D. M. Smith

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.

Water soluble organic compounds formed by oxidation of soot

The water soluble organic compounds (WSOC) in soot samples as a function of the extent of ozone oxidation have been measured by a new methodology which utilises ion exchange chromatography, total carbon analysis and proton nuclear magnetic resonance. These results have been compared with the same analyses of various atmospheric aerosol samples. The WSOC produced from oxidation of soot particles increase rapidly with ozone exposure and consist primarily of aromatic polyacids which are found widely in atmospheric aerosols and which are frequently referred to as macromolecular humic-like substances (HULIS). This work demonstrates that the atmospheric oxidation of soot can produce HULIS in aerosols. The cloud condensation nuclei effectiveness of soot aerosol likely has its origin in these oxidation processes.

The Structure of Hexane Soot I: Spectroscopic Studies

Soot produced from the combustion of fossil fuels, widely distributed in the atmosphere, is significantly different from most carbons for which the surface structure and/or reactivity have been studied. The composition and surface structure of soot derived from the combustion of n-hexane have been examined by FT-IR, Raman, UC CP/MAS NMR, and EPR spectroscopies as well as through desorption measurements. Carbon-oxygen functionalities on the fresh carbon surface include acid anhydride, a carbonyl conjugated with an aromatic segment, an alkyl-ketone, and aryl ether linkages. Also present, confirmed by isotopic substitution, is a quantity of unsaturated C-H, dependent upon the combustion conditions. The degree of aromaticity and the graphitic nature of this soot have been determined.

The surface structure and reactivity of black carbon

Abstract Fourier transform infrared (FT-IR) spectroscopy has been the most definitive analytical tool in a comprehensive program on the structure and reactivity of black carbon (in the form of n -hexane soot). In combination with other techniques, it has revealed the soot structure, as produced by high temperature incomplete combustion, to be predominantly aromatic with a surface coverage by oxygen-containing functional groups of about 0.5. Particularly well suited to following net changes in surface groups, and gas phase reactant/product concentrations, FT-IR has been the key technique in determining the kinetics and mechanisms of some important heterogeneous reactions of black carbon with gas phase oxidant molecules. For example, the reaction of NO 2 /N 2 O 4 with soot follows a dual path mechanism, down to 2 p.p.m., which is reflected in the rate law: initial rate = (k 1 + k 2 [ soot ] 1 2 ) P NO2 . On the other hand, catalytic decomposition initiates the reaction with ozone, followed by the formation of surface carboxylic groups and gaseous CO 2 and H 2 O. The evidence suggests that dissociation of ozone yields a steady-state concentration of excited oxygen atom which is actually the oxidant. FTIR combined with chemical measurements has proven that a high solubility observed for carbon particles exposed to ozone has its origin in the hydrolysis of the surface carboxylics. Significant effects of simulated solar radiation on the reactions, especially in the soot/SO 2 /H 2 O/O 2 system, has been revealed by FTIR. Infrared will continue its central role in the examination of increasingly complex systems containing black carbon, particularly through its interface with ancillary techniques.

The Structure of Hexane Soot II: Extraction Studies:

Successive extraction of soluble components of hexane soot with a suite of solvents has been followed by CGC/MS, FT-IR, UV/Vis, fluorescence, and NMR spectroscopic analysis of these extracts. From these studies, additional understanding of the structure and reactivity of this material has emerged. A significant portion of the soot is extractable as polynuclear aromatic and aliphatic compounds, while the nonextractable solid structure, with both aromatic and aliphatic portions, contains such carbon-oxygen functionalities as acid anhydride, carbonyl, and ether linkages. A model of hexane soot as formed in flame is proposed on the basis of this work.

Spectroscopic and Solubility Characteristics of Oxidized Soots

Spectroscopic and solubility 1 studies of reaction products of soot (black carbon) with O3, NO2/N2O4, and SO2 have revealed a relationship between reactivity and product solubility and structure. A remarkably high solubility of ozonated n-hexane soot has its origin in the formation of anhydride and lactone surface structures and their subsequent hydrolysis to carboxylic acid species. Calculations indicate that the rate of surface carboxylation of 0.1-μm diameter spheroidal soot particles, in the presence of 50 ppbv ozone at ambient temperature, is such that solubilization may occur within a 30-minute time frame. Measurements on ambient air aerosol samples in metropolitan Denver are consistent with these observations and demonstrate the high reactivity of soot with ozone even at very low levels in natural systems. 1A “solution” of soot and water refers to a system that passes through Whatman 934-AH glass filter, and does not imply complete solid-solvent interaction at the molecular level.

Hydration of black carbon

Hydration studies of n-hexane soot particles in the relative H 2 O pressure range 0.33-0.52 have revealed the nature of the processes between water molecules in the vapor phase and primary reaction sites at the particle surface. Initially, about 40% of the carbon-oxygen functionalities (most likely the carboxylics) on freshly prepared soot reacts irreversibly with water vapor, while at P/P o ≥ 0.48, the adsorption follows the Dubinin-Radushkevich equation. Determination of these parameters enables calculation of surface coverage at limiting adsorption and at the chemisorption limit for all materials studied. Limiting surface coverage of fresh soot is consistent with a 50% oxygen coverage determined earlier ; it increases to about 100% for nitrated soot, which is the most extensively hydrated. Hydration has been shown to increase with soot aging, a phenomenon which has revealed an important role of physisorbed O 2 in hydration. Trace-metal incorporation at the 30-110 ppm level significantly increases particle hydration and demonstrates a role of metal centers in the process.

Carbonaceous particle hydration

Abstract Microgravimetric measurements of the hydration of several different black carbons or soots and a series of commercial carbon blacks have been carried out, over a relative humidity range of 20–85%, in an extension of earlier work with the model n-hexane soot. All adsorption isotherms are of type III and were analyzed by the use of the Dubinin–Radushkevich (DR) equation which, although applicable over a limited range of intermediate relative humidity values, allows identification of chemisorption limit and onset of multilayer formation. While surface area determines the maximum adsorption possible for a given type, surface functionalities are determinative at lower humidity and are characteristic of the soot-producing fuel. Aging of carbon particles and oxygen chemisorption as well as O2 physisorption strongly influence the extent of hydration for those soots studied, such as JP-8 aviation and diesel fuels. Infrared spectra confirm the surface oxidation of JP-8 soot by its reaction with O3, a reaction of probable atmospheric importance, as underlying its increased hydration.

The Structure of Hexane Soot. Part III: Ozonation Studies

The gravimetric estimation of CO2-evolved, FT-IR monitoring of the rate of formation of surface species, as well as several other measurements during the n-hexane soot/ozone reaction, has led to the conclusion that soot is a layered structure whose layers are chemically bonded, unlike the graphite structure where the layers are ordered by van der Waals forces. Fourier transform infrared spectroscopy, electrochemical analysis, microgravimetry, scanning electron microscopy (SEM), and the classical BET surface area determination technique have been employed in this work.

Soot-Ozone Reaction Kinetics: Spectroscopic and Gravimetric Studies

Studies on the kinetics of soot-O3 reactions, at various soot and ozone concentrations, have been conducted under flow conditions with ozone ranging from 50 to 15,000 ppm and soot from 2 to 350 mg. At lower concentrations, the initial rates of CO2 and CO formation are found to be half order with respect to soot and first order with respect to ozone. At higher concentrations, CO2 formation exhibits a more complex pattern. The initial rate for the formation of CO2 for a first stage is half order with respect to soot and 1.5 order with respect to O3, while the second stage is zero order in both species. Differences between data at higher and lower concentrations are discussed, and mechanisms for the formation of CO2 CO, and carboxylics during ozonation are suggested. Mass balance calculations on low concentration data reveal that only a small portion of the ozone is used to produce CO2, CO, H2O, and carboxylic species, most of it being decomposed catalytically over soot. At higher concentrations of O3 the rate of formation of carboxylic functionalities during the hexane soot-ozone reaction under static conditions has been examined. The initial rate, as determined by the Elovich equation, suggests that the soot-ozone reaction is nearly 6 times faster under equivalent conditions than the sool-NO2/N2O4 reaction reported earlier from this laboratory.