 610°C for 60 s; step 4 in He, 835°C for 72 s. For steps 1-4 in O 2 -He, the conditions are 550°C for 180 s, 700°C for 60 s, 850°C for 60 s, and 900°C for 90 s to 120 s, respectively.

Standard test data for estimating peak parameter errors in x-ray photoelectron spectroscopy III. Errors with different curve-fitting approaches

We present results in the final part of a three-part study employing standard test data (STD) to estimate errors in peak parameters derived from data analysis procedures used in x-ray photoelectron spectroscopy (XPS). The XPS-STD are simulated doublet and singlet XPS spectra based on spline polynomial models of measured C 1s spectra. The XPS-STD contained 10 sets of spectra, each of which consisted of 18 doublets and 4 singlets. The doublet spectra in each set were created from a factorial design with three factors: peak separation; relative intensity of the component peaks; and fractional Poisson noise. Each of the XPS-STD sets contained the same complement of doublet and singlet spectra and Poisson noise at the same levels; however, the noise distributions were unique in each spectral set. Seven curve-fitting approaches were used by a group of 20 analysts to represent single peaks: a single Gaussian (G), a Gaussian–Lorentzian (G–L), a Voigt function, a G function with an asymmetry term, a G–L function with an asymmetry term and dual G or G–L functions. We statistically analyzed errors in binding energies and intensities by the type of curve-fitting approach using the Kruskal–Wallis test and the Mann-Whitney U-test. Bias was used to measure the accuracy of a curve-fitting approach, whereas random error was used to measure the precision of the approach. Despite the fact that individual peaks were nearly symmetrical, curve-fitting approaches that accounted for peak asymmetry proved to be the most accurate for determining both peak intensities and binding energies. For the doublets exhibiting the larger peak at the higher binding energy, the use of dual G–L functions to fit individual peaks, as a way to account for peak asymmetry, produced the highest accuracy in determining peak binding energy. This dual peak-shape approach for this particular spectral condition is also preferable for determining peak intensities and produces better precision. The XPS-STD and the statistical analysis methods presented here provide a means to distinguish differences in the accuracy of various curve-fitting approaches for spectral conditions that resemble those of the XPS-STD. To obtain the XPS-STD and to have an online evaluation of curve-fitting accuracy and precision, consult the following web site: http://www.acg.nist.gov/std. Copyright

NIST Data Resources for Surface Analysis by X-Ray Photoelectron Spectroscopy and Auger Electron Spectroscopy

Abstract A description is given of data resources that are available from the National Institute of Standards and Technology (NIST) for X-ray photoelectron spectroscopy (XPS) and Auger-electron spectroscopy. NIST currently has three databases available: an XPS Database, an Electron Elastic-Scattering Cross-Section Database, and an Electron Inelastic-Mean-Free-Path Database. NIST also offers Standard Test Data (STD) for XPS, a set of simulated XPS data designed to evaluate algorithms and procedures for detecting, locating, and measuring the intensities of overlapping peaks in a doublet. The XPS database and the XPS-STD are available over the internet.

Carbon 13 composition of tropospheric CO in Brazil: A model scenario during the biomass burn season

The stable isotopes of carbon and oxygen are potentially powerful tools for distinguishing sources of CO in the troposphere due to isotopic differences among source emissions that are caused by isotope fractionation in formation or reaction. It is incorrect, however, to assume that the CO source strengths estimated using isotopic measurements on single-day air samples truly represent a season and region. Atmospheric transport and dispersion models are useful for selecting representative sampling locations, dates, and duration to adequately reflect isotopic variation. Here a three-dimensional transport and dispersion model was used to predict surface-level 13 CO/ 12 CO ratios at four remote sites in the rain forest and savanna of Brazil during the 1992 burn season. The purpose was to demonstrate the scope of surface-level CO isotopic variation due to isotopically distinct source emissions and changing meteorology. The model included 13 C signatures of four classes of CO sources: biomass burning, oxidized vegetative nonmethane hydrocarbon (NMHC) emissions, atmospheric methane oxidation, and fossil fuel combustion. Among the four model locations, sites 1 and 2 were well within the burn region, site 3 was at the edge of it, and site 4 was well north of it. The model employed the program HY-SPLIT to track air masses and calculate CO concentrations from emissions at satellite-detected burn sites which were mainly in the Brazilian savanna. An average CO δ 13 C value for burned biomass (-21.3‰ versus PDB) was determined from our δ 13 C measurements of savanna biomass, reported fuel loadings, and the distribution of savanna plant communities in Brazil. Two model scenarios were created, based mainly on the level of CO from fossil fuel combustion. Scenario A had a low CO contribution from this source (15 ppbv), and scenario B had a higher CO contribution (100.1 ppbv). Both model scenarios used -32.2, -48.3, and -25‰ for CO δ 13 C values for oxidized vegetative NMHC emissions, CH 4 oxidation, and fossil fuel combustion, respectively, based on data reported by others. Sensitivity studies showed that at sites closest to the burn region the model was influenced largely by the 13 C composition of burned biomass for both scenarios. At the site farthest from the burn region the model was influenced moderately by the amount of CO emitted per fire, a greater rate of CH 4 oxidation, and a higher 13 CO/ 12 CO ratio for fossil fuel combustion, particularly for scenario B. For the model scenario with minimal CO from fossil fuel combustion (scenario A), results showed surface-level δ 13 C values for August 5, 1992, averaging about -23‰, close to the average δ 13 C value for biomass burning CO. Model results for August 11, 1992, showed 13 CO/ 12 CO that ratios at sites 1-3 were, again, close to the ratio for biomass burning CO (δ 13 C = -22.6‰ to -24.7‰). The more 13 C-enriched values match closely with the most 13 C-enriched measurements that have been reported for July/August in the tropics and southern hemisphere when elevated CO levels are driven by emissions from large-scale biomass burning. At site 4 for August 11, 1992, the calculated surface-level δ 13 C average was -32.6‰. Thus results indicate that 13 CO/ 12 CO ratios may be highly variable from week to week in the Amazon region during the biomass burn season. Model results suggest that on August 5, 1992, fossil fuel combustion probably did not alter significantly the 13 CO/ 12 CO ratio in surface-level air at sites 1-4, given the low and high levels of CO from fossil fuel combustion defined in the two model scenarios. In addition, measurements taken at sites 1-3 probably would have been indistinguishable from the 13 C composition of the biomass burning source. At site 4 on August 11, however, other CO sources probably altered significantly the 13 CO/ 12 CO ratio in surface air from that of CO from biomass burning.

The isotopic characterization of carbon monoxide in the troposphere

Abstract The distributions of stable isotopes in trace atmospheric species are controlled mainly by the isotopic compositions of precursor molecules and isotope fractionation effects during production and removal of the species. Distributions of radioactive isotopes are controlled mainly by the isotopic compositions of precursor molecules and radioactive decay processes. As a result, through their isotopic compositions, atmospheric species are traceable to sources and sinks. Thus, isotopic compositions provide useful information for estimating source strengths and for understanding the importance of removal processes in the cycling of the species. The use of radioactive 14C and the stable isotopes (13C and 18O) are reviewed here for understanding production and removal processes of CO in the troposphere. Carbon monoxide is a critical component in atmospheric chemistry because of its large effect on levels of OH, the principal oxidant in the atmosphere. In the troposphere, this is due to relatively high concentrations of CO and a short lifetime (2–4 months). Initially, 14CO measurements were instrumental in estimating accurately the tropospheric lifetime. Since seasonal 14CO variation is controlled largely by OH, 14CO serves as an important surrogate measure of tropospheric OH. Global 14CO measurements have also been used to estimate the biogenic component of the global CO budget, specifically contributions from biomass burning, oxidized non-methane hydrocarbon (NMHC) emissions, oceans and plants. Research using 14CO measurements is also active in quantifying fossil and non-fossil urban emissions. Kinetic isotopic fractionation during production of 13CO and C18O from reduced precursors allows one to distinguish, at least qualitatively, different varieties of CO based on seasonal tropospheric isotopic measurements. Difficulties in interpreting the stable isotopic record arise from large fractionation effects that obscure source isotopic signatures (in particular the oxygen kinetic isotope effect for the CO+OH reaction), and large seasonal/latitudinal variability in source fluxes. While dual-isotopic CO source signatures do not allow a direct mathematical determination of contributions from several sources, plots of δ18O vs δ13C may help estimate relative proportions of dominant sources. In addition to understanding CO sources, the pressure dependence of the carbon kinetic isotope effect (KIE) has helped elucidate pathways for the CO+OH reaction. Since formaldehyde (HCHO) is an important intermediate in hydrocarbon oxidation to CO and CO2, the stable carbon and oxygen isotopes in atmospheric HCHO may help distinguish different hydrocarbon sources to regional CO budgets and elucidate the importance of different oxidative reaction pathways.

Factors Affecting the Ambient Physicochemical Properties of Cerium-Containing Particles Generated by Nanoparticle Diesel Fuel Additive Use

Despite the use of cerium oxide nanoparticles (nCe) in some regions as a diesel fuel additive, the physicochemical properties of the resulting exhaust particles in the ambient atmosphere are not well known. The mixing state of ceria with other exhaust particles is one such physicochemical property that has been shown to potentially affect ecosystem/human health. In this study, cerium-containing particles associated with an nCe additive were collected in the laboratory and in Newcastle-upon-Tyne, UK where the local bus fleet uses the Envirox nCe additive. Electron microscopy of laboratory-generated exhaust samples indicated both individual ceria and soot particles (external mixture) and ceria contained within soot agglomerations (internal mixture). Low ambient concentrations prevented quantification of the ceria particle mixing state in the atmosphere; therefore, a multicomponent sectional aerosol dynamic model was used to predict the size, chemical composition, and mixing state of ceria particles as a function of distance from an idealized roadway. Model simulations predicted that most ceria particles remain nonmixed in the ambient atmosphere (300 m downwind from the roadway) due to slow coagulation, with the mixing rate most sensitive to the ceria content of emitted nuclei-mode particles and the particle concentration upwind of the road. Although microscopy analysis showed both external and internal mixtures of ceria and soot in freshly emitted particles, the ambient mass concentration, and size distribution of ceria particles predicted by the model was insensitive to the emitted mixing state. Copyright 2015 American Association for Aerosol Research

Filter Material Effects on Particle Absorption Optical Properties

Absorption enhancement and shadowing effects were investigated for nigrosin-laden quartz (fibrous), Teflon (matted), and polycarbonate (membrane) filters in inert surroundings at different sample steady-state temperatures and particle mass loadings. Sample absorptivity was determined using a novel laser-heating technique, which is based on perturbing the sample steady-state temperature and monitoring the thermal response during decay back to steady state, along with a model for thermal energy conservation. In addition, transmissivity measurements were carried out to enable determination of the sample absorption coefficient. The results indicated that the isolated-nigrosin absorption coefficient decreased with steady-state temperature and increased with mass loading and filter pore size. Comparing the absorption coefficient for both the isolated nigrosin and nigrosin-laden filters, indicated that absorption enhancement was most significant for the Teflon filters and least significant for the polycarbonate filters. The effect became more significant as the pore size decreased, steady-state temperature increased, and particle mass loading decreased. The decrease in the isolated-nigrosin, mass-specific absorption cross-section with heavier sample loadings was attributed to shadowing effects. Copyright 2014 American Association for Aerosol Research

Absorption/transmission measurements of PSAP particle-laden filters from the Biomass Burning Observation Project (BBOP) field campaign

ABSTRACT Absorptivity measurements with a laser-heating approach, referred to as the laser-driven thermal reactor (LDTR), were carried out in the infrared and applied at ambient (laboratory) nonreacting conditions to particle-laden filters from a three-wavelength (visible) particle/soot absorption photometer (PSAP). The particles were obtained during the Biomass Burning Observation Project (BBOP) field campaign. The focus of this study was to determine the particle absorption coefficient from field-campaign filter samples using the LDTR approach, and compare results with other commercially available instrumentation (in this case with the PSAP, which has been compared with numerous other optical techniques). Advantages of the LDTR approach include (1) direct estimation of material absorption from temperature measurements (as opposed to resolving the difference between the measured reflection/scattering and transmission), (2) information on the filter optical properties, and (3) identification of the filter material effects on particle absorption (e.g., leading to particle absorption enhancement or shadowing). For measurements carried out under ambient conditions, the particle absorptivity is obtained with a thermocouple placed flush with the filter back surface and the laser probe beam impinging normal to the filter particle-laden surface. Thus, in principle one can employ a simple experimental arrangement to measure simultaneously both the transmissivity and absorptivity (at different discrete wavelengths) and ascertain the particle absorption coefficient. For this investigation, LDTR measurements were carried out with PSAP filters (pairs with both blank and exposed filters) from eight different days during the campaign, having relatively light but different particle loadings. The observed particles coating the filters were found to be carbonaceous (having broadband absorption characteristics). The LDTR absorption coefficient compared well with results from the PSAP. The analysis was also expanded to account for the filter fiber scattering on particle absorption in assessing particle absorption enhancement and shadowing effects. The results indicated that absorption enhancement effects were significant, and diminished with increased filter particle loading.

Thermochemical Characterization of Materials using a Novel Laser-Heating Technique

This article reviews the application of a novel rapid laser-heating technique (referred to as the laserdriven thermal reactor) for characterizing multiphase, multicomponent substances. The technique provides quantitative measurements of various relevant thermochemical properties, including sample heat release rate, chemical kinetics rates, total heat value, specific heat release, and chemical reaction byproduct identification. The technique is currently being used to measure the absorption coefficient of particle-coated filters for atmospheric aerosol research. The optical properties of individual particle-laden droplets are also being studied in the laboratory under tropospheric conditions. In addition, a forensic science investigation of energetic materials is underway to develop a database of their thermal and chemical signatures. Past studies include characterization of simulant hazardous organic wastes and propellants for improving storage safety, and planned future studies are to focus on thermochemical characterization of biomass/biofuels/biodiesel. Results presented demonstrate the capability of this technique to address different thermochemical-related issues associated with a wide variety of applications.