Link-based vs NEI 

On-road Emissions Impac

On Air Quality Model Predict

2008 Emission Inventory Conference

Session 11- Mobile Sources

Portland, Oregon

June 5, 2008

Madeleine Strum, Karen Wesson, Sharon Phillips – EPA OADavid Brzezinski, Harvey Michaels, Rich Cook – EPA OTAQAlison Pollack, Michele Jimenez, Stella Shepard - ENVIRONAllan Beidler, Melanie Wilson, Computer Sciences Corporat
Purpose

• Compare AQ Model-ready link-based on

emissions from CONCEPT MV with NEI 

– NEI is county-level, used less accurate data– CONCEPT MV is link-based, hourly, gridded

• Quantify/understand differences in VMT, emissions, spatially and temporally using 

data

• Determine differences in modeled concen

and impact of data inputs versus method
Comparison of 

Emission Estimation Approac

Local data from MPOLink-specific speeds, by hourHour and grid-specific temperatures from 12km met data used for AQ modelingDetailed temporal profiles by hour/day/month from traffic classification monitoring dataLink-level, hour-specific, from Transportation Demand Model, adjusted using hour/day/month temporal profiles from traffic counter data

CONCEPT MV

Uniform across all hours, Same adays. County database: uses national defaults where data not submitted by State.

VMT mix

Local daCONCECONCEdistributby roadVMT-we(across within eUses CO

NMIM 

County database: uses national defaults where data not submitted by StateCounty database: uses national defaults where data not submitted by StateMonthly average 24-hr temperature by county from NWS dataCounty-level, monthly, by road type; either provided by State or allocated to county from MSA-level VMT using population

NEI

Other M6 inputsSpeedTemperatureVMT
NEI Processing for 

AQ Modeling

• Processed county-level monthly emissions f

modeling using SMOKE:

– Spatially allocated using miles of roadway (or 

population for local roads)

• 12 km grid cells for CMAQ• 1 km grid cells for AERMOD

– Temporally allocated from monthly to day-of-w

hour of day using temporal profiles 

– Speciated emissions for use in photochemical 
CONCEPT MV 

Link-Based Method

Link-level VMT and speeds from SEMCOG transpodemand model (TDM)

– VMT and speed on ~15,000 links, 

by period of day

SEMCOG Roadway Network
Link vs NEI Comparisons: Det

• Link emissions based on CONCEPT MV f

comparison to NEI:

– Estimated for seven-county SEMCOG transportation network – One representative week for each of four sea

3⁄4January, April, July, October 2002

– Emissions estimated for 8 MOBILE5 vehicle c– Results were used to estimate monthly and annualized emissions
County-level VMT Similar –

Michigan Provided VMT Inputs to 

Annual VMT (millions) NEI

0

5,000

10,00015,00020,00025,000

Livingston

Macomb

Monroe

Oakland

St. Clair

Washtenaw

LINKNEI
Spatial Distribution of VMT for Grid 

Modeling is Very Different When 

Link VMT is Used

Link VMT gridded to 1kmVMT distributed across more roads – more detailed spatial coverage

Max VMT = 301

Max VMT = 89

NEI County VMT gridded to 1kmAllocation based on Census TIGER roadway surrogates; creates VMT “hot spots”
Hourly Temporal Allocation 

for AQ Modeling is Different

When using the NEI, SMOKE temporally allocates monthly emissions to day-ofweek and hour-of-day.Example:Monday Benzene for Urban Interstates, April

-

200,000400,000600,000800,000

1,000,0001,200,0001,400,0001,600,000

0 2 4 6 8 10 12 14 16 18 

Hour

VMT

CONCEPT temporally VMT for different time the day to hourly baseanalysis of traffic coun

Example:Monday VMT for Urban Inters

0.0000.0200.0400.0600.0800.1000.1200.140

0 2 4 6 8 10 12 14 16 18 20 22

Hour

Tons Benzene
In CONCEPT VMT 

Mix Varies by Hour

HDDV VMT fraction is lower during am and pm peak traffic 

SEMCOG Freeway VMT Mix by Hour of Day - Weekdays

0%

10%20%30%40%50%60%70%80%90%

100%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 

Hour of Day

Percent of Total VMT

NEI Ave
NEI Benzene Emissions 

Higher Than Link

0

100200300400

JANUARY APRIL JULY OCTO

Benzene (Tons) Link NEI

Sum across 7 Counties

17%

29%

26%

5

Differences most likely dominated by differences in the MOBILE6 inputs. Differences due to methodology appear to be small. 

Much closer when NMIM 

is run with Local Inputs

One day/m

05

101520

January April 

LN
Other Pollutants Emissions –

NEI Greater Than Link

0%5%

10%15%20%25%30%35%40%45%

Acetaldehyde

Benzene

Butadiene

Formaldehyde

Toluene

P

Percent difference from link
Reasons for Higher Emission

• Other than VMT, inputs to NEI were natio

defaults and didn’t match the local inputs the link based approach

• Speeds – speed values and spatial distribu• Temperatures – County/monthly average 

versus day-hour and grid cell-specific

• These and further differences (e.g., age 

distribution) are currently being investigate
Link Based Emissions Have Little 

Impact on 12 km Model Evaluation

8 hr Ozone: Macomb county site - 260990009

observed

8 hr Ozone Macomb county site - 260990009

Daily PM2.5 Wayne county site – 261630001

observed
Total Benzene Gridded to 1 km 

(On-road Sector is 66% of Total Emission

using NEI on-road using link-based o

26 tons

NEI on-road emissions 

gridded using Census TIGER 

roadways surrogates

On-road emissions 

from CONCEPT MV 

(gridded link-based)
CMAQ results Benzene 

12 km Annual Average

2.048

0.12

1.807

1.5661.325

1.084

0.8430.6020.361

NEI Benzene ug/m3 (NEI – Link) Benzene 

Largest Negative Difference relaNEI underestimating risk by 1 in (NEI 16; Link 15)Largest Positive Difference relate

overestimating risk by 3 in a milLink 11)

Orange

/Red Cells: 

Risk is 12-16 in a millionRisk computed based on concentration * Unit Risk Estimate (URE)Benzene URE =0.0000078 (1/[ug/m3]) 

Differences vary across gr
Benzene 

risk differences: 

NEI – link

-1.2 to -.255-.255 to -.17-.17 to .529.529 to 1.261.26 to 3.1

Link Concentrations 

Are Higher Than NEI for Cells with Few Major Roads
Conclusions

• Compared NEI to CONCEPT/TDM (link) for De

– VMT: totals are similar– Emissions: NEI benzene roughly 20% higher (across 

area)

– Concentrations: vary over geographic regions, maxim

benzene differences caused 1-3 in a million change in 

NEI base of roughly 12-16 in a million)

• Differences in aggregate emissions are primarily 

inputs, not methods

• Use of NEI could result in inaccurate representa

spatial distribution of certain pollutants due to thallocation approach for the emissions

• No significant differences in model performance 

no link and link methods at 12 km resolution for PM2.5, toxics. Expect to see differences at fineresolution.
Next Steps

• Perform analyses for more pollutants• Isolate MOBILE6 input parameters of impo• Run AERMOD at1 km resolution and comp

fine scale modeling results 

• Run 1 km resolution through exposure an

benefits models and compare results to 12