APPENDIX E

URBAN DIESEL TRANSIT BUS EMISSIONS INVENTORY
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The EMFAC model used by the Air Resource Board (ARB) to obtain on-road motor

vehicle emissions also calculates an emissions inventory for urban buses.

However, for two reasons staff believes that the urban diesel bus inventory in

EMFAC may not be suitable, without modification, for developing regulations that

address only urban diesel transit buses. First, the population of the urban diesel

bus vehicle class in EMFAC is derived from the Department of Motor Vehicles

(DMV) registration database and contains urban transit buses as well as other

categories of non-public transit buses, such as Greyhound and tour buses. The

population reported by transit bus agencies is smaller than that used in the EMFAC

model. Second, the urban bus fleet in EMFAC consists of 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 agencies show that the

transit bus fleet consists of only 22 model years and buses of different ages accrue

different mileage as a function of the age of the buses.

In support of the ARB transit bus fleet rule regulation amendments, staff has

constructed an inventory model specifically for diesel-powered transit buses that

uses the population and activity data reported by transit agencies and emission

rates from the EMFAC model. The following sections discuss the transit bus activity

and emission rate estimate and present a revised urban diesel transit bus inventory.

Urban Diesel Transit Bus Activity Data

The following urban diesel 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 diesel transit 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.

A statewide diesel transit bus population (POP) of 6,303 vehicles was reported for

2002 by transit bus agencies. The age distribution (the number of vehicles by age)

for diesel transit buses was calculated from the 2002 population data.

Population for future years was projected based the following assumptions:

• No growth in 2003;

• Forty new diesel buses each year for 2004, 2005, and 2006;

• One percent growth for 2007, 2008, 2009, and 2010.
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The population of diesel transit buses for 2002+ was estimated using 2002

population as the base year. The projected population for each future year was

adjusted with the survival rate (the fraction of the new vehicles that remains in the

fleet after certain years) used for urban diesel buses in the EMFAC model.

The diesel transit bus accrual rate, survival rate, and population distribution for years

2002 and 2010 are given in Table 1.

Table 1. Diesel Transit Bus Accrual Rate, Survival Rate, and

Population Distribution

Age

Accrual Rate

(mi/year) Survival Rate

Year 2002

Population*

Year 2010

Population**

0 30,868 1.0000 467 107

1 31,679 1.0000 945 160

2 32,332 1.0000 539 144

3 32,824 0.9930 269 96

4 33,158 0.9930 400 40

5 33,332 0.9930 506 40

6 33,346 0.9930 230 40

7 33,201 0.9894 188 52

8 32,897 0.9894 144 462

9 32,434 0.9878 326 933

10 31,811 0.9877 496 532

11 31,028 0.9840 462 267

12 30,087 0.9840 412 396

13 28,986 0.9791 362 499

14 27,725 0.9791 340 227

15 26,305 0.9746 18 185

16 24,726 0.9329 69 136

17 22,987 0.9329 45 308

18 21,089 0.9329 2 468

19 19,032 0.9251 10 434

20 16,815 0.9151 23 383

21 14,439 0.9099 9 336

22 11,904 0.9032 41 314

* Reported by transit bus agencies.

** Projected from year 2002 population and growth rate.

The diesel transit bus annual VMT for any given year was estimated from the

population (POP) and accrual rate using the following equation:
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VMT = Σ (POPage x Accrual Ratei), age = 1 to 22. (1)

Figure 1 shows the estimated diesel transit bus fleet annual VMT for 2001 to 2010.

Figure 1. Urban Diesel Transient Bus Annual Mileage VMT

The diesel transit bus fleet shows a decline in VMT from 2001 to 2010. According

to transit agency reporting, the population of diesel transit buses decreases from

6,738 vehicles in 2001 to 6,303 in 2002. From 2003 to 2006, the population is

projected to remain essentially constant at 2002 level and then grow slightly each

year from 2007 to 2010. However, from 2002 to 2010, the average transit bus fleet

is projected to transform from mostly newer buses to mostly older buses (see Table

1). Since the annual mileage data shows that newer buses accrue more mileage

than older buses, the annual VMT is projected to decline from 2001 to 2010.

It is believed that this decline in diesel transit bus fleet VMT should be compensated

by buses powered by alternative fuels and therefore the total VMT of the transit bus

fleet should remain essentially constant or show an overall increase.

Transit Bus Emission Rates

The transit bus emission rates used for this analysis are the same as those used in

EMFAC2001 version 2.08. Table 2 shows the speed-corrected NOx and PM

emission rates obtained from EMFAC2001.

150,000

160,000

170,000

180,000

190,000

200,000

210,000

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Calendar Year

VMT (x 1000 mi/yr)
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Table 2. EMFAC2001 Diesel Urban Bus Emission Rates (g/mi)

Model Year Group NOx PM

Pre 1987 23.2 0.310

1987-1990 20.3 0.294

1991-93 12.9 0.278

1994-95 15.1 0.339

1996-98 19.8 0.407

1999-02 10.3 0.139

2003 5.15 0.028

2004-06 1.2 0.024

2007 0.478 0.037

2008+ 0.426 0.032

Transit Bus Emission Inventory

Table 3 shows the diesel transit bus baseline inventories for selected calendar

years.

Table 3. Statewide Transit Bus Baseline Emissions Inventory

2001 2002 2004 2006 2008 2010

NOx (ton/day) 10.3 8.82 8.49 7.98 7.31 6.49

PM (lb/day) 372 321 309 290 267 239

The inventory given in the table does not include the ARB’s 2002 low sulfur diesel

fuel and adopted retrofit regulations for transit buses and the U.S. EPA 2006 low

sulfur diesel fuel regulation.