Abstract
Black carbon (BC), produced by incomplete combustion of fossil fuels and vegetation, occurs ubiquitously in soils and sediments. BC exists as a continuum from partly charred material to highly graphitized soot particles, with no general agreement on clear‐cut boundaries of definition or analysis. In a comparative analysis, we measured BC forms in eight soil samples by six established methods. All methods involved removal of the non‐BC components from the sample by thermal or chemical means or a combination of both. The remaining carbon, operationally defined as BC, was quantified via mass balance, elemental composition or by exploiting benzenecarboxylic acids as molecular markers or applying 13C MAS NMR (magic angle spinning nuclear magnetic resonance) spectroscopy. BC concentrations measured for individual samples vary over 2 orders of magnitude (up to a factor of 571). One possible explanation for this wide range of results is that the individual BC methods rely on operational definitions with clear‐cut but different boundaries and developed for specific scientific questions, whereas BC represents a continuum of materials with widely contrasting physicochemical properties. Thus the methods are inherently designed to analytically determine different parts of the continuum, and it is crucial to know how measurements made by different techniques relate to each other. It is clear from this preliminary comparative analysis that a collection of BC reference materials should be established as soon as possible 1 ) to ensure long‐term intralaboratory and interlaboratory data quality and 2) to facilitate comparative analyses between different analytical techniques and scientific approaches
Highlights
Black carbon(BC), producedby incompletecombustionof fossil fuels and vegetation fires, is relatively resistant to degradationand occursubiquitouslyin naturalenvironments, including soils, sediments,seawater,and the atmosphere [Goldberg,1985 BC existsas a continuumfrom partlycharred plant materialthroughchar and charcoalto soot and graphite particleswith no general agreementon clear-cut boundaries [Sellerand Crutzen, 1980].BC can form in two fundamentally differentways.The solidresiduesof planttissuesform charBC, whereasvolatilesformedwithin flames comprisehighly graphitizedsootBC
Basically,the problemleadingto the observeddifferencesin resultsobtainedby the variousmethodsis that BC existsas a continuumof thermallyalteredmaterial,whereasmanymethods rely on operational definitions with clear-cut but different boundariesW. ithin the continuum,the analyticalwindowsof individual methods strongly depend on the objectives of a particularstudy(e.g., quantificationof BC for carboncyclingin soils,reconstructionof fire history),and vary with the associated matrix.At present,very little is known about the comparabilityof the chemicaland physicalpropertiesof the BC componentsmeasuredby the individual methods.Systematiccomparativeanalysison welldefinedstandardmaterialscouldhelp immenselyto circumvent theseproblemsand allow a better understandingof what is actuallybeingdeterminedby thedifferentmethodsandhowthese resultsrelate to each other, leadingto more accurate measurements of BC in soils and sediments
At presenthereareno referenceBC sampleswidely available for interlaboratorycomparisonor instrumentstabilitytestsnor is there a consensuson how suchsamplesmight bestbe prepared andpreservedO. verall, it is clearfi'omthis preliminaryexcercise that a collection of BC reference materials should be established as soonas possibleto facilitatetheir comparativeanalysisby a range of commonlyusedtechniquesin a numberof different laboratories.These efforts will provide a more thorough understandinogf what is actuallybeingdeterminedby different BC methodsand the impact of different matriceson these methods
Summary
Black carbon(BC), producedby incompletecombustionof fossil fuels and vegetation fires, is relatively resistant to degradationand occursubiquitouslyin naturalenvironments, including soils, sediments,seawater,and the atmosphere [Goldberg,1985 BC existsas a continuumfrom partlycharred plant materialthroughchar and charcoalto soot and graphite particleswith no general agreementon clear-cut boundaries [Sellerand Crutzen, 1980].BC can form in two fundamentally differentways.The solidresiduesof planttissuesform charBC, whereasvolatilesformedwithin (and recondensedfrom) flames comprisehighly graphitizedsootBC. In recentyears,increasing attentionhasbeengivento geochemicaal ndbiologicalstudiesof differentformsof BC owing to their potentialimportancein a wide rangeof biogeochemicaplrocessesA. S examples,BC may represenat significanst inkin theglobalcarboncycle[Kuhlbusch, 1998a], affect the Earth's radiativeheat balance [Crutzenand.
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