Brown and black carbon: Light absorbing carbonaceous matter in

atmospheric aerosols

M. O. Andreae, T. W. Andreae, P. Artaxo,

A. Gelencser, B. Graham, P. Guyon, G. Helas,

W. Maenhaut, and O. Mayol-Bracero

Black carbon (or soot) has arrived on the political scene:

„Kyoto also failed to address two major pollutants that have an impact on warming: black soot and tropospheric ozone. Both are proven health hazards. Reducing both would not only address climate change, but also dramatically improve people's health.“

G. W. Bush, 11 June 2001

But, is policy getting ahead of science here?

• There is no rigorous definition of „black carbon“

• There is no unique relationship between BC mass concentration and it optical effect

• There are numerous analytical techniqes, but they provide conflicting results

• There are two major sources, but they release quite different material– Fossil fuel burning (6.6 Tg/yr, IPCC 2001)– Biomass burning (5.7 Tg/yr, IPCC 2001)

Soot =“Black Carbon”

??

SEM

TEM

HRTEMSoot-type „Black“ Carbon:• can only be formed in flames (including

those in automobile engines)

• is the main light-absorbing component in fossil fuel emissions

• consists almost exclusively of carbon (aka „elemental carbon“)

• can be determined with reasonable reliability with combustion analyzers

• has optical absorption properties that vary over at least an order of magnitude, depending on size, mixing state, etc.

• shows little spectral dependence of absorption properties „black“

Soot-type black carbon in flames

Char particles

„Brown carbon“ in pyrolysis and smoldering stages

Biomass burning produces:

A big char/ash particle

These particles are: - highly light-absorbing - aerodynamically light, and can travel great distances - chemically and optically quite different from “soot” carbon

•The distinction of OC from BC using thermal combustion methods is particularly

difficult for biomass burning samples since these have a large organic component.

•Acetone extraction removes much of what appeared to be BC!

•45 to 60% of the material corresponding to the BCapparent peak can be removed by

water extraction (extraction of water-soluble OC and catalytic inorganic ions).

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CO

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Front Filter

Back Filter

Front after Acetone

Front after Water

BCapparent

BCwater

Thermal analysis of biomass smoke before and after solvent-extraction

•About 1/3 of all organic functionalities still remain at TBC/OC

•Only traces of organics remain at TBC/OC if filter is previously extracted in water

Effect of thermal separation on organics by Py-GC-MS

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AbundanceTIC: 610W6TPL.D

Heating stopped at TBC/OC

Water extracted before heating to TBC/OC

Thermal combustion analysis can over- or under-estimate „black carbon“...

• organic compounds soluble in water or organic solvents

can be refractory enough to show up in the „BC“ peak • light-absorbing organic compounds can be present in the

„OC“ region • the combustion of „BC“ can be catalyzed by inorganic

cations (K, Na, ..) • non-absorbing organics can be converted to BC by

„charring“ during the analysis

Biomass smoke contains light-absorbing substances other than „soot“

The evidence:• non-flaming combustion produces a dark aerosol, even

though soot formation is not possible• UV-VIS spectra of smoke aerosols show absorption in

the visible, increasing into the UV• the colored substances in biomass smoke can be partially

oxidized by chemical agents, such as hypochlorite• lignin pyrolysis products can be polymerized into

colored substances in free radical reactions with OH

Laboratory smoke aerosols from flaming and smoldering combustion

Flaming

Smoldering

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wavelength (nm)

abso

rban

ce

UV-VIS spectrum of water-soluble organic carbon extract of a biomass smoke aerosol against pure water

Conversion of 3,4-dihydroxi-benzoic acid in OH radical reactions

A puzzle: What material is on this aerosol filter, shown with its thermogram?

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Temperature, °C

CO

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CO2

What appears like a textbook sample of soot carbon, is actually…

Light-absorbing organics come not only from combustion processes...

The thermal behavior of humic acids

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CO

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2O,

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CO2

- strongly resembles that of „elemental carbon“ or „soot“- but simultaneous measurement of the H2O from the com- bustion of the hydrogen atoms indicates that we are looking at organic material, not „elemental“ carbon

But not all light-absorbing organics are thermally refractory, e.g. lignin:

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CO2

Lignin

Conclusions• Combustion aerosols, especially from biomass burning,

contain soot-like („black“) and light-absorbing organic („brown“) fractions

• Light-absorbing organics can be formed not only in combustion, but also– from biogenic substances

– by chemical aging („humics“)

– by photochemical processes in hydrometeors

• There is no unique relationship between thermal refractive character and light absorption

• Thermal methods are not reliable for the quantitative determination of black/brown carbon

• Optical methods are not reliable for the determination of the chemical identity of light-absorbing carbonaceous matter