Donna B. Klinedinst

Sources of Urban Contemporary Carbon Aerosol

Emissions from the major sources of fine carbonaceous aerosol in the Los Angeles basin atmosphere have been analyzed to determine the amounts of the ^(12)C and ^(14)C isotopes present. From these measurements, an inventory of the fossil carbon and contemporary carbon particle emissions to the Los Angeles atmosphere has been created. In the winter, more than half of the fine primary carbonaceous aerosol emissions are from sources containing contemporary carbon, including fireplaces, charbroilers, paved road dust, cigarette smoke, and brake lining dust, while in the summer at least one-third of the carbonaceous particle emissions are contemporary. Using a mathematical model for atmospheric transport, predictions are made of the atmospheric fine particulate fossil carbon and contemporary carbon concentrations expected due to primary source emissions. Model predictions are in reasonable agreement with the measured radiocarbon content of the fine ambient aerosol samples. It is concluded that the high fraction of contemporary carbon measured historically in Los Angeles is not due to local emission sources of biogenic material; rather, it is due to a combination of local anthropogenic pollution sources and background marine aerosol advected into the city.

Distinguishing the Contributions of Residential Wood Combustion and Mobile Source Emissions Using Relative Concentrations of Dimethylphenanthrene Isomers

As part of the United States Environmental Protection Agencys Integrated Air Cancer Project, air particulate matter samples collected in Boise, ID, were analyzed by gas chromatography with mass spectrometric detection (GC-MS) and apportioned between their two main sources : residential wood combustion (RWC) and motor vehicle (MV) emissions. The technique used for distinguishing the source contributions involved comparison of the concentration of 1,7-dimethylphenanthrene (1,7-DMP), a polycyclic aromatic hydrocarbon (PAH) emitted primarily by burning soft woods (e.g., pines), with that of a PAH emitted in modest concentrations by both RWC and MV sources, 2,6-dimethylphenanthrene (2,6-DMP). These results were then compared with the mean 1,7-DMP/2,6-DMP ratio of 48 samples collected in a roadway tunnel, with any enrichment in the Boise sample ratios over the mean tunnel ratio attributable to the RWC source. These resulting RWC contributions were compared with fraction RWC results obtained by radiocarbon measurements ( 14 C/ 13 C) of the same extracts from Boise, with generally good correlations between the two techniques observed, suggesting that the methods are comparable when used to distinguish emissions of MVs from RWC of soft woods.

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.

Fossil- and bio-mass combustion: C-14 for source identification, chemical tracer development, and model validation

Abstract Carbonaceous gases and aerosols emitted during fossil- and bio-mass combustion processes have significant impacts on regional health and visibility, and on global climate. 14 C accelerator mass spectrometry (AMS) has become the accepted standard for quantitatively partitioning individual combustion products between fossil and biospheric sources. Increased demands for source apportionment of toxic gases/vapors such as carbon monoxide and benzene, and toxic aerosol species such as polycyclic aromatic hydrocarbons, however, have led to increased needs for chemical source tracers. As a result, the application of atmospheric 14 C measurements has been extended to the discovery of new chemical tracers and the validation of the related apportionment models. These newer applications of 14 C are illustrated by recent investigations of: 1) sources of excessive concentrations of carbon monoxide and benzene in the urban atmosphere during the winter, as related to combustion source control strategies; and 2) the development/validation of potassium and hydrocarbon tracer models for the apportionment of mutagenic aerosols from biomass (wood) burning and motor vehicle emissions. Among the important consequences of these studies are new insights into potential limitations of elemental tracer models for biomass burning, and the impact of bivariate (isotopic, mass) chemical blanks on atmospheric 14 C-AMS data.

Calibration of a Condensation Particle Counter Using a NIST Traceable Method

This work presents a calibration of a commercial condensation particle counter using National Institute of Standards and Technology (NIST) traceable methods. By the nature of the metrology involved, this work also compares the measurement results of three independent techniques for measuring aerosol concentration: continuous flow condensation particle counter (CPC); aerosol electrometer (AE); and the aerosol concentration derived from microscopic particle counting. Because of the transient nature of aerosol, there are no concentration artifact standards such as exist for particle diameter standards. We employ a mobility classifier to produce a nearly monodisperse, 80 nm, polystyrene latex aerosol. The test aerosol is used as a challenge for the CPC and the AE, and is subsequently filter sampled for electron microscopy. Our test stand design incorporates a continuous CPC aerosol concentration monitor to verify the aerosol stability. The CPC determines particle concentration by single particle counting at a constant sample flow rate. The AE has been calibrated to a NIST traceable current standard. The subsequent aerosol concentration measurement is obtained by determining the electrical current produced by a charged aerosol transported to the detector by a controlled aerosol flow rate. We have NIST traceability for flow rates for all methods and a methodology to calibrate the AE to NIST traceable electrical standards. The latter provides a calibration and a determination of the uncertainty in the aerosol derived current measurement. A bias in the measurements due to multiple charged particles was observed and overcome by using an electrospray aerosol generator to produce the challenge particles. This generator was able to produce aerosol concentrations over the range of 100 particles/cm 3 to 15 000 particles/cm 3 with lower number of dimer particles (≈1%). In our work, independent measurement of aerosol concentration is obtained by quantitatively collecting samples of the airborne polystyrene latex spheres on a small pore filter material and determining the number of particles collected by electron microscopy. Electron micrograph images obtained using a field-emission scanning electron microscope are analyzed using particle counting. We found the relative uncertainty in the aerosol electrometer measurements to be in excess of 100% for particle concentrations of approximately 120 particles/cm 3 and approximately 5% for concentrations above 6000 particles/cm 3 . The uncertainty found by the microscopy method was approximately 3%.

Comparative study of Fe-C bead and graphite target performance with the National Ocean Science AMS (NOSAMS) facility recombinator ion source

Abstract An accelerator mass spectrometry (AMS) experiment was designed to investigate 14 C target performance for two target types over a range of isotopic concentrations and sample sizes, with a special focus on the ability to measure 14 C in environmental samples having only microgram amounts of carbon. The findings were positive, showing that precision, accuracy, and stability were adequate to determine 14 C to 1% or better in samples containing as little as 25 μg carbon. Satisfactory Poisson uncertainty and target stability were demonstrated down to a level of 7 μg carbon, but experimental data showed that accurate measurements at that level require detailed knowledge of blank variability and mass dependence of the modern carbon calibration factor.

Iron-Manganese System for Preparation of Radiocarbon AMS Targets: Characterization of Procedural Chemical-Isotopic Blanks and Fractionation

We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10-100 mu g carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures. Chemical and isotopic ( (super 13) C, (super 14) C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 mu g (central mean) with a dispersion of 0.50 mu g (standard deviation). Reductions of 10 mu g carbon onto targets were complete in 3-6 h with all targets, after correction for the blank, reflecting the (super 13) C signature of the starting material. The 100 mu g carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the (super 13) C data suggested the presence of kinetic isotope effects during the reduction. The observed CO (sub 2) -graphite (super 13) C fractionation factor was 3-4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%. Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the (super 12) C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to (super 14) C uncertainties of 5-1% for 10-100 mu g carbon samples, respectively. A bias of 5-7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.

14C measurements of sub-milligram carbon samples from aerosols

Accelerator mass spectrometry (AMS) at the milligram level is routinely performed, but it is difficult to go substantially below 100 mu g of carbon. We discuss various approaches for sample preparation, machine operation and data evaluation, to meet the special requirements of (super 14) C AMS measurements at the microgram-carbon level. Furthermore, we present first results obtained at the Vienna Environmental Research Accelerator (VERA) from (super 14) C measurements of a snow sample from Gaithersburg, Maryland, USA, prepared at the National Institute of Standards and Technology (NIST).

Characterization of SiGe films for use as a National Institute of Standards and Technology Microanalysis Reference Material (RM 8905).

Bulk silicon-germanium (SiGe) alloys and two SiGe thick films (4 and 5 microm) on Si wafers were tested with the electron probe microanalyzer (EPMA) using wavelength dispersive spectrometers (WDS) for heterogeneity and composition for use as reference materials needed by the microelectronics industry. One alloy with a nominal composition of Si0.86Ge0.14 and the two thick films with nominal compositions of Si0.90Ge0.10 and Si0.75Ge0.25 on Si, evaluated for micro- and macroheterogeneity, will make good microanalysis reference materials with an overall expanded heterogeneity uncertainty of 1.1% relative or less for Ge. The bulk Ge composition in the Si0.86Ge0.14 alloy was determined to be 30.228% mass fraction Ge with an expanded uncertainty of the mean of 0.195% mass fraction. The thick films were quantified with WDS-EPMA using both the Si0.86Ge0.14 alloy and element wafers as reference materials. The Ge concentration was determined to be 22.80% mass fraction with an expanded uncertainty of the mean of 0.12% mass fraction for the Si0.90Ge0.10 wafer and 43.66% mass fraction for the Si0.75Ge0.25 wafer with an expanded uncertainty of the mean of 0.25% mass fraction. The two thick SiGe films will be issued as National Institute of Standards and Technology Reference Materials (RM 8905).

Radiocarbon blank correction: Methodologies and limitations in a major urban study of carbonaceous aerosols

The Northern Front Range Air Quality Study (NFRAQS) was the latest and most ambitious of a series of efforts to apportion sources of carbonaceous aerosol “soot” in the Denver, Colorado metropolitan area. The study was mandated by the Colorado State Legislature as a result of the continuing impact of aerosol carbon on visibility in the region. Apportionment of fossil and biomass carbon was based on blank-corrected values of carbon mass concentrations (μg/m3) and 14C data (fM, fraction of modern carbon) of a selected subset of the samples collected in conjunction with this program. Over 100 14C measurements were made on size segregated (⩽2.5 μm diameter) atmospheric aerosol samples collected during the summer of 1996 and the winter of 1996–1997. The reported fM values required correction for both the mass and fM of the overall carbon blank. Lack of direct fM data for the field blanks had a substantial effect on the estimated uncertainty of the final results, and in a few of the most extreme cases blank-corrected fM data had to be designated as “indeterminate”. Blank correction procedures and limitations will be illustrated with quantitative data from the NFRAQS study.