J I U M E N G L I U

E A S 8 8 0 2 : C L O U D S , A E R O S O L S A N D C L I M AT E

M A R . 2 5 , 2 0 1 3

Light Absorbing Carbonaceous Aerosols (BC and BrnC)

and their Climate Impacts

Terminology

Light absorbing carbonaceous aerosols are divided into two categories: black carbon (BC) and brown carbon (BrnC).

BC: a strong absorber of visible and near-IR light; generally implied to have optical properties and composition similar to soot carbon.

BrnC: Light-absorbing organic matter in atmospheric aerosols of various origins, e.g., soil humics, humic-like substances (HULIS), tarry materials from combustion, bioaerosols, etc.

Current progress on BC

Microphysical properties of black carbon

Constraints on BC atmospheric abundance: acceptable and explainable bias among models

Climate impacts: large uncertainty, BrnC involved

(Hansen et al., JGR, 2005)

Scientific questions for BrnC

What is the current measurement capability of BrnC?

What do we know about the optical properties of BrnC?

How optically important is BrnC comparing to BC?

What do we know about the climate impacts of BrnC?

0.6

0.5

0.4

0.3

0.2

0.1

Ab

s,

Mm

-1

600500400300

wavelength, nm

Some background of BrnC

350 nm 800 nm

appears

brown

Jaoui et al., JGR, 2008

o Heterogeneous (aqueous) reactions of carbonyls (eg. glyoxal), BSOA with amine, NH3, NH4

+

o Aromatic SOA formed under high Nox

o ……

Sources of Brown carbon

Sources of

Brown carbon

Primary

Sources

Secondary

Sources

Biomass Burning

Coal Combustion

Vehicles

Formed in heterogeneous reactions

from dienes.(Limbeck et al., 2003).

Other Primary

sources

Formed in multiphase reactions with OH

radicals in cloud water.(Gelencser et al.,

2003).

Produced by Anthropogenic/biogenic

VOC under high-NOx condition (Zhang

et al., 2011 )

Other Mechanisms…

Scientific questions for BrnC

What is the current measurement capability of BrnC?

What do we know about the optical properties of BrnC?

How optically important is BrnC comparing to BC?

What do we know about the climate impacts of BrnC?

Remote sensing

o Single-angle techniques: cannot distinguish the scattering component of extinction from the absorption component o Eg., mono-static lidaror simple sun photometry

o Multi-angle and multi-wavelength observations o Eg., AERONET and ARM o Successful in characterizing aerosol light absorption for

specific aerosol types during specific events(such as biomass burning)

o Pros: atmospheric distribution o Apply radiative inversion methods to infer optical properties

o Yield column-integrated, ‘‘effective’’ values, potentially including different aerosols and mixing states o Eg. Ångstrom exponent ~1.2 from AERONET

In-situ, no filter based

o Convert optical signal into other signals o Photoacoustic technique

o eg., PAS, creating a pressure increase

o Refractive index-based techniques o refractive index change due to the absorption of a laser beam

o Thermal in-situ techniques o particle incandescence, rely upon the heating and expansion

of the air surrounding absorbing particles

o Extinction-minus-scattering techniques o measure the sum of the absorption coefficients of the particle

ensemble and the surrounding air

o Interferences from gaseous absorption

o Temperature/pressure sensitive

In-situ, filter-based

o Direct measurement of aerosol light absorption concentrate

and deposit aerosols on particle filters

o Pro: highly time-resolved (seconds)

o Usually with several fixed wavelengths

o Potential systematic errors due to

multiple scattering by filter medium

and deposits of aerosols, angular

distribution of scattered light, etc.

o Measure BrnC+BC absorption

o ‘effective’ BC [Moosmuller et al., 2009]

[Moosmuller et al., 2009]

Filter-based, in Lab

o Highly wavelength resolved

o Chemically-resolved o Heterogeneous (aqueous) reactions of carbonyls (eg. glyoxal),

BSOA with amine, NH3, NH4+ o Aromatic SOA formed under high NOx

Data courtesy: Xiaolu Zhang

0.6

0.5

0.4

0.3

0.2

0.1

Abs, M

m-1

600500400300

wavelength, nm

Filter-based, in Lab

o Comprehensive BrnC bap (λ)

Absorption of solution

Absorption of particles in

the atmosphere

Molecule in

solution

Particles

in air

o Insight into chemical properties of BrnC o No black carbon interferences o Whole spectra (vs. several fixed wavelengths) o Limited time resolution o Only applies for dissolved components

m=n+ik,

Ångstrom

exponent,

etc.

Scientific questions for BrnC

What is the current measurement capability of BrnC?

What do we know about the optical properties of BrnC?

How optically important is BrnC comparing to BC?

What do we know about the climate impacts of BrnC?

Absorption Ångstrom exponent (AAE)

Mass Absorption Coefficients

Refractive index

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Absorp

tion

1.21.00.80.60.40.20.0

WSOC

MAC=0.12 g/m2

m2/g

AAE

Remote sensing: 0.8-1.6 (AERONET, mix of BC, BrnC, dust, etc.) 4.55±2.01 (OC)

In situ measurements Ambient AAE: 0.6-3.7 (mix of BC, BrnC, dust, etc.)

Treated BrnC AAE: 3-6 (a suite of assumptions, eg., BC AAE =1)

In lab measurements from solution spectra From single compounds: 4-6 for organic solvents, 6-8 for water-

solubles

Semi-controlled: up to 15 reported (pine burning)

Ambient bulk organics: 1-7

[Bahadur et al., PNAS, 2012]

Mass Absorption Coefficients

0.6

0.5

0.4

0.3

0.2

0.1

Abs, M

m-1

600500400300

wavelength, nm

Wavelength dependent Large variation

Aerosol properties Variety of locations Analytical methods Sources Single compound vs. bulk

Measurement vs. model

Sample λ (nm) MAC (m2/g) Reference

Brown carbon produced by aging SOA with 100 ppb NH3 (lab) 500 0.001-0.1 Updyke et al., 2012

“Tar balls” from smoldering combustions of wood (lab) 532 0.01-0.07 (calculated

from k=0.0005-0.003) Chakrabarty et al.,2010

Humic-LIke Substances (HULIS) extracted from filter samples from

various sites in Europe 532

0.07-0.1 (calculated

from k = 0.003-0.05) Dinar et al., 2008

Methanol extracts from wood combustion particles (lab) 500 0.1-0.5 Chen and Bond, 2010

Refractory organic carbon from biomass burning in North America

(INTEX/ICARTT) 530 0.1 Clarke et al., 2007

Brown carbon in particles collected in Asia (EAST-AIRE) 520 0.6 Yang et al., 2009

Acetone extracts from biomass burning aerosols in Africa (SAFARI 2010) 500 0.9 Kirchstetter et al., 2004

Refractive Index

Alexandre et al., Science, 2008

Refractive indices are ALWAYS inferred by assuming a theory and applying it to optical measurements.

i.e., for imaginary part,

Wavelength dependent Large variation

Reported values: 0.002-0.27 at 500-550nm

Assumptions applied Aerosol mixing state Aerosol density

Scientific questions for BrnC

What is the current measurement capability of BrnC?

What do we know about the optical properties of BrnC?

How optically important is BrnC comparing to BC?

What do we know about the climate impacts of BrnC?

Seasonally

Regionally

Globally

Vertically

Organic carbon

EC

Sulfate

Nitrate

Ammonium

Undefined

Seasonally

Major differences: Seasonal difference of organics

abundance

Different sources result in

difference in optical properties

Organic carbon

EC Sulfate

Nitrate

Ammonium

Undefined

Winter

Summer

[Bahadur et al., PNAS, 2012]

Figure courtesy: Zhenyu Du

Regionally

Wavelength (nm)

brnC fraction

Location Reference

300 Up to 50% Rondˆonia, Brazil Hoffer et al., 2006

400 ~40% Mexico City Barnard et al., 2008

400 ~30% Xianghe, China Yang et al., 2009

550 ~10% Xianghe, China Yang et al., 2009

440 ~20% CA Bahadur et al., 2012

405 13% Los Angeles region, CA Cappa et al., 2012

532 6% Los Angeles region, CA Cappa et al., 2012

550 20% Chung et al., 2012

Limited measurements/data available

Values vary among different locations Sources, eg., anthropogenic vs. biomass

burning, etc.

Vertical profile

Interpolation from models

Run model with different mix-

ratios of BrnC and BC to get

vertical profiles

Constrain with observations Why? No direct measurement of

“pure” BrnC

Better agreement, BrnC to BC

ratio increases with altitude

Increasing importance of BrnC

with higher altitude

Absorption (x10-5m-1)

Park et al., 2010

Globally

Two ways Remote sensing measurement Global models

Constrained with satellite data Chung et al. 2012: 20% contribution from BrnC to carbonaceous

aerosols at 550nm Constrained with AERONET data Partition BC and BrnC assuming no BrnC absorption at 675nm

Global models Feng et al., 2013: 8–26% % at 450nm and up to 56% at 350 nm Park et al., 2010: 27% contribution at 550nm

Problems: Very limited studies: needs further comparison between

measurements and model results Large uncertainty from both measurement and models

eg. Optical properties of BrnC unclear

Scientific questions for BrnC

What is the current measurement capability of BrnC?

What do we know about the optical properties of BrnC?

How optically important is BrnC comparing to BC?

What do we know about the climate impacts of BrnC?

Park et al., 2010

Climate impacts

Very limited studies

Currently only focusing on direct forcing indirect forcing involves more complex BrnC properties such as CCN

activity, hygroscopicity, aerosol mixing state, etc.

Estimations varied a lot Probably because the optical properties applied into models are

largely uncertain

BrnC forcing at TOA, Wm-2 BrnC forcing at surface, Wm-2 Reference

0.24 -2.4 Park et al., 2010

0.8~1.4 -1.5~-0.75 Chung et al., 2012

0.11 -0.14 Feng et al., 2013

Conclusions and the way forward…

Measurement capabilities: further validation, including inter-comparison among instruments needed

Optical properties: more information needed regarding to chemical/microphysical properties of organics

Optical importance and climate impacts: models

On the particle level

a. Mixing state of the aerosols (internal/external) b. Water-uptake properties (hygroscopicity) c. CCN/IN activity of organics (wettability, surface tension) d. Size distribution of BrnC e. Optical properties

On the model scale

a. Three-dimensional mass abundance/mixing of carbonaceous aerosols b. Cloud processing with carbonaceous aerosols

Any Questions?

References

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