OC/EC Analysis with Thermal-Optical Methods

Effects of temperature protocol and non-carbonaceous compounds

R SUBRAMANIAN†, Andrey Khlystov#, Allen Robinson*Carnegie Mellon University, Pittsburgh, PA

† now at the University of Illinois, Urbana-Champaign, IL# now at Duke University, Raleigh, NC* alr@andrew.cmu.edu/ 412-268-3657

Platform 5C-1: AAAR Specialty Conference, February 2005

Thermal-Optical Analysis for OC/EC

0

200

400

600

800

1000

0 200 400 600 8000.00

0.02

0.04

0.06

0.08

0.10

Analysis Time (s)

Car

bon

(µg-

C/c

m2 )

870 °C

Helium He/Ox910 °C

Pyrolysis

ECOC-3

-2

-1

0

1

Lase

r ATN

Tem

pera

ture

( °C

)

Temperature protocol affects EC

0.00

0.10

0.20

0.30

0.40

0.00 0.10 0.20 0.30 0.40

EC/TC (He4 870 C)

EC/T

C (H

e4 7

00 C

)Correlated pointsHigh-EC points

1:1

Y = 1.022X + 0.030R2 = 0.84Through (0,0):Y = (1.23±0.02)*X

Also seen by Chow et al. (2001); Schauer et al. (2003); Conny et al. (2004)

Differences: He4-870 v/s He4-700

0 200 400 600 8000.00

0.03

0.06

0.09

0.12

Analysis Time (s)

Car

bon

Helium He/Ox

-3

-2

-1

0Sam

ple ATN

He4-870: B

lueH

e4-700: Red

Objectives of our research

• Develop a mechanistic understanding of the thermal/optical method:– Evaluate the assumptions

– Carbon evolution pattern

– Optical properties of evolving carbon

• Estimate the potential uncertainties

• Examine possible improvements to the EPA thermal/optical method

Method assumptions

PCtotal * k_PC ~ PCpre-split * k_PC + ECpre-split * k_EC

Assumption 1: ECpre-split = 0(PC evolves completely before EC)

orAssumption 2: k_PC = k_EC

(Optical properties of PC and EC are similar)

0 200 400 600 8000.00

0.02

0.04

0.06

0.08

0.10

870 °C 910 °C

Pyrolysis

ECOC-3

-2

-1

0

1

He4-870: Co-evolution of PC and EC

0 200 400 600 8000.00

0.05

0.10

0.15

0.20

Analysis Time (s)

Car

bon

Helium He/Ox

Soluble Carbon

Insoluble Carbon"pure EC"

-3

-2

-1

0

Sample A

TN

Solvent-rinsed: Blue

Untreated: R

ed

He4-700: Co-evolution of PC and EC

0 200 400 600 800 1000 1200 1400 16000.00

0.05

0.10

0.15

0.20

0.25

Analysis Time (s)

Car

bon

Helium Extended He/Ox

Soluble Carbon

Insoluble Carbon"pure EC"

-3

-2

-1

0Sam

ple ATN

Solvent-rinsed: Blue

Untreated: R

ed

Assumption 1 not valid

PCtotal * k_PC ~ PCpre-split * k_PC + ECpre-split * k_EC

Assumption 1: ECpre-split ≠ 0(PC co-evolves with EC)

orAssumption 2: k_PC = k_EC

(Optical properties of PC and EC are similar)

0 200 400 600 8000.00

0.02

0.04

0.06

0.08

0.10

870 °C 910 °C

Pyrolysis

ECOC-3

-2

-1

0

1

Calculating k_EC with the solvent-rinsed filters

Lambert-Beer law:LN(I/Io) = EC * kEC

650 700 750 800 8500.00

0.02

0.04

0.06

Analysis Time (s)

Car

bon

LN(I/Io)

EC

He/Ox

-1.5

-1.0

-0.5

0.0

Sample A

TN

k_ECrinsed25.8 ± 2.8 m2/g-C

k_EC similar to the findings of Gundel et al. (1984)Squirrel Hill tunnel samples: k_EC ~ k_He/Ox = 20.6 ± 1.8 m2/g

Specific attenuation of untreated carbon

0 200 400 600 8000.00

0.05

0.10

0.15

0.20Helium He/Ox

Soluble Carbon

Insoluble Carbon"pure EC"

-3

-2

-1

0

HeOx2 HeOx3 HeOx4 HeOx50

10

20

30

40

50

Spec

ific

Atte

nuat

ion

Coe

ffici

ent (

m2 /g

) He4-700 He4-870

kECmax = 25 m2/g

“EC”

Den. 2

4-h

Bare 24

-h

Bare 8-

hBare

mult

i-day

0

10

20

30

40

50

Spec

ific

Atte

nuat

ion

Coe

ffici

ent (

m2 /g

)

He4-700 He4-870

kECmax = 25 m2/g

Instrument-defined EC (k_“EC”)

Assumption 2: also not valid!

PCtotal * k_PC ~ PCpre-split * k_PC + ECpre-split * k_EC

Assumption 1: ECpre-split ≠ 0(PC co-evolves with EC)

orAssumption 2: k_PC >> k_EC

(Optical attenuation by PC is much greater than that by EC)

0 200 400 600 8000.00

0.02

0.04

0.06

0.08

0.10

870 °C 910 °C

Pyrolysis

ECOC-3

-2

-1

0

1

Uncertainties due to different optical properties

(a) Ideal: PC evolves completely before EC(b) Worst case: EC evolves completely before PC(c) Actual: Co-evolution of PC and EC

0 200 400 600 8000.00

0.05

0.10

0.15

0.20Helium He/Ox

Soluble Carbon

Insoluble Carbon"pure EC"

-3

-2

-1

0

PCtotal * 55.8 = PCpre-split * 55.8 + ECpre-split * 15.7

Ideal: PC evolves before EC

-1.50

-0.50

0.50

1.50

0 100 200 300 400 500 600 700 800 9000.00

0.05

0.10

0.15

0.20

PC-first (correct) split

(Initial Transmittance = 0)

Actual EC = 5.5 µg-C/cm2

Worst case: EC evolves completely before PC

-1.50

-0.50

0.50

1.50

0 100 200 300 400 500 600 700 800 9000.00

0.05

0.10

0.15

0.20

PC-first (correct) split

EC-first (worst) split

(Initial Transmittance = 0)

Measured EC = 1.7 µg-C/cm2

Error = - 68%

Actual under-estimation by He4-870 protocol

-1.50

-0.50

0.50

1.50

0 100 200 300 400 500 600 700 800 9000.00

0.05

0.10

0.15

0.20

PC-first (correct) split

Actual split

(Initial Transmittance = 0)

Measured EC = 4.4 µg-C/cm2

Error = - 20%

Improving the EPA thermal/optical method

• Reducing the peak Helium temperature reduces the negative bias

• He4-700 increases the EC compared to the He4-870

• What about reducing the peak Helium temperature further?– e.g. IMPROVE has a peak Helium temperature of

550 °C– Potential positive bias with unpyrolyzed organics?

Improving the EPA thermal/optical technique: He4-550?

0

0.01

0.02

0.03

1 501 1001 1501

A n alysis T ime (s)

Car

bon

H elium H elium /Oxygen

-0 .2

0

0 .2

0 .4

0 .6

0 .8

1

1 .2

1 5 0 1 1 0 0 1 1 5 0 1

A n a lys is T ime (s )

Sam

ple

tran

smitt

ance

H eliu m H eliu m /O xyg en

Vehicular exhaust:TC 14.2 µg-C/cm2

EC 5.4 µg-C/cm2

Veh. exhaust + Levoglucosan:TC 23.9 µg-C/cm2

EC 7.4 µg-C/cm2

He4-550: Positive bias with leonardite?

Vehicular exhaust:TC 6.83 µg-C/punchEC 2.28 µg-C/punch

Veh. exhaust + leonardite:TC 22.2 µg-C/punchEC 5.89 µg-C/punch

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0.01

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0.06

0 200 400 600 800 1000 1200 1400-2.4

-2

-1.6

-1.2

-0.8

-0.4

0

Carbon Tunnel-550 Carbon Tunnel+Leo-30uL-550-1Tunnel-550 Transmittance Tunnel-550 ReflectanceTunnel+Leo-30uL-550-1 Transmittance Tunnel+Leo-30uL-550-1 Reflectance

"EC"

Conclusions• The thermal/optical method assumptions are not valid

– PC and EC co-evolve; k_PC >> k_EC for Pittsburgh samples

• He4-870 under-estimates EC– Premature EC evolution coupled with different k_EC and k_PC

• He4-700 EC is 23±2 % higher than He4-870 EC– Reduces premature EC loss

• He4-550 may be too low, possibly subject to positive bias– Need to investigate large polymeric and colored organic compounds

Acknowledgments• US EPA• US DOE/NETL

• AAAR travel grant for R.

• Spyros Pandis, Cliff Davidson• Eric Lipsky, Sarah Rees, Juan Cabada, Charles Stanier• Erin Casgren-Tindall, Prakash Rao, Leonard Lucas• Barbara Turpin, David Smith, Robert Cary, Jeff Brook• Paul Solomon• Tami Bond