Vehicle generated nanoparticles are not an artifact!

D. B. Kittelson, W.F. Watts, and J.P. JohnsonCenter for Diesel Research

University of Minnesota

8th CONFERENCE ON ENVIRONMENTAL SCIENCE AND TECHNOLOGY

PARTICULATES WORKSHOP

11th SEPTEMBER 2003

Lemnos Island, GREECE

Outline

• Size and composition

• On-road measurements– On-road measurements in mixed urban traffic

– Weekday/weekend determinations of on-road emission factors

– On-road characterization of post 2007 system

• Conclusions

Typical Engine Exhaust Particle Size Distributions, Number, Surface Area, and Mass Weightings Are Shown

0

0.05

0.1

0.15

0.2

0.25

1 10 100 1,000 10,000

Diameter (nm)

No

rmal

ized

Co

nce

ntr

atio

n (

1/C

tota

l)dC

/dlo

gD

p

Number Surface Mass

Fine ParticlesDp < 2.5 m

Ultrafine ParticlesDp < 100 nm

NanoparticlesDp < 50 nm

Nuclei Mode - Usually forms from volatile precursors as exhaust dilutes and cools

Accumulation Mode - Usually consists of carbonaceous agglomerates and adsorbed material

Coarse Mode - Usually consists of reentrained accumulation mode particles, crankcase fumes

PM10Dp < 10 m

In some cases this mode may consist of very small particles below the range of conventional instruments, Dp < 10 nm

Nuclei mode (~3 to 30 nm diameter)

• The nuclei mode contains most of the particle number• Nuclei mode particles form mainly from volatile precursors1

– The nuclei mode typically consists mainly of heavy hydrocarbons, mainly from lubricating oil, and sulfates

– Although although the mode is mainly hydrocarbon, its formation is facilitated by sulfur in the fuel

– Its formation is very dependent on dilution conditions, especially dilution rate and dilution air temperature

– Its formation is favored by low solid carbon and high precursor concentration

• Solid nuclei mode particles may form from metals in the lube oil or fuel– Formed from oil under engine conditions that lead to little solid carbon

formation. – Formed from fuel when metallic additives or high metal fuels are used.

• A volatile nuclei mode will be all that is left when solid particles are removed by exhaust filtration

1 - Sakurai, Hiromu, Herbert J. Tobias, Kihong Park, Darrick Zarling, Kenneth S. Docherty, David B. Kittelson, Peter H. McMurry, and Paul J. Ziemann, 2002. “On-Line Measurements of Diesel Nanoparticle Composition, Volatility, and Hygroscopicity,” Submitted to Atmospheric Environment

Accumulation mode (~30 to 500 nm)

• The accumulation mode contains most of the particle mass and “soot”• Consists primarily of carbonaceous agglomerates and adsorbed OC• Particles in this mode are strongly light absorbing and may influence

global warming• Most of the lubricating oil ash is found in this mode – this enhances soot

oxidation in exhaust filters but eventually plugs them• Density of accumulation mode particles decreases with increasing size1

– Fractal like behavior– Low densities cause size to be underestimated by some methods

• Accumulation mode particles have been reduced sharply by better engine technology and will be eliminated by efficient filtration

1 - Park, Kihong, Feng Cao, David B. Kittelson, and Peter H. McMurry, 2002. “Relationship Between Particle Mass and Mobility, and Between Aerodynamic and Mobility Size Distributions for Diesel Exhaust Particles,” Submitted to Environmental Science & Technology

U of M Mobile Laboratory built to study formation of nanoparticles in the atmosphere for the CRC E-43 project

• Instruments (primary instruments highlighted in blue)– SMPS to size particles in 9 to 300

nm size range– ELPI to size particles in 30 to 2500

nm size range– CPC to count all particles larger

than 3 nm– Diffusion Charger to measure total

submicron particle surface area– Epiphaniometer to measure total

submicron particle surface area– PAS to measure total submicron

surface bound PAH equivalent– CO2 , CO, and NO analyzers for

gas and dilution ratio determinations

On-highway measurements made on urban freeways in Minnesota show a large nuclei mode even in the absence of significant Diesel traffic

• Traffic speed has at least as much influence on the size of the nuclei mode as the presence of Diesel traffic

• Particle number increases and size decreases as traffic speed increases• Particle volume (mass) is higher under low speed congested conditions• It appears that slow moving congested traffic leads to storage of volatile materials

in the exhaust system• As vehicles speed up the exhaust system heats leading to the release of the

materials which subsequently form nanoparticles

0

100000

200000

300000

400000

500000

1 10 100 1000

Midpoint Diameter (nm)

dN

/dlo

gDp

(p

art.

/cm

3)

Diesel=Y, MPH >=0 <=10 Diesel=Y, MPH >10 <50 Diesel=Y, MPH >=50

Diesel=N, MPH >=0 <=10 Diesel=N, MPH >10 <50 Diesel=N, MPH >=50

0.0E+00

1.0E+04

2.0E+04

3.0E+04

4.0E+04

5.0E+04

6.0E+04

0 10 20 30 40 50 60

Speed, mph

N/V

(pa

rt./ m

3 )

0

6

12

18

24

30

36

DG

N (

nm)

N/V

DGN

N/V of 1 part./m3 =1012 part./gm for spherical unit density particles

Diesel

No Diesel

Nuclei Mode Decays Rapidly Downwind of Roadways• Modeling (Capaldo and

Pandis, 2001) indicates– For typical urban conditions,

characteristic times and transit distances for 90 % reduction of ultrafine concentrations are on the order of a few minutes and 100-1000 m, respectively.

– For a given wind speed, ultrafine particles are expected to survive and travel a factor of ten greater distances in a rural flat area as compared to an urban downtown location.

• Mobile particle sources will influence the aerosol particle number concentrations mainly near roadways.

Nanoparticles Are Composed of Mainly Volatile Material

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1 10 100 1000

Diameter, nm

dN/d

logD

p, p

art/

cm3

12 Sept 02 UMN SMPS, N=25516 Sept 02 UMN SMPS, N=23312 Sept 02 TDSMPS, N=25616 Sept 02 TDSMPS, N=233

SMPS - no thermal denuder

SMPS - thermal denuder at 300o C

Large Reduction in Nuclei Mode

87-95% Reduction

On-Road Characterization of Real World Fleets: Weekday / Weekend On-Highway Apportionment Experiments

• Summertime urban freeway measurements

• Over-the-road aerosol, corrected for the background is contributed by vehicles in proportion to their traffic volume.

• Measuring traffic volumes and aerosol concentrations on days with differing SI to Diesel ratios gives a system of equations that can be solved for average Diesel and SI contribution on a per unit traffic volume basis

• Presented on a fuel specific (per kg of fuel) basis

• Principal uncertainties are in traffic counts and background corrections

Diesel/SI Ratio By Route And Day

Ratio Diesel/SI Separated by Route and Day

0.00

0.01

0.02

0.03

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0.12

0.13

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0.15

62 & 169 (red) 100 & 169 (blue) 94 & 494 (green)

Route

Rat

io D

iese

l/SI

Sunday May 12, 2002

Tuesday May 21, 2002

Route selected

Average Weekend vs. Weekday Size Distributions

SMPS Average Size Distributions - Sunday vs. Weekday + MnDOT

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1 10 100 1000

Diameter, nm

dN

/dlo

gD

p,

part

/cm

3

MnDOT, N = 457

Week Day Average, N = 605

Sunday Average, N = 189

DOE data uncorrected for background and particle lossesMnDOT data uncorrected for particle losses

Diesel and SI Apportioned Size Distributions on a Fuel Specific Basis

Fuel Specific Contribution to On-Highway Aerosol by Vehicle Type

1.0E+10

1.0E+11

1.0E+12

1.0E+13

1.0E+14

1.0E+15

1.0E+16

1.0E+17

1 10 100 1000

Dp (nm)

dN

/dlo

gD

p p

er k

g o

f fu

el b

urn

ed,

pa

rt/c

m3 /k

g

SI, On-Hwy

Diesel, On-Hwy

Diesel On-Highway Compared to CRC E-43 Fleet Average On a Fuel Specific Basis

Fuel-Specific Comparison of On-Highway Diesels to CRC E-43 Diesel Fleet Averages for Various Conditions

1.0E+10

1.0E+11

1.0E+12

1.0E+13

1.0E+14

1.0E+15

1.0E+16

1.0E+17

1 10 100 1000

Dp (nm)

dN

/dlo

gD

p p

er k

g o

f fu

el b

urn

ed,

pa

rt/c

m3 /k

g

Diesel, On-Hwy

CRC E-43, Diesel, Acceleration

CRC E-43, Diesel, 60 Cruise

CRC E-43, Diesel, 55 Cruise

Source Apportioned Highway Cruise SI Fleet Average Compared to Chase and Chassis Dyno Measurements

Fuel-Specific Comparison of On-Highway SI Vehicles to SI Fleet Averages, Highway Conditions for both CVS and Chase Testing

1.0E+10

1.0E+11

1.0E+12

1.0E+13

1.0E+14

1.0E+15

1.0E+16

1.0E+17

1 10 100 1000

Dp (nm)

dN

/dlo

gDp

per

kg

of f

uel

bu

rned

, p

art/

cm3 /kg

SI, On-Hwy

SI, Acceleration, Chase

SI, 70 Cruise, CVS

SI, 65 Cruise, CVS

• The proposed (by Swiss) EU standard of 1011 particles/km corresponds to about 2 x 1012 particles/kg fuel of solid particles

• If we assume all particles above 30 nm are solid our fleet emission factors are 5 x 1014 and 9 x 1013 particles/kg fuel for Diesel and SI, respectively

• These are a snapshot of Minnesota urban freeway fleets – more work is needed

Fuel Specific Emissions – Diesel and SI Summertime Highway Cruise

Mean SDOM Mean SDOM Mean SDOM Mean SDOMDiesel 1.34E+16 1.86E+15 2.13E+15 3.04E+14 3.15E+12 6.15E+11 1.45E+13 2.15E+12

SI 7.10E+15 1.55E+15 3.88E+14 6.11E+13 1.77E+11 3.91E+10 9.41E+11 1.52E+11

Mean SDOM Mean SDOM Mean SDOM Mean SDOMDiesel 2.91E+12 4.04E+11 6.21E+11 1.40E+11 22.7 1.7 195.7 12.1

SI 5.69E+11 1.26E+11 1.44E+10 4.44E+09 19.3 0.7 104.0 24.3

Mean SDOM Mean SDOM Mean SDOMDiesel 3.44E+03 7.76E+02 0.76 0.13 0.008 0.002

SI 2.70E+04 7.69E+03 0.77 0.09 0.059 0.017

Apportioned Source

Apportioned Source

PAS (fA/kg)

N/V Ratio

CPC (part/kg) SMPS Number (part/kg) DC (µm2/kg)Apportioned Source

N30/N Ratio

SMPS Active Surface (µm2/kg)

V30/V Ratio

DGN (nm) DGV (nm)SMPS Volume (µm3/kg)

Dp > 3 nm Dp > 10 nm

On-road characterization of post a 2007 aftertreatment system – we sniff our own exhaust plume

Driver side sample point

Passenger side sample point

Background sample point

Stacks

Stacks

On road tests of particle filtration device – volume (mass) distributions

0.0E+00

5.0E+00

1.0E+01

1.5E+01

2.0E+01

2.5E+01

3.0E+01

3.5E+01

4.0E+01

1 10 100 1000

Dp, nm

dVdl

ogD

p,

m3 /c

m3

Engine out with low S oil

Filter with standard oil, August

Filter with standard oil, October

All tests done with 15 ppm S fuel (post 2006) with standard or low sulfur lubricant

On-road nanoparticle measurements for real world fleets

• On-road nanoparticles are real• On-road submicron on-road aerosols are found in two principal size modes

– A nuclei mode in the nanoparticle range containing most of the particle number– An accumulation mode in the ultrafine and fine range containing most of the particle

mass.

• Most nuclei mode particles are volatile and very sensitive to dilution conditions, both on-road and in the laboratory

• Both Diesel and SI engines are important contributors to the on-road aerosol– Both engine types produce significant nuclei mode (nanoparticles)– SI particles are usually smaller– SI particle emissions are much more load dependent

• On-road measurements with post 2007 systems may show large nuclei mode with some systems

• More work should be done to determine nanoparticle formation with new engines and fuels and for a range of ambient and operating conditions

Acknowledgements

• We have had help from many collaborators– In the Center for Diesel Research

• Feng Cao, Marcus Drayton, Jason Johnson, Hee Jung Jung, Duane Paulsen, Winthrop Watts, Robert Waytulonis, Qiang Wei, Darrick Zarling

– At Paul Scherrer Institute• Nick Bukowiecki, Urs Baltensperger, Adelheid Kasper

• And many sponsors– Coordinating Research Council, U.S. Office of Heavy Vehicle

Technologies, Engine Manufacturers Association, Southcoast Air Quality Management District, California Air Resources Board, BP/Amoco, Castrol, Corning, Cummins, Caterpillar, Perkins, and Volvo.