Lloyd A. Currie

Comparison of quantification methods to measure fire‐derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified most of the carbon in all three materials as BC, whereas other methods identified only soot carbon as BC. The different methods also gave widely different BC contents for the environmental matrices. However, these variations could be understood in the light of the findings for the other two groups of materials, i.e., that some methods incorrectly identify non-BC carbon as BC, and that the detection efficiency of each technique varies across the BC continuum. We found that atmospheric BC quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of BC as light-absorbing material irrespective of its origin, leading to biases when applied to terrestrial and sedimentary materials. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. A major goal of this ring trial was to provide a basis on which to choose between the different BC quantification methods in soil and sediment studies. In this paper we summarize the advantages and disadvantages of each method. In future studies, we strongly recommend the evaluation of all methods analyzing for BC in soils and sediments against the set of BC reference materials analyzed here.

Nomenclature in evaluation of analytical methods including detection and quantification capabilities1 (IUPAC Recommendations 1995)

Abstract This IUPAC nomenclature document has been prepared to help establish a uniform and meaningful approach to terminology, notation, and formulation for performance characteristics of the chemical measurement process (CMP). Following definition of the CMP and its performance characteristics, the document addresses fundamental quantities related to the observed response and calibration, and the complement to the calibration function: the evaluation function. Performance characteristics related to precision and accuracy comprise the heart of the document. These include measures for the means or “expected values” of the relevant chemical quantities, as well as dispersion measures such as variance and standard error. Attention is given also to important issues involving: assumptions, internal and external quality control, estimation, error propagation and uncertainty components, and bounds for systematic error. Special treatment is given to terminology and concepts underlying detection and quantification capabilities in chemical metrology, and the significance of the blank. The document concludes with a note on the important distinction between the sampled population and the target population, especially in relation to the interlaboratory environment.

Detection and Quantification Limits: Origins and Historical Overview

Detection and quantification capabilities represent fundamental performance characteristics of measurement processes, yet there have been decades of confusion and miscommunication regarding the underlying concepts and terminology. New, coordinated documents prepared for the International Union of Pure and Applied Chemistry (IUPAC) [L.A. Currie, IUPAC Commission on Analytical Nomenclature, Recommendations in Evaluation of Analytical Methods including Detection and Quantification Capabilities, Pure Appl. Chem. 67 (1995) 1699–1723] and the International Organization for Standardization (ISO) [P. Wilrich, Chairman, ISO/DIS 11843-1,2 (1995), Capability of Detection, ISO/TC69/SC6, ISO Standard, 11843-1, 1977] promise to alleviate this situation by providing, for the first time, a harmonized position on standards and recommendations for adoption by the international scientific community. The text begins with (1) a brief historical summary of detection limits in chemistry, illustrating the critical need for the development of a sound and uniform system of terms and symbols; and (2) a review of the ISO–IUPAC deliberations and the ensuing harmonized position on concepts and nomenclature. In the following text a number of special topics are introduced, including: specification of the measurement process, attention to the meaning and evaluation of “sigma”, special considerations for calibration (or regression)-based detection and quantification limits, the central role of the blank, and finally, some challenges for the future.

Accelerator Radiocarbon Dating at the Molecular-Level

Molecular level 14C dating is the isolation of specific classes of molecules for their 14C dating by accelerator mass spectrometry (AMS). Complex matrices such as fossil bone are difficult to date due to their extreme chemical heterogeneity. By isolating individual amino acids, contaminants (humates) are removed and crystalline amino acids result. Bones with ≥0·1–0·2% N and collagenous compositions can be dated accurately because structural collagen is present; contaminants are removable with XAD resin. Bones with ⩽0·1% N and non-collagenous compositions yield dates hundreds to thousands of years too young because most of the preserved organic matter is exogenous. Accelerator 14C dates on collagenous and non-collagenous bones are not comparable due to intrinsic dating inaccuracies. AMS 14C dating of amino acids demonstrated that (1) post 10,800 year ages for North American megafauna are due to sample contamination, not Holocene ages on extinct fauna, (2) a Clovis age (10,900 years) was established for a human fossil from the Anzick site, Montana, (3) Holocene ages cannot be established absolutely for many North American human fossils because the bones were non-collagenous, (4) accurate ages are attainable on vertebrate fossils as small as passerine birds from Pacific Island localities, (5) well preserved bones are datable without their destruction by extracting protein with water at high temperatures, and (6) stratigraphic anomalies to 45,000 years in European Upper Paleolithic rock shelters are recognizable by dating bone directly.

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.

Radiocarbon measurements of black carbon in aerosols and ocean sediments

Black carbon (BC) is the combustion-altered, solid residue remaining after biomass burning and fossil fuel combustion. Radiocarbon measurements of BC provide information on the residence time of BC in organic carbon pools like soils and sediments, and also provide information on the source of BC by distinguishing between fossil fuel and biomass combustion byproducts. We have optimized dichromate-sulfuric acid oxidation for the measurement of radiocarbon in BC. We also present comparisons of BC 14C measurements on NIST aerosol SRM 1649a with previously published bulk aromatic 14C measurements and individual polycyclic aromatic hydrocarbon (PAH) 14C measurements on the same NIST standard.Dichromate-sulfuric acid oxidation belongs to the chemical class of BC measurement methods, which rely on the resistance of some forms of BC to strong chemical oxidants. Dilute solutions of dichromate-sulfuric acid degrade BC and marine-derived carbon at characteristic rates from which a simple kinetic formula can be used to calculate concentrations of individual components (Wolbach and Anders, 1989). We show that: (1) dichromate-sulfuric acid oxidation allows precise, reproducible 14C BC measurements; (2) kinetics calculations give more precise BC yield information when performed on a % OC basis (vs. a % mass basis); (3) kinetically calculated BC concentrations are similar regardless of whether the oxidation is performed at 23°C or 50°C; and (4) this method yields 14C BC results consistent with previously published aromatic 14C data for an NIST standard.For the purposes of intercomparison, we report % mass and carbon results for two commercially available BC standards. We also report comparative data from a new thermal method applied to SRM 1649a, showing that thermal oxidation of this material also follows the simple kinetic sum of exponentials model, although with different time constants.

Evolution and Multidisciplinary Frontiers of 14c Aerosol Science

A review is given of some critical events in the development of radiocarbon aerosol science, and the profound influence of radiocarbon accelerator mass spectrometry (AMS) on its current applications and future prospects. The birth of this discipline occurred shortly after the initial development of 14C dating. Unlike dating, which is founded on the continual decay of 14C and the resulting full range of 14C/12C ratios in once-living matter, 14C applications to atmospheric aerosol research relate primarily to the determination of mixing ratios of fossil and biomass components. Such determinations have come to have major importance in work ranging from the resolution of woodburning and motor vehicle components of urban particulate pollution, to the apportionment of radiation-forcing (black) particulate carbon from natural wildfires and anthropogenic regional plumes. The development of this area has paralleled that of AMS itself, with the one or the other alternately serving as the driving force, in a sort of counterpoint. The remarkable million-fold improvement in sensitivity made possible by AMS has become critical in meeting rapidly emerging societal concerns with the origins and effects of individual carbonaceous species on health and climate.

Analysis of insoluble carbonaceous materials from airborne particles collected in pristine regions of colorado

Abstract Atmospheric particulate samples collected from remote mountainous locations in Colorado were extracted in sequence with methanol, methylene chloride, acetone and hexane. The resulting insoluble material was investigated using pyrolysis-mass spectrometry (Py-MS) and accelerator mass spectrometric 14 C analysis. The pyrolysis produced a complex mixture of aromatic and aliphatic hydrocarbons. The aliphatic hydrocarbons were postulated as being derived from vegetation. The 14 C analysis and the GC/MS results suggested that the aromatics occurred from combustion of living materials and also from long-range transport of industrial pollution. Appropriate samples from power plants and controlled forest fires were used to substantiate the conclusions.

Assessment of contemporary carbon combustion source contributions to urban air particulate levels using carbon-14 measurements.

w Measurement of carbon-14 activities with new low-level counting methods has been demonstrated to be an effective tool for assessing the contribution of contemporary carbon combustion sources to the mass collected with typical highvolume air samplers. This study represents the first time that radiocarbon measurements have been applied to fine particles (<2 pm) and used to assess the contribution of specific sources to urban air quality. Radiocarbon analysis of fine particles minimized interferences from large particles such as pollen, spores, wood fiber, etc., and improved the method’s ability to assess the impact of burning vegetative material such as field and slash burning and space heating with wood. Slash burning contributed between 39% and 70% of the fine particulate mass while field burning contributed 50% of the total suspended particulates (TSPs) on high-impact days in the Portland and Eugene, OR, airsheds. Radiocarbon analysis of filters selected for high impact from residential wood combustion shows that this source is a substantial contributor to fine particulate mass during winter months in Portland, OR.