Latest Revisons to the On-Road Emission Model

This page last reviewed April 14, 2010

This page summarizes the major revisions associated with the development of EMFAC2002. Each major revision has a brief description, background information, resulting inventory effects, detailed technical documentation, and a staff contact.

Revised Implementation Schedule for LEVII Evap
Correct Usage Rates for School Buses
Correct Monthly Average Gasoline RVP
Extended Idle for School Buses
Correction to 2007+ HDD PM Emission Rates
Extended Idle for Heavy-Duty Trucks
Modification of Passenger Car Mileage Accrual Rates
Update Speed Distribution
Update Vehicle Miles Traveled
Update Population and Registration Distributions

Other Revisions


Dated September 2002

Dated April 2003


Major Revision: Revised Implementation Schedule for LEVII Evap

  • Align Evaporative and Exhaust Tech Group Fractions
  • Proposed change would shift assumed new-car sales from near-zero evaporative emissions to zero-evaporative emission vehicles (PZEV)


DESCRIPTION

This revision updates the EMFAC model with respect to the technology group fractions for 2003+ passenger cars and light-duty trucks (T1). Although the previous version of EMFAC did correctly model exhaust technology group fractions for the Low Emission Vehicle II (LEVII) program, evaporative technology groups were not adjusted. These changes rectify this omission and result in a very modest decrease in emissions.

BACKGROUND

  • Exhaust and Evaporative Emissions Technology Groups are modeled independently in EMFAC
  • EMFAC2001 v2.06 was updated to include modifications in response to changes to ARB’s LEVII and U.S. EPA’s Tier II programs
  • Exhaust Tech Fractions Were Properly Modified - EVAP Tech Fractions Were Not
  • EMFAC2001 v2.06-2.08 has too few PZEVs


INVENTORY EFFECTS

ROG Evaporative Emissions - South Coast Air Basin (tons per day)
Calendar Previous Revised Change
Year LDA LDT1 LDA LDT1 LDA LDT1
2010 54.04 12.14 54.00 12.16 -0.04 0.02
2020 32.58 8.58 31.23 8.35 -1.35 -0.24


Technical Documentation: Revised Implementation Schedule for LEVII Evap

Staff Contact: Ben Hancock




Major Revision: Correct Usage Rates for School Buses

  • Apply School Bus VMT and Trips only to the appropriate hours of operation (morning/afternoon peak), for all portions of the state.
  • Affects Running Exhaust, Starts, Hot Soak, and Running Loss Emissions

DESCRIPTION

This revision updates to the EMFAC model with respect to how daily school bus activity is modeled. The previous versions of EMFAC effectively "dropped" much of the school bus activity, resulting in an underestimation of the school bus emissions inventory. The changes rectify this undercounting of emissions.

BACKGROUND

  • EMFAC2000 assumes School Buses activity is limited to morning and afternoon peaks. However, VMT and Trips were assumed to occur throughout the day.
  • EMFAC2001 v2.06-2.08 properly applied speed profiles (morning/afternoon) Statewide but did not restrict VMT and Trips.
  • Implication - VMT and Trips were spread throughout the day, but because there was no activity outside the morning/afternoon peaks, the associated emissions were effectively “dropped”.


INVENTORY EFFECTS

Statewide Emissions Increases for School Buses - Summer Episodic 2010

  tons per day
School Bus Inventory 		% Increase
School Bus Inventory 		% Increase
Total Inventory
ROG 1.04 168% 0.2%
CO 11.58 180% 0.2%
NOx 10.91 235% 1.1%
PM10 0.46 235% 0.8%


Technical Documentation: Correct Usage Rates for School Buses

Staff Contact: Dilip Patel



Major Revision: Correct Monthly Average Gasoline RVP

  • Reduce Predicted Monthly RVPs to below 7.0 psi during control periods (RVP = 6.8 psi)
  • Redefine RVP for winter episodes to be the mean of the RVPs for winter months
  • Affected Areas Include
    • Placer/Humboldt/Trinity/San Mateo/Fresno
    • San Luis Obispo/ Imperial
  • Recalculate episodic and annual average RVPs for all areas of the state


DESCRIPTION

This revision updates the EMFAC model with respect to seasonal RVP. During the control season, these changes bring the RVP levels down to values more consistent with Title 13, California Code of Regulations §2262.4. We have also changed the manner in which we calculated the winter-average RVP. Previously, we used the maximum RVP values found during the winter for use in calculating the winter-average day emissions before and after 1996. We are now changing this approach to use an average of the December, January, and February monthly RVP values.

BACKGROUND

  • EMFAC calculates monthly, summer (ozone episodic), winter (CO episodic), and annual average inventories
  • The volatility of the dispensed gasoline is stored in EMFAC as a function of calendar year, month and county. The RVPs are predicted from historical relationships between Nominal and Empirical RVPs.
  • State regulations (13 CCR §§2262.4) require refiners starting in 1996 to produce and sell low-volatility gasoline (less than 7 psi RVP) in the summer ozone months.
  • EMFAC currently predicts RVPs exceeding 7 psi for some phase-in months.
  • The maximum monthly RVP was being used for the winter episodes rather than an average RVP of the winter months
  • Changing the RVPs of individual months necessitates a change in the episodic and annual average RVPs
  • Summer Episodic Estimates unaffected


INVENTORY EFFECTS

ROG Evaporative Emissions - Statewide (tons per day)
  Previous Revised Change
Season 1990 2010 1990 2010 1990 2010
Annual 871.2 289.6 871.2 287.2 0.0 -2.4
Winter 1073.0 382.3 974.8 336.7 -98.2 -45.6


Technical Documentation: Correct Monthly Average Gasoline RVP

Staff Contact: Ben Hancock


Major Revision: Extended Idle for School Buses


  • Incorporate idle activity based on chase studies into the inventory
  • This would add about 32 minutes of idle per bus per day


Pick up Students:
5.75 Stops * 1.38 Minutes of Idle/Stop = 7.94 (Morning)
Unload Bus at School 7.94 (Morning)
Load Bus at School 7.94 (Afternoon)
Drop off Students:
5.75 Stops * 1.38 Minutes of Idle/Stop = 7.94 (Afternoon)
31.76 Min. of Idle/Day


DESCRIPTION

This revision updates the EMFAC model with respect to adding idle emissions for school buses. Although idle emissions for heavy-duty trucks are currently estimated in EMFAC, no such estimation is made for school buses. Staff has included approximately 32 minutes of idle per bus per day based upon an analysis performed by Valley Research Corporation for the ARB. Given that most school buses are medium-heavy duty vehicles, the idle emission rates for this vehicle class is used for school buses.

BACKGROUND

  • Measures are being considered to limit idle time for school buses
  • Idle emissions for school buses are not currently included in EMFAC


INVENTORY EFFECTS

Addition of School Bus Idle - Statewide 2002 (tons per day)
Fuel Type HC CO NOx PM10
Gasoline 0.10 0.60 0.01 0.00
Diesel 0.05 0.36 1.10 0.02
Total 0.15 0.96 1.11 0.02


Technical Documentation: Extended Idle for School Buses

Staff Contact: Connie Leong


Major Revision: Correction to 2007+ HDD PM Emission Rates

  • Reflect impact of standards on PM


DESCRIPTION

This revision updates the EMFAC model with respect to calendar year 2007+ particulate matter emission rates. Although the emission factors for hydrocarbon, carbon monoxide and oxides of nitrogen were updated in EMFAC2000 to reflect the 2007 standards, no change was made for particulate matter. Staff has modified the 2007+ particulate matter emission rates for California certified heavy-heavy and medium-heavy duty diesel trucks, as well as diesel powered school buses and motor homes.

BACKGROUND
  • Recently adopted standards call for significant reduction in emission from Heavy-Duty-Diesel Vehicles
  • Although current model reflects standard change for HC, CO, and NOx - Changes in PM not currently included


INVENTORY EFFECTS

Correction to 2007+ HDD PM Emission Rates - Statewide PM Emissions (tons/day) in 2010
Heavy Duty Diesel Vehicles Previous Revised Increase
Medium Heavy-Duty Diesels 3.46 2.82 0.64
Heavy Heavy-Duty Diesels 6.61 5.13  1.48
School Bus 0.64 0.60 0.04
Motor Home 0.05 0.04 0.01
All Diesels 11.3 9.16 2.17


Technical Documentation: Correction to 2007+ HDD PM Emission Rates

Staff Contact: Lei Zhou

Major Revision: Extended Idle for Heavy Heavy-Duty Diesel Trucks


  • Increase the per trip idle time for heavy heavy-duty diesel trucks from 5 minutes per trip to 21 minutes per trip
  • Decrease the number of trips per day from approximately 10 to 5
  • This change would ascribe an average of 105 minutes of idle per vehicle per day


DESCRIPTION

This revision incorporates extended idle events that better reflect activity associated with truck stops, rest areas, and distribution centers. EMFAC2000 included only that activity associated with "idle trips" defined as key-on to key-off events with no appreciable distance traveled. This activity is indicative of a truck in queue to either pick up or drop off a shipment. Instrumented truck activity data has shown that considerable amounts of idle occur between trips. This revision increase heavy heavy-duty diesel truck idle from 13 minutes to about 105 minutes per vehicle per day.

BACKGROUND

  • EMFAC included only idle activity associated with "idle trips" (key-on to key-off events with no appreciable movement)
  • Total idle time was restricted to avoid possible double counting
  • Analysis of second by second activity data from 84 instrumented trucks suggest that idle is underestimated in EMFAC


INVENTORY EFFECTS

Extended Idle for Heavy Heavy- Duty Diesel Trucks - Statewide 2010 (tons per day)
Statewide 2010 HC CO NOx PM10
Previous 0.19 1.16 3.57 0.11
Revised 1.51 9.52 29.21 0.87
Difference 1.32 8.36 25.64 0.76


Technical Documentation: Extended Idle for Heavy Heavy-Duty Diesel Trucks

Staff Contact: Hector Maldonado



Major Revision: Modification of Passenger Car Mileage Accrual Rates

  • Returned the passenger car mileage accrual rates to those originally estimated and documentation in the EMFAC2000 technical support document (Section 7.1


DESCRIPTION

This revision reverts to the original EMFAC2000 mileage accrual rates for passenger cars. In April 2000, an adjustment was made to the mileage accrual rates of all passenger cars (gas, diesel, and electric) to better match the overall statewide VMT estimate. Basically, the mileage accrual rates for passenger cars were made equal to those of light-duty trucks. This change tended to overstate the travel of older passenger cars in some areas of the state.

BACKGROUND

  • Mileage accrual rate is an estimate of the miles per year traveled and is based on odometer data from the Smog Check Program
  • VMT is mostly reported by either Councils of Governments or Metropolitan Planning Organizations.
  • VMT can also be calculated as VMT = Vehicles * Mileage accrual rates by age and vehicle
  • For those calendar years where the calculated and reported VMTs do not agree, the mileage accrual rates are adjusted until the reported VMTs are achieved.


INVENTORY EFFECTS

Passenger Cars Statewide 2010 (tons per day)
Statewide 2010 ROG CO NOx PM10
Previous 80.5 2088.9 177.5 7.3
Revised 65.9 1869.5 160.5 7.1
Difference -18.1 -10.5 -9.6 -2.7


Technical Documentation: Modification of Passenger Car Mileage Accrual Rates

Staff Contact: Jeff Long


Major Revision: Updated Speed Distribution

  • Incorporated new speed distribution data from some planning agencies - AMBAG, Amador, MTC, SACOG, SANDAG, SBCAG, SCAG, Fresno, Kern, Kings, Madera, Merced, San Joaquin, Stanislaus, Tulare
  • For the first time, SCAG has submitted separate speed distributions for heavy-duty trucks based on their heavy-duty truck travel demand model


DESCRIPTION

This revision updates the speed distribution data submitted by some of the planning agencies. The submission typically includes new baseline and projected speeds. Table 1 in the technical memorandum lists the areas that submitted new speed distribution data incorporated into this version of EMFAC.

BACKGROUND

  • Planning agencies periodically revise their motor vehicle activity estimates in response to an upcoming State Implementation Plan or Transportation Plan updates.


INVENTORY EFFECTS

Change in Statewide Summer Episodic Emissions (tons per day)
Calendar Year ROG CO NOx PM10
2000 -8.22 -26.13 41.99 -0.81
2010 -4.29 -30.83 22.78 -1.02


Change in SCAG Region Summer Episodic Emissions (tons per day)
from Heavy-Duty Truck Speed Distribution Update
Calendar Year ROG CO NOx PM10
2000 -8.01 -61.04 37.68 -0.77
2010 -2.88 -18.78 21.82 -0.61


Technical Documentation: Updated Speed Distribution

Staff Contact: Agnes Dugyon



Major Revision: Updated Vehicle Miles Traveled

  • Incorporated new vehicle miles traveled data from some planning agencies - AMBAG, MTC, SACOG, SCAG, Kern, Madera, Merced, San Joaquin


DESCRIPTION

This revision updates the vehicle miles traveled (VMT) submitted by some of the planning agencies. For the areas that have chosen not to update their forecasts, the existing information is used. These new VMT forecasts affect the total statewide VMT and indirectly the vehicle population. It is important to note that the total change in VMT reflects significant increases in VMT in some areas, which are partially offset by decreases in other areas. Table 1 in the technical memorandum lists the areas that submitted new VMT incorporated into this version of EMFAC.

BACKGROUND

  • Planning agencies periodically revise their motor vehicle activity estimates in response to an upcoming State Implementation Plan or Transportation Plan updates.
  • The EMFAC model maintains an internal relationship between mileage accrual rates, vehicle population, and vehicle miles traveled: VMT = vehicle population * mileage accrual rates


INVENTORY EFFECTS

Area Specific VMT and Emissions Changes (tons per day) in 2005
Agency VMT*1000 %VMT ROG CO NOx PM10
MTC 415 0.2 0.60 3.47 -0.23 -0.01
AMBAG -20 -0.1 -0.1 -0.74 -0.01 -0.01
SJCOG 1243 2.5 1.57 18.21 1.28 0.04
SACOG -633 -1.4 2.59 18.5 1.03 -0.03
Subtotal 975 0.3% 4.66 39.44 2.07 0.01

Note: Although the San Joaquin Valley is represented by the six COGs that are responsible for forecasting travel in eight counties; the VMT data were submitted by Mike Bitner from Fresno COG who is the main contact for all data from San Joaquin area COGs. Hence, the data are referenced in this document as being from SJCOG.

Technical Documentation: Updated Vehicle Miles Traveled

Staff Contact: Dilip Patel

SCAG Specific VMT and Emissions Changes (tons per day) in 2010
Agency VMT*1000 ROG CO NOx PM10
SCAG 1969 -1.95 -13.3 -1.64 0.05


Technical Documentation: Updated Vehicle Miles Traveled (SCAG)

Staff Contact: Jeff Long



Major Revision: Revisions to the Population and Registration Distributions

  • Incorporated vehicle population and registration distribution information for calendar years 1999 and 2000


DESCRIPTION

This revision incorporates vehicle population and registration distributions by age for calendar years 1999 and 2000. The Air Resources Board receives extractions from the California Department of Motor Vehicles (DMV) twice each year. The over 30,000,000 records in the data set were grouped by vehicle classes listed below:

Class Description Weight (GVW) Abbreviation
PC Passenger Cars All LDA
T1 Light-Duty Trucks 0-3,750 LDT1
T2 Light-Duty Trucks 3,751-5,750 LDT2
T3 Medium-Duty Trucks 5,751-8,500 MDV
T4 Light-Heavy Duty Trucks 8,501-10,000 LHDT1
T5 Light-Heavy Duty Trucks 10,001-14,000 LHDT2
T6 Medium-Heavy Duty Trucks 14,001-33,000 MHDT
T7 Heavy-Heavy Duty Trucks 33,001+ HHDT
UB Urban Bus All UB
MC Motorcycles All MCY
SB School Buses All SBUS
MH Motor Homes All MH


BACKGROUND

  • EMFAC2001 contains vehicle population and registration distribution data for calendar years 1997 and 1998.
  • EMFAC forecasts based on the latest data available, 1998, and backcasts vehicle populations based on the earliest data available, 1997.


INVENTORY EFFECTS

Change in Statewide 2010 Emissions (tons per day)
  ROG CO NOx PM10
Previous 608.7 5613 1022.3 27.4
Revised 538 5198 1043 27.7
Difference -70.7 -415 20.7 0.3
% Difference 11.6 7.4 2.0 1.1


Technical Documentation: Revisions to the Population and Registration Distributions

Staff Contact: Alice Tong

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