www.cert.ucr.edu 1

Influence of dilution and particle fractal dimension of diesel exhaust on measured

SOA formation in a smog chamber

Shunsuke Nakao (1, 2), Manish Shrivastava (2, 3), Ahn Nguyen(2, 4), Li Qi (1, 2), Heejung Jung (2, 4), David Cocker III* (1, 2)

University of California, Riverside, Department of Chemical and Environmental EngineeringCollege of Engineering – Center for Environmental Research and Technology (CE-CERT)

Currently at Pacific Northwest National LaboratoryUniversity of California, Riverside, Department of Mechanical Engineering

SOA from POA?

Robinson et al., 2007, Science

• Significant fraction of POA volatilize at higher dilution ratio (~ambient condition)

• Traditional SOA precursors cannot explain SOA from diesel exhaust

SOA from “Intermediate/Semi”-volatility Organic Compounds (IVOCs, SVOCs)

www.cert.ucr.edu 3

Recent studies on SOA from diesel exhaust

High SOA formation

- Weitkamp et al. (2007), CMU chamber

Low/No SOA formation

*without addition of radical source or hydrocarbons

- Samy and Zielinska (2010), EUPHORE chamber- Chirico et al. (2010), PSI chamber

CMU: Carnegie Mellon UniversityEUPHORE: European photoreactorPSI: Paul Scherrer Institute

1) Weitkamp et al., Environ. Sci. Technol., 41, 6969-6975, 20102) Samy and Zielinska, Atmos. Chem. Phys., 10, 609-625, 20103) Chirico et al., Atmos. Chem. Phys. Discuss., 10, 16055-16109, 2010

www.cert.ucr.edu 4

SOA condensation onto agglomerates

• In some studies1)2), SMPS was used to quantify SOA (ρ=1g/cm3)

? ?

• How spherical/fractal is the PM after SOA formation?• Is density uniform?

Question:

SMPS: Scanning Mobility Particle Sizer1) Weitkamp et al., Environ. Sci. Technol., 41, 6969-6975, 20102) Samy and Zielinska, Atmos. Chem. Phys., 10, 609-625, 2010

aging

www.cert.ucr.edu 5

Particle measurement methods

1. SMPS- volume (based on dm)

2. APM – SMPS- mp, dm ρeff

3. HR-ToF-AMS- Non refractory organics

dm: mobility diametermp: mass of particleρeff: effective density

SMPS: Scanning Mobility Particle SizerAPM: Aerosol Particle Mass AnalyzerHR-ToF-AMS: High Resolution-Time-of-Flight Aerosol Mass Spectrometer

CE-CERTmezzanine chamber

FEP, 2mil, 12m3

Dilution method: Ejector

www.cert.ucr.edu 7

Particle volume evolution80

60

40

20

0Vo

lum

e c

on

cen

tra

tion

(m

3/c

m3)

121086420Hours after lights on

Little volume increase little SOA formation?

Run 1 (engine idle,short transfer line, DR: 114)

www.cert.ucr.edu 8

Particle effective density evolution

How significant is this density increase?

Run 1 (engine idle,short transfer line, DR: 114)

1.5

1.0

0.5

0

Eff

ect

ive

de

nsi

ties

(g/c

m3)

12840Hours after lights on

0.11 fg 0.26 fg 0.51 fg 0.88 fg 1.40 fg

(c)

Larger particles more fractal

www.cert.ucr.edu 9

Size dependent density60

50

40

30

20

10

0

dV

/dln

Dp

3 4 5 6 7 8 9100

2 3 4 5 6 7

mobility diameter (nm)

0.6

0.4

0.2

0.0

Effe

ctive d

en

sity (g/cm

3)

volume density

(b)

• V(dm)• ρ(dm)

Mass concentration

Power function fit Fractal dimension (Df)*

*Park et al., ES&T, 2003, Xue et al., ES&T 2009Run 11 (medium load, long transfer line, DR:519))

www.cert.ucr.edu10

Particle volume and mass concentration

Mobility diameter measurement was insufficient to observe SOA formation

Run 1 (engine idle,short transfer line, DR: 114)

80

60

40

20

0Vo

lum

e c

on

cen

tra

tion

(m

3/c

m3)

or

Inte

gra

ted

Ma

ss c

on

c. (g

/m3)

12840

3.0

2.8

2.6

2.4

2.2

2.0

Df

Volume Mass Df

(a)

Hours after lights on

www.cert.ucr.edu 11

Measurement of SOA by AMS4

3

2

1

0

Org

an

ics

(g

/m3)

121086420Hours after lights on

0.30

0.25

0.20

0.15

0.10

0.05

0.00

C4 H

9+ (

g/m

3) Organics

C4H9+

Run 1 (engine idle,short transfer line, DR: 114)

C4H9+: tracer of primary organic aerosol*

POA

SOA

*Chirico et al. 2010, APCD

www.cert.ucr.edu 12

Run 1 (engine idle,short transfer line, DR: 114)

Measurement of SOA by AMS

Comparable to Robinson et al. (2007)

Robinson et al. (2007) Science

100

80

60

40

20

0

Fra

ctio

n o

f O

A m

ass

(%)

121086420Hours after lights on

POA

SOA

CE-CERTmezzanine chamber

FEP, 2mil, 12m3

www.cert.ucr.edu 14

Injection/dilution methods and initial particle density in a smog chamber*

Raw (plume injection) possibly higher density

*Idle, short transfer line

0

0.2

0.4

0.6

0.8

1

1.2

Run

2

Run

4

Run

3

Run

1Bul

k ef

fect

ive

dens

ity (

g/cm

3)

050

100150

200250

300350

400

Dilution ratio

density

dilution ratio

Raw injectionDilute injection

Dilute injection: Ejector dilutor

CE-CERTmezzanine chamber

FEP, 2mil, 12m3

www.cert.ucr.edu 16

100

80

60

40

20

0

Vo

lum

e c

on

cen

tra

tion

(m

3/c

m3)

or

Ma

ss c

on

cen

tra

tion

(g

/m3)

86420Hours after injection

Volume Mass

Long transfer line (Run 9), Dark

Particle volume/mass evolution (transfer line effect)

Idle, diluted exhaust

Longer transfer line (Run 5~11)

Increase in PM volume(Decrease in bulk ρ)

More particle coagulation

www.cert.ucr.edu 17

Discrepancy in diesel-SOA studies

High SOA formation: CMU1)

THIS STUDY

Low/No SOA formation: EUPHORE2), PSI3)

*without addition of radical source or hydrocarbons

1) Weitkamp et al., Environ. Sci. Technol., 41, 6969-6975, 20102) Samy and Zielinska, Atmos. Chem. Phys., 10, 609-625, 20103) Chirico et al., Atmos. Chem. Phys. Discuss., 10, 16055-16109, 2010

• In addition to the difference in engine, different injection/dilution methods may in part explain the gap

• Use of SMPS2) might have lead to underestimation

www.cert.ucr.edu 18

• Mobility diameter measurement is shown to underestimate SOA formation from diesel exhaust in a smog chamber. Hence, mass based measurement is necessary for evaluating SOA from diesel exhaust (e.g., APM, AMS, TEOM)

• Experimental conditions possibly have strong impacts on particle physical evolution in a smog chamber– Raw exhaust injection enhanced particle density– Ejector dilutor speeds evaporation: lower particle density– Longer transfer line enhanced particle coagulation to such an extent that

particle volume increased in dark

• Difference in experimental methods (injection, dilution) may in part contribute to current discrepancy of diesel-chamber studies

Conclusion

www.cert.ucr.edu 19

Acknowledgements

• Graduate advisor: Dr. David Cocker• Current/former students: Christopher Clark,

Ping Tang, Xiaochen Tang, Dr. Quentin Malloy, Dr. Li Qi

• Support staff: Kurt Bumiller, Chuck Bufalino• Funding sources: NSF, W.M. Keck Foundation,

and University of California, Transportation Center

www.cert.ucr.edu 20

Expt.

Run loadPM volume

(μm3/cm3)a

initial NOx

(ppm)a

initial ethene

(ppb)a

calculated OH

(molec cm3 s-1)b

Df

(Ave.±S.D.)c

bulk ρeff

(g/cm3)ddilution

ratio

Dilute exhaust, short transfer line

1 4/17/2010 idle 68 0.76 N.A. N.A. 2.52 → 2.79 0.75 114

Raw exhaust, short transfer line

2 3/20/2010 idle 195 1.60 150 8.4 x 107 2.97 ± 0.05 1.16 54

3 4/23/2010 idle 22 0.24 32 9.7 x 107 2.65 → 2.83 0.84 362

4 5/7/2010 idle 50 0.54 71 9.3 x 107 2.79 → 2.90 0.99 161

Dilute exhaust, long transfer line

5 11/13/2009 medium 103 0.65 N.A. N.A. 2.39 ± 0.03 0.49 359

6 11/16/2009 medium 36 0.23 N.A. N.A. 2.39 ± 0.03 0.49 1015

7 11/18/2009 medium 53 0.30 N.A. N.A. 2.35 ± 0.05 0.55 778

8 2/1/2010 low 113 0.91 45 6.6 x 107 2.44 ± 0.04 0.51 176

Dilute exhaust, long transfer line, dark

9 2/25/2010 idle 75d 0.30d 76 N.A. 2.55 → 2.45 0.51 290

10 5/9/2009 medium 90d 0.92d N.A. N.A. 2.38 ± 0.03 0.55 254

11 7/6/2010 medium 49d 0.45d N.A. N.A. 2.40 → 2.20 0.62 519

Note: N.A.: not acquireda values when blacklights were turned onb calculated based on ethylene decayc calculated by power curve fit on effective density v.s. mobility diameter (see text)d values when injection finished

www.cert.ucr.edu 21

Particle fractal dimension (Df)

Df = 1: straight chain, 2: sheet, 3: sphere

fD

g

a

RCN

3' fDmeff dC

If a is constant, N is proportional to mass

(Park et al. 2003, Xue et al. 2009)