Effects of Temperature on Gasoline Exhaust VOC speciation

Anirban Roy1, Darrell Sonntag2, Charles Schenk2, Joseph McDonald2, Rich Cook2, Catherine Yanca2, David Hawkins2, Yunsoo Choi1

1 University of Houston2 USEPA, Ann Arbor

USEPA’s Annual Emissions Inventory ConferenceApril 16th, San Diego

The VOC ozone problem

• VOC major contributor to ozone in urban regions, which are NOx-saturated i.e. VOC sensitive.

Choi et al., 2012

NOx-sensitiveVOC-sensitive

What drives ozone?

Motor Vehicle VOC emissions

• Two urban counties – New York (NYC-Manhattan) and Harris (Houston)• Major contributor to urban VOC emissions.• Gasoline VOCs- comprise of Mobile Source Air Toxics (MSATs)• Gasoline dominates the split for both emission cases.

New York, NY Harris, TX 0

5

10

15

20

25

30

VO

C, k

tons

per

yea

r

Gasoline Diesel

New York, NY Harris, TX 0

1

2

3

4

5

6

MS

AT

VO

Cs,

kto

ns p

er y

ear Gasoline

Diesel

(a) Total VOCs (b) MSATs

USEPA NEI, 2011

Temperature dependence of VOC emissions

Total hydrocarbons vs. T

Temperature Catalytic efficiency Emissions

Stump et al .

Motivation for current work

• Previous studies focused on total hydrocarbons and MSATS at different temperatures.

• Little quantification of uncertainty in emission factors• No comprehensive evaluation of speciation profiles -

imperative for air quality modeling.• This work intends to build on previous work to develop

comprehensive temperature dependent speciation profiles.

Key questions

• How do the speciation profiles for gasoline exhaust vary under temperature?

• What is the uncertainty in the emission factors for species classes?

• How does the MSAT content (both species and total) in VOCs vary with temperature?

• What implications do the results have for air quality modeling?

Vehicle fleet

• Driving conditions

• FTP (urban conditions)• US-06 (aggressive highway)

Temperatures

• 0 oF• 20oF• 75oF

Name Year Technology Standard Mileage Configuration

Buick Lucerne 2010 MPFI Tier 2/Bin 4 22000 3.9L V-6

Honda Accord 2010 MPFI Tier 2/Bin 5 24000 2.4L I-4

Jeep Patriot 2010 MPFI Tier 2/Bin 5 22000 2.0L I-4

Kia Forte EX 2010 MPFI Tier 2/Bin 5 25000 2.0L I-4

Mazda 6 2010 MPFI Tier 2/Bin 5 24000 2.5L I-4

Mitsubishi Galant 2010 MPFI Tier 2/Bin 5 38000 2.4L I-4

Additional details

• Testing done at the USEPA’s OTAQ premises.• All vehicles used 10% ethanol by volume.• 160 compounds, grouped into following species classes

AromaticsAlkanesCyclic AlkanesAlkenesAlkynesEthersAldehydesKetones

• Methane evaluated separately• FTP – 3 phases: Cold Start, Running and Hot Start.• Cold Start evaluated separately

Bootstrap Method

• Example with FTP Cold Start at 0oF.• Distribution of means from random sampling.• Uncertainty shown by 95% confidence interval.

0 1 2 30.0

0.1

0.2

0.3

0.4

0.5

R

elat

ive

freq

uenc

y

Aromatics EF, g mi-1

Draw SampleCalculate Mean

Repeat 1000 times

0.8 1.0 1.2 1.4 1.6 1.8 2.00.00

0.05

0.10

0.15

0.20

Rel

ativ

e fr

eque

ncy

Aromatics EFs, g mi-1

0.6 0.8 1.0 1.2 1.4 1.6 1.80.0

0.2

0.4

0.6

0.8

1.095% C.I

Mean

Per

cent

ile

Aromatics EFs, g mi-1

FTP Composite Emissions: Monte Carlo method

0.43*Cold Start+Running+0.57*Hot Start

Means, Cold Start Means, Running Means, Hot Start

0.1 0.2 0.3 0.40

20

40

60

80

100

Per

cent

ile

Composite Aromatics EFs, g mi-1

Temperature dependence of mean emission factors

(a) FTP (b) US-06

• Significant decrease with temperature due to increasing catalyst efficiency.

• US-06 emissions significantly lower than FTP-Composite.

0

1000

2000

3000

4000

75oF20oF0oF75oF20oF0oF

Composites

Cold Start

TOG

EFs

, mg

mi-1

0

10

20

30

40

50

20oF 75oF0oF

Methane Ketones Aldehydes Alcohols Ethers Alkynes Alkenes Cyclic Alkanes Alkanes Aromatics

TOG

EFs

, mg

mi-1

Uncertainty in mean emission factors

• Example with FTP composite emissions, units in mg mi-1.• Broadly factor of 2, uncertainty.

0 oF 20 oF 75 oF

Aromatics 265 (188-356) 51 (76-99) 12 (9-15)

Alkanes 256 (189-317) 61 (46-74) 8 (7-10)

Cyclic Alkanes 41 (25-59) 11 (7-14) 1.1 (0.8-1.3)

Alkenes 125 (100-150) 36 (26-46) 5 (4-6)

Alkynes 19 (14-24) 3.3 (2.3-4.3) 0.4 (0.2-0.6)

Alcohols 54 (39-69) 18 (10-28) 7 (2-13)

Aldehydes 9 (8-11) 7 (6-8) 3 (4-11)

Ketones 1.2 (1-1.3) 1.7 (1-2.5) 0.6 (0.5-0.7)

Methane 34 (29-39) 12 (9-14) 5 (3-7)

What influences composite emissions?

• Two scenarios:

• CASE 1 : Cold Start dominates by orders of magnitude.• Applicable to most hydrocarbons at all temperatures .

• CASE 2: Hot Start and Running comparable to Cold Start.• Applicable to all oxygenates - alcohols, aldehydes a nd ketones.

Cold S

tart

Hot Sta

rt

Running

100

101

102

103

Aro

mat

ics

EFs

, mg

mi-1

(a) FTP, 0F

Cold S

tart

Hot S

tart

Running

0

2

4

6

8

10

Alc

ohol

EFs

, mg

mi-1

(b) FTP, 75F

Speciation profiles

• Significant increases seen in speciation for alcohols, aldehydes and methane.• Alkanes show definitive decrease with temperature for both Cold Start and composite

phases of the FTP cycle.• Aromatics significantly fall with temperature for the US-06 cycle.

Cold S

tart,

0F

Cold S

tart,

20F

Cold S

tart,

75F

Composit

e, 0F

Composit

e, 20

F

Composit

e, 75

FUS-0

6, 0F

US-06,

20F

US-06,

75F

0

20

40

60

80

100 US-06

Composites, FTPCold Start, FTP

Wei

ght %

TO

G Methane Ketones Aldehydes Alcohols Ethers Alkynes Alkenes Cyclic Alkanes Alkanes Aromatics

MSAT fraction in TOG profile

• Three distinct patterns.• FTP, Cold Start – MSAT fraction increases marginally with temperature.

Speciation constant.• FTP, Composite – Substantial change in MSAT fraction and speciation.• US06- MSAT fraction unchanged, substantial change in speciation.

0.0

0.1

0.2

0.3

0.4

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0.6

US06

75F75F20F 20F0F0F

FTP, CompositeFTP, Cold Start

75F20F0F

M

SA

T fr

actio

n in

TO

G

Other MSATs

Ethanol

Benzene, Toluene, Ethyl benzene, Xylene (BTEX)

Ozone forming potential: the Carter MIR scale

• US-06: definitive changes with temperature, especially at 75oF.• Weaker trends for FTP-Composite profiles.• Need detailed air quality modeling representing varying NOx

conditions to accurately understand the impacts.

Cold

Start,

0F

Cold S

tart,

20F

Cold

Start,

75F

Compo

site,

0F

Compo

site,

20F

Compos

ite, 7

5FUS06

, 0F

US06, 2

0FUS06

, 75F

0

1

2

3

4

Ozo

ne fo

rmed

, ton

s/da

y

http://www.cert.ucr.edu/~carter/emitdb/

Ozone potential = VOC mass fraction x MIR x emission rate ( 1 ton/day)

Conclusions

• Significant difference in speciation across temperatures and driving conditions.

• Cold Start emissions were the dominant FTP phase for most hydrocarbons, greater by at least 2 orders of magnitude than Running and Hot Start emissions .

• For alcohols and carbonyls, Cold Start, Running and Hot Start emissions were comparable.

• Three distinct patterns for MSAT fractions with temperature.Cold Start: Marginal change in MSAT fraction, speciation unchangedComposite : Significant change in MSAT fraction and speciationUS06: Marginal change in MSAT fraction, significant for speciation

• Ozone forming potential decreased at 75oF for Composite and US-06 profiles.

Future directions

• Speciation results will be available at the USEPA’s SPECIATE database for the next version.

• Use these profiles to inform and update MOVES model to build motor vehicle emissions inventories.

Acknowledgements

• University of Houston-College of Natural Sciences and Mathematics for funding.

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