APPENDIX C

INVENTORY AND EMISSIONS MODELING
I. Methodology

The emissions inventory model EMFAC2002, which is used by the Air Resource Board

(ARB) to estimate on-road motor vehicle emissions inventories, also calculates an

inventory for urban buses. However, for a number of reasons staff believes that,

without modification, the EMFAC model may not be suitable for developing regulations

that address only urban transit buses. First, the population of the urban bus vehicle

class in EMFAC is derived from the Department of Motor Vehicles (DMV) registration

database and contains urban buses as well as other categories of buses (examples are

Greyhound and tour buses). The urban bus population reported by transit agencies is

much smaller than the urban bus population in EMFAC. Second, the urban bus fleet in

EMFAC contains 45 model years of vehicles and buses of all ages are assumed to

accrue 37,700 miles per year on average. Data reported for the years 2000-2002 by

transit agency show that the transit bus fleet consists of only 23 model years and that

mileage accrual rate is a function of vehicle age. Finally, the EMFAC model does not

include alternative-fueled vehicles, but alternative-fueled buses have increasingly

become an important part of every transit fleet.

In support of the ARB’s amendments to the Fleet Rule for Transit Agencies, staff has

constructed an inventory model specifically for diesel and alternative-fueled urban

buses. The model uses the population and activity data reported by transit agencies,

diesel bus emission rates from the EMFAC model, and alternative-fueled bus emission

rates estimated from recent test data. The following sections discuss the urban bus

activity and emission rate estimates and present an urban bus specific inventory.

II. Urban Bus Activity Data

The Fleet Rule for Transit Agencies, adopted in 2000, has required that transit agencies

provide an annual report of their urban buses since 2001. The population data were,

therefore, based on reported data.

The following urban transit bus activity data were obtained and analyzed:

• Annual mileage accrual rate;

• Population (POP) and age distribution;

• Total vehicle miles traveled (VMT).

The annual mileage accrual rate for urban buses was estimated from the annual

mileage data provided by transit agencies. The average annual mileage data by model

year was statistically fit to obtain a relationship between annual mileage accrual rate

and vehicle age. Alternative-fueled urban buses were assumed to accrue the same

annual miles as diesel urban buses at the same age.
C-2

A. Statewide

The statewide population of urban buses, based on the 2002 reports from transit

agencies, was 10,142, or 6,476 diesel buses and 3,666 alternative-fuel buses. Staff

calculated the age distributions (number of vehicles by age) for diesel and alternativefueled urban buses using these data. For the statewide alignment analysis, only the

diesel population was analyzed. Alternative fuel urban buses are assumed not to be

affected by engine availability as described below.

Staff projected the population for future years for the statewide baseline emissions,

alignment scenario emissions, and statewide alternative fuel scenario based on the

following assumptions:

• For the baseline scenario, no growth between 2004 and 2009 and growth

resuming starting 2010 at a 1.6 percent growth rate. As stated, in the Staff

Report, no engine manufacturer plans to certify a diesel engine for urban buses

during this time frame. Therefore, staff assumes that transit agencies on the

diesel path will maintain their current fleets through repowering until 2010 when

diesel engines become available. The slow down in turnover is reflected in the

reported data.

• For the alignment scenario, no growth between 2004 and 2006 and growth

resuming starting in 2007 at a 1.6 percent growth rate. Diesel engines will be

available at 1.2 g/bhp-hr NOx for 2007 through 2009. Fleets will start buying

diesel buses again at their usual rate in 2007, however, once the 2010 diesel

buses are available at 0.2 g/bhp-hr NOx, it is expected there will be larger

purchases made in 2010 and 2011 to catch up to their needed fleet sizes.

• For the statewide alternative fuel mandate, staff assumed the emissions would

follow the same declining rate as was projected in the original rule since it

followed the same engine standards path, i.e., 0.2 g/bhp-hr engines starting in

2007. However, the emissions starting points were adjusted to the 2006

emission values of 10.6 tons per day NOx and 420 pounds per day PM

consistent with the baseline values. All purchases will be alternative fuel buses

beginning in 2007 and continuing through 2015 when the alternative fuel path

mandate expires, but all engines whether diesel or alternative fuel will meet the

0.2 g/bhp-hr NOx standard from 2010 on.

Staff estimated the populations of diesel urban buses for 2003 and later model years

using reported 2002 populations as the base year. The projected populations for future

years were adjusted using the survival rates (the fraction of the new vehicles that

remains in the fleet after certain years) for urban diesel buses in EMFAC2002 and

modified to reflect the reported urban bus population survival rate.

The urban bus daily vehicle miles traveled (VMT) for any given year was estimated from

the population (POP) and accrual rate using the following equation:

VMT = Σ (POPage x Accrual Ratei), where age = 0 to 22. (1)
C-3

The urban bus mileage accrual rate, survival rate, and population distribution for years

2002 and 2020 are found in Table 1.

Table 1. Statewide Urban Bus Accrual Rate, Survival Rate and

Population Distribution

Age

Accrual Rate

(mi/year) Survival Rate

2002 Diesel

Population1

2020 Diesel

Population2

0 30,868 1.00000 501 647

1 31,679 1.00000 675 658

2 32,332 1.00000 796 670

3 32,824 0.99301 364 657

4 33,158 0.99301 438 637

5 33,332 0.99301 558 615

6 33,346 0.98943 237 588

7 33,201 0.98774 215 556

8 32,897 0.98399 154 496

9 32,434 0.97909 165 466

10 31,811 0.93291 610 461

11 31,028 0.93291 360 444

12 30,087 0.84530 355 392

13 28,986 0.71960 398 344

14 27,725 0.60220 336 287

15 26,305 0.49520 7 250

16 24,726 0.37055 59 182

17 22,987 0.24590 41 120

18 21,089 0.13950 161 70

19 19,032 0.07180 20 48

20 16,815 0.02960 25 24

21 14,439 0.00070 0 0

22 11,904 0.00070 1 0

1 Reported by transit agencies.

2 Projected from year 2002 original rule population.

B. South Coast Air Quality Management District (SCAQMD)

The SCAQMD urban bus population of 4,632 urban buses is based on data reported for

2004 by the 17 transit agencies located within the jurisdiction of SCAQMD, which

reported populations of 1,865 diesel fuel and 2,767 alternative-fuel buses. Staff

calculated the age distributions (number of vehicles by age) for diesel and alternativefueled urban buses using the survival rates (the fraction of the new vehicles that

remains in the fleet after certain years) based urban diesel buses in EMFAC2002. A 95
C-4

percent fleet turnover rate of the oldest diesel buses to be replaced with alternative-fuel

buses was used, except for years 2005 to 2009.

For model years 2005 to 2009, staff conducted a telephone survey of the six diesel path

transit agencies to obtain specific plans for purchasing, including replacement and

growth vehicles (Table 2). All other assumptions regarding growth rate and VMT

remain the same as in the statewide model. The population distribution for years 2005

and 2020 are given in Table 3.

Table 2. Reported Plans for Bus Purchases, SCAQMD Diesel Path Agencies

Year GHEB1 CNG Bus2

2005 71 14

2006 23

2007 55 9

2008 64

2009 51 1

Gasoline Hybrid Electric Bus

2

Compressed Natural Gas Bus
C-5

Table 3. SCAQMD Transit Bus Population Distribution

Age

2005 Diesel

Population1

2020 Diesel

Population2

2005 Alt Fuel

Population1

2020 Alt Fuel

Population2

0 2 394 63 402

1 90 63 122 72

2 135 103 210 87

3 112 105 1054 134

4 233 138 322 75

5 24 42 321 68

6 42 78 266 70

7 255 92 359 70

8 99 106 1 72

9 127 75 17 52

10 48 76 32 247

11 25 2 6

12 387 88 6

13 19 131 7

14 59 110 7

15 68 228 7

16 101 22 59

17 0 39 114

18 9 238 197

19 0 92 982

20 0 116 296

21 0 44 294

22 30 22 242

1 Reported by transit bus agencies.

2 Projected from year 2005 population.

III. Emission Rates of Diesel and Alternative-Fueled Urban Buses

The diesel urban bus emission rates used for this analysis are the same as those used

in EMFAC2002 version 2.2 (Table 4). The emission rates are based on the currently

adopted exhaust emission standards for urban bus engines and were adjusted for the

modeled scenarios. Changes to the emission factors for the scenarios were calculated

using a ratio between the adopted emission standard and the "projected" emission

standard to determine the emission rate for that year.
C-6

Table 4. EMFAC2002 Diesel Urban Bus Emission Rates (g/mi)1

Model Year Group HC NOx PM

Pre 1987 2.06 46.18 1.29

1987-1990 2.05 40.20 1.22

1991-93 2.02 25.49 1.16

1994-95 1.99 29.84 1.41

1996-98 1.98 39.17 1.69

1999-02 1.98 20.39 0.58

2003 0.84 10.20 0.12

2004-06 0.84 2.55 0.12

2007 0.84 1.022,3 0.12

2008+ 0.75 0.902,3 0.10

1. Values assumed for baseline, if all engines were available. 2. The difference in the emission factors for 2007 and 2008+ is based on the

implementation Zero Emission Bus purchasing requirements.

3 To model the alignment scenario, staff used emission factors of 6.14 for 2007 and 0.90

for 2010, to reflect certification values of 1.2 g/bhp-hr in 2007 and 0.2 g/bhp-hr (+ZEB) in

2010.

Emission rates for alternative-fueled transit buses were estimated from emission data

collected by West Virginia University (WVU) (Table 5). The WVU data include emission

data for 71 1991-1998 model year CNG-powered urban buses. The emission data were

first divided into model year groups corresponding to the model year groups of diesel

transit buses and the data in each group were then averaged. Emission rates for model

years after 1998 were estimated using the ratio of the standards. As with the statewide

analysis, changes to the emission factors for the scenarios were calculated using a ratio

between the adopted emission standard and the "projected" emission standard to

determine the emission rate for that year.
C-7

Table 5. Alternative-Fuel Urban Buses Emission Rates (g/mi)

Model Year Group HC NOx PM

1991-93 14.6 25.4 0.02

1994-95 15.5 11.2 0.02

1996-98 20.7 20.0 0.02

1999-02 20.7 20.0 0.02

2003 7.96 10.0 0.004

2004-06 0.80 7.51 0.004

2007 0.80 1.0 0.004

2008+ 0.80 1.0 0.004

1 The range of the certification values for alternative-fuel buses is 0.6 to 2.5 g

NOx+NMHC/bhp-hr. For the Baseline calculation, staff used the average value of 1.5 g

NOx/bhp-hr certification level to determine the emission factor of 7.5 g/mi. For the

scenarios, staff used the emission factor of 2.5g/mi (0.5 g/bhp-hr) because the

predominant urban bus purchased in these years is the gasoline hybrid certified at the

0.6 g NOx+NMHC level.

For comparison, Table 6 shows the currently adopted emission standards for

urban buses.

Table 6. Urban Bus Standards (g/bhp-hr)

Model Year HC CO NOX PM HC+NOx

1973-74 --- 40.0 --- --- 16.0

1975-76 --- 30.0 --- --- 10.0

1977-79 1.00 25.0 7.5 --- ---

1980-83 1.00 25.0 --- --- 6.0

1984-86 1.30 15.5 5.1 --- ---

1987-90 1.30 15.5 6.0 0.60 ---

1991-93 1.30 15.5 5.0 0.10 ---

1994-95 1.30 15.5 5.0 0.07 ---

1996-98 1.30 15.5 4.0 0.05 ---

1999-02 1.30 15.5 4.0 0.05 ---

10/2002-03 --- 15.5 2.5 (NOx+NMHC) 0.01 ---

2004-06 15.5 Diesel: 0.5

Alt Fuel:2.5 (NOx+NMHC) 0.01 ---

2007 15.5 0.2 0.01 ---

2008+ 15% of new purchases are

ZEBs for large fleets
C-8

IV. Urban Bus Emission Inventory

Using the data presented above, staff calculated baseline inventories for the statewide

and SCAQMD urban buses. Tables 7 and 8 show the diesel urban bus baseline

inventories for selected calendar years for the statewide and SCAQMD inventories and

Tables 9 presents the alternative-fueled urban buses for the SCAQMD inventories.

Table 7. Statewide Diesel Urban Bus Baseline Emissions (tons/day)

Pollutant 2000 2005 2010 2015 2020

HC 0.56 0.52 0.41 0.25 0.09

NOx 13.5 10.6 7.93 3.72 1.37

PM (lbs/day) 540 420 320 142 100

Table 8. Statewide Alignment Scenario Emissions (tons/day)

Pollutant 2000 2005 2010 2015 2020

HC 0.56 0.52 0.39 0.26 0.10

NOx 13.5 10.6 7.79 4.91 1.86

PM (lbs/day) 540 420 300 142 100

Table 9. Statewide Alternative Fuel Path Mandate Scenario Emissions

(tons/day)

Pollutant 2000 2005 2010 2015 2020

HC 0.56 0.52 0.55 0.56 0.56

NOx 13.5 10.6 7.18 3.72 1.37

PM (lbs/day) 540 420 300 142 100

Table 10. SCAQMD Diesel Urban Bus Baseline Emissions (tons/day)

Pollutant 2000 2005 2010 2015 2020

HC 0.23 0.15 0.13 0.11 0.062

NOx 5.72 3.03 2.64 2.11 1.19

PM (lbs/day) 239 125 107 85.3 48.8
C-9

Table 11. SCAQMD Alternative-Fuel Urban Bus Baseline Emissions

(tons/day)

Pollutant 2000 2005 2010 2015 2020

HC 2.61 5.42 5.19 4.17 2.38

NOx 2.50 5.27 5.06 4.09 2.38

PM (lbs/day) 5.1 10.5 10.3 8.6 5.6