Recommendations for the Interpretation of "Black Carbon" Measurements

A. Petzold1, J. A. Ogren2, M. Fiebig3, P. Laj4, S.-M. Li5, U. Baltensperger6, Th. Holzer-Popp7, S. Kinne8, G. Pappalardo9, N. Sugimoto10, C. Wehrli11, A.

Wiedensohler12, and X.-Y. Zhang13

Contact: Andreas Petzold, Institut für Energie- und Klimaforschung IEK-8: Troposphäre, FZ Jülich a.petzold@fz-juelich.de

Recommended Terminology

Photo by courtesy of Alexander Karmazin

Affiliations of GAW SAG Aerosol Members1 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung IEK-8, 52425 Jülich, Germany2 NOAA/ESRL Global Monitoring Division, Boulder, CO 80305, USA3 Norwegian Institute for Air Research (NILU), N-2027 Kjeller, Norway4 Laboratoire de Glaciologie et Géophysique de l'Environnement, Université de Grenoble I - CNRS, 38402 - Saint Martin d'Hères cedex, France5 Environment Canada, Processes Research Section, Toronto, ON M3H 5T4, Canada6 Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland7 Deutsches Fernerkundungsdatenzentrum, DLR, 82234 Oberpfaffenhofen, Germany8 Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany9 Istituto di Metodologie per l'Analisi Ambientale (CNR-IMAA), Potenza, I-85050, Italy10 National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan11 Physikalisch-Meteorologisches Observatorium Davos (PMOD/WRC), 7260 Davos, Switzerland12 Leibniz Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, Germany13 Chinese Academy of Meteorological Sciences, 46 Zhong-Guan-Cun S. Av., Beijing 100081, China

GAW Scientific Advisory Group Aerosol

SAG website: http://gaw.tropos.de

Chairman: J. A. Ogren , NOAA john.a.ogren@noaa.gov

These recommendations are a result of discussions carried out in the context of the Scientific Advisory Group (SAG) for Aerosols of the Global Atmosphere Watch (GAW) program of the World Meteorological Organization (WMO).

However, the authors express their own views and do not act on behalf of, or commit, their institutions, ministries or WMO.

Black carbon (BC) is carbon that is black The formation process is excluded from this definition because of the variety of potential processes. While BC is mostly formed in incomplete combustion, it can be a pro-duct of pyrolysis of carbonaceous matter, dehydration of sugar, or heating of wood in an oxygen-free atmosphere

Elemental carbon (EC) is formally defined as a substance containing only carbon The formal term “elemental carbon” refers to a set of materials with different optical and physical properties instead of a given material with well-defined properties. Examples of “true” EC are diamond, carbon nanotubes, graphite or fullerenes.

Strict definitions are not particularly useful in practice because

carbonaceous matter appears in atmospheric aerosols under no circumstances as pure matter;

it occurs as a highly variable mixture of different carbonaceous compounds with different material properties.

What is Black Carbon

Classification and molecular structure of carbonaceous aerosol components (Pöschl, Anal Bioanal Chem 2003)

Structure of Black Carbon in the atmosphere (Ogren & Charlson TELLUS 1983)

Delhaye, 2009

Interpreting “BC” Measurements

Property Characteristics Consequences

Microstructure graphitic-like structure containing a high

fraction of sp2-bonded carbon atoms

low chemical reactivity in the atmo-

sphere; slow removal by chemical

processes; strong optical absorption

Morphology aggregates consisting of small carbon

spherules of < 10 - 50 nm in diameter

high specific surface area; high capacity

for sorption of other species

Thermal stability refractory material with a volatilization

temperature near 4000K; gasification is

possible only by oxidation at T > 340°C

high stability in the atmosphere; longer

atmospheric life time

Solubility insoluble in organic solvents including

methanol and acetone, in water, and in the

other components of atmospheric aerosol

Slow removal by clouds and precipi-

tation, unless coated with water-soluble

compounds; longer atmospheric life time

Light absorption uniformly absorbing in the VIS; characterized

by a significant, non-zero and wavelength-

independent imaginary part of the refractive

index over VIS and NIR spectral regions

Reduction of the albedo of clouds, snow,

and ice; atmospheric heating; surface

cooling – all of which lead to effects on

solar radiation and climate

Properties defining Black Carbon and their consequences for effects and atmospheric lifetime Evolved Carbon

CO2 evolved from thermal or thermo - optical methods; applied analytical protocols, e.g. IMPROVE / EUSAAR

BC properties: chemical composition, thermal stability

Light Absorption Filter-based: Aethalometer, PSAP, MAAP, COSMOS In situ: photo-acoustic, ext. minus scat. BC properties: light absorption

Laser Incandescence Laser heating of particles, e.g., SP2, LII BC Properties: refractory, chemical composition

Aerosol Mass Spectrometry Vaporization and detection of carbon ion clusters in mass

spectra: ATOFMS, SP-AMS BC properties: chemical composition

Raman Spectrometry Detection of graphite-like ordered and disordered carbon BC properties: graphite-like microstructure

Electron microscopy Detection of particle microstructure and morphology, e.g. TEM BC properties: morphology

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“Black Carbon” Measurement Methods

Black carbon (BC) is a useful qualitative description when referring to light-absorbing carbonaceous substances in atmospheric aerosol; for quantitative applications the term requires clarification of the underlying determination.

In the absence of a method for uniquely determining the mass of BC, the term "BC" should be used as a qualitative and descriptive term when referring to material that shares some of the characteristics of BC in particular its carbonaceous composition combined with its light-absorbing properties.

Equivalent black carbon (EBC) should be used instead of BC for data derived from optical absorption methods, together with a suitable MAC for the conversion of light absorption coefficient into mass concentration.

In the absence of a standard reference material, it is recommended to report such measurements as aerosol light absorption coefficient. If reported as EBC, it is crucial to report the MAC (mass-specific absorption coefficient) value used for the conversion of light absorption coefficient into mass conc.

Elemental carbon (EC) should be used instead of BC for data derived from methods that are specific to the carbon content of carbonaceous matter.

It is recommended to report data from evolved carbon methods and aerosol mass spectrometry as EC. Also, data from Raman spectroscopy, which addresses the graphite-like structure of carbon atoms, should be reported as EC. One strong recommendation, however, is to avoid using the terminology BC or EBC for evolved carbon methods.

Refractory black carbon (rBC) should be used instead of BC for measurements derived from incandescence methods.

For incandescence-based methods like LII, SP2 and SP-AMS it is recommended to report data as refractory black carbon, rBC, since they mainly address the thermal stability of the carbonaceous matter.

Soot is a useful qualitative description when referring to carbonaceous particles formed from incomplete combustion.

The term soot generally refers to the source mecha-nism of incomplete combustion rather than to a material property. Since atmospheric research usually treats mixed and aged particles, the recommendation is to avoid using this term for atmospheric aerosol.