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APPENDIX
Transit Air Benefits Calculator:
Description and User's Manual
Air Pollutants
To conduct a simple evaluation, one often looks at the "criteria pollutants." These pollutants--
defined by the Clean Air Act--include oxides of nitrogen (NOx), sulfur dioxide (SO2), carbon
monoxide (CO), ground-level ozone (O3), particulate matter (PM), and lead. Following is a
description of the criteria pollutants and other significant pollutants.
Oxides of Nitrogen
NOx cause respiratory problems and contribute to ozone formation, acid rain formation,
nutrient overloading of lakes and streams, regional atmospheric haze, and global warming.
Ozone
Ozone is primarily formed through a chemical reaction involving sunlight, NOx, and volatile
organic compounds (VOCs). Motor vehicles are the largest source of NOx and the second largest
source of VOCs, accounting for 36% of all NOx and 22% of all VOCs emitted nationally.1 Ozone
is linked to a variety of respiratory problems including lung irritation, wheezing, coughing, dif-
ficulty breathing, aggravated asthma, and reduced lung capacity.
Particulate Matter
PM is a term referring to small, respirable particles in solid and liquid form. PM can be com-
posed of any number of materials including dust, organics, metals, and acids. Smaller, fine par-
ticles are more toxic than larger ones because they can be inhaled deeper in the lungs and
absorbed in the bloodstream instead of being exhaled. Buses emit a greater portion of smaller or
fine particles than do motor vehicles. About 50% of PM from cars is considered fine (i.e., less
than or equal to 2.5 micrometers, or "PM2.5"); about 90% of diesel particulate (from buses) is
fine.2 Highway vehicles3 generate about 1% to 3% of all particulate matter.
1 Source: U.S. EPA data for 2006, www.epa.gov/ttn/chief/trends/trends06/nationaltier1upto2006basedon2002finalv2.1.xls as
of October 2007.
2 Based on National Mobile Inventory Model (NMIM) output for Burlington, VT, 2008 model run.
3 Source: U.S. EPA data for 2006, www.epa.gov/ttn/chief/trends/trends06/nationaltier1upto2006basedon2002finalv2.1.xls as
of October 2007.
76
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Appendix 77
Oxides of Sulfur
SO2 is generated primarily by electric utilities. With fuel sulfur standards dropping to 15 ppm,
very little is generated by highway vehicles (about 1%).4 SOx compounds contribute to acid depo-
sition, create atmospheric haze, and can cause respiratory illness, especially in children and the
elderly.
Carbon Monoxide
CO has many side effects because it displaces oxygen in the bloodstream. As a result, it can cause
dizziness, affect the central nervous system, affect people with heart disease, and contribute to
smog. Oxidation catalysts have reduced highway vehicle CO emissions by 66% since 1970 despite
the large increase in VMT over that time period.5 As a result, only three cities in the United States
are in non-attainment for CO.
Lead
Lead damages the kidneys, liver, brain, and nervous system; causes heart disease; and can last
a long time in the environment. Lead was a serious concern when the 1970 Clean Air Act was
established. Consequently, a phase-out of lead in gasoline was initiated. Since that time, overall
lead emissions from motor vehicles have been virtually eliminated.
Air Toxics/Hazardous Air Pollutants
There is another category of pollutants called "air toxics," also referred to by the U.S. EPA as
hazardous air pollutants (HAPs). These are known to or suspected in causing cancer or other
serious health effects such as birth defects, respiratory disease, immune system defects, neuro-
logical problems, and other problems through chronic and/or acute exposure. There are no fed-
eral limits to air toxics concentrations in ambient air; however, many states have adopted their
own standards. According to the U.S. EPA, motor vehicle air toxics are expected to decline by
75% between 1990 and 2020 due to the introduction of reformulated gasoline and tailpipe emis-
sion limits.
Greenhouse Gases
Greenhouse gases are a separate category of pollutants that contribute to global warming. Com-
monly known greenhouse gases are water vapor, carbon dioxide, tropospheric ozone, nitrous
oxide, and methane. Motor vehicles directly emit or contribute to the creation of each of these
greenhouse gases. Water vapor and carbon dioxide emissions are a function of vehicle fuel effi-
ciency, while nitrous oxide and methane are functions of both fuel efficiency and combustion
method (e.g., diesel or spark ignition). For example, buses have seven times the carbon dioxide
emissions of cars/light trucks6, but about 10% of the nitrous oxide and 3% of the methane emis-
sions per mile.7
4 Source: U.S. EPA data for 2006, www.epa.gov/ttn/chief/trends/trends06/nationaltier1upto2006basedon2002finalv2.1.xls as
of October 2007.
5 Source: U.S. EPA data for 2006, www.epa.gov/ttn/chief/trends/trends06/nationaltier1upto2006basedon2002finalv2.1.xls as
of October 2007.
6 2008 NMIM model runs for Burlington, VT.
7 "Update of Methane and Nitrous Oxide Emission Factors for On-Highway Vehicles," U.S. EPA, EPA420-P-04-016, Novem-
ber 2004.
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78 Planning for Offsite Airport Terminals
State Implementation Plans
As noted previously, VOCs and NOx are ozone precursors and also contribute to atmospheric
haze. While there are National Ambient Air Quality Standards (NAAQS) that limit concentra-
tions of criteria pollutants in ambient air, there are also often limits established for NOx and VOC
emissions in certain regions due to non-attainment of ozone standard(s). These limits are set in
State Implementation Plans (SIPs).
SIPs are regulations, adopted by a state and approved by the EPA, that establish a plan to meet
requirements of the Clean Air Act. In particular, for large metropolitan areas, SIPs may contain
emissions budgets, transportation conformity requirements, and limits on industrial sources.
"Transportation conformity" refers to a set of rules that requires the transportation system and
transportation plans to meet the requirements of the SIP. Transit systems and bus retrofits are
often part of a region's transportation plan and are used to meet emissions reduction budgets
specified in SIPs. Emissions reductions that are credited must be quantified using methods and
models approved by the U.S. EPA.
Transit Air Benefits Calculator
The Transit Air Benefits Calculator, a spreadsheet program, was developed by the project team
to estimate the change in various air pollutants and overall air pollution resulting from the air-
port transportation link serving an offsite airport terminal. It calculates emissions of 40 pollu-
tants from cars, pickup trucks, SUVs, and buses for 24 cities between the years 2006 and 2028. It
also aggregates pollutants to assess the net change in overall non-cancer health risk and green-
house gas emissions.
When first started, an opening screen is displayed (see Figure A-1). This shows the user the
project name and the funding source. On the initial screen, the user clicks on the start button to
move to the main calculations page.
Figure A-1. Transit benefits calculator start page.
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Appendix 79
The Main calculations sheet has portions which accept user inputs (shaded in gray) and the
results (Figure A-2). The process for running a scenario is as follows:
1. Select an analysis year--the user can select 2006, 2007, or any other even year from 2008 to
2028. The analysis year is used to take into account reductions in emissions due to retirement
of older, dirtier vehicles in the fleet.
2. Select the nearest city--the calculations of bus and car/light truck emissions are based on
runs from the U.S. EPA's National Mobile Inventory Model (NMIM), which, in turn is based
on the U.S. EPA's Mobile6.2 motor-vehicle emissions model. Different cities and counties
have different emission rates due in part to
· Temperature and humidity,
· Gasoline formulation and vapor pressure,
· Inspection/maintenance programs,
· Anti-tampering programs, and
· Fleet-vehicle-type mix.
The Calculator considers each of these different factors for each city and computes valid
motor vehicle emission rates specific to the county in which the city is located. At present,
Source: Resource Systems Group, Transit Air Benefits Calculator.
Figure A-2. Transit Air Benefits Calculator: main calculations page.
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80 Planning for Offsite Airport Terminals
Source: Resource Systems Group, Transit Air Benefits Calculator.
Figure A-3. Simplified displaced passenger VMT calculator.
24 cities in 23 states are represented in the calculator. Note that two cities in the Calculator
are in California. NMIM is not approved for use in calculating authorized SIP reductions in
California; therefore, results for California should be viewed with caution. California uses the
EMFAC model,8 which is specific to that state.
3. Enter displaced passenger vehicle VMT--the VMT for passenger vehicles displaced by the
airport transportation link is entered in Cell E11. This VMT may be for an hour, week, month,
or year so long as the user uses a consistent unit throughout the worksheet. We recommend
the use of annual VMT since emissions are based on the average of January and July Mobile6.2
emission rates. The VMT savings should be calculated using the methodology described in
Chapter 6 of this report, taking into account all low-occupancy-vehicle modes including taxi,
limousine, private automobile pick-up/drop-off, and private automobile long-term parking.
However, if the data is not available for calculating VMT savings at this level of detail, the
Transit Air Benefits Calculator includes a simplified VMT calculator method that simply con-
siders the number of buses per day, average number of passengers per bus, the automobile
occupancy of the displaced passenger cars, the number of round trips per bus, and the round
trip miles per bus (Figure A-3).
To access the calculator click on the calculator button to the right of the VMT entry cell
(Cell E11). The Total Daily VMT would be equal to
(Average Passengers per Bus)
(Number of Buses per Day) ×
(Automobile Occupancy)
× (Round Trips per Bus) × (Miles per Round Trip).
The Weekly VMT is calculated as follows:
(Weekday VMT) × 5 + (Weekend VMT) × 2.
The Annual VMT is as follows:
(Weekly VMT) × 52.
8 www.arb.ca.gov/msei/onroad/latest_version.htm as of October 2007.
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Appendix 81
Table A-1. Types of transit vehicles and emissions in the calculator.*
Type of Transit Vehicle NOx PM HC** CO CO2
Conventional Diesel (15 ppm Sulfur) 0% 0% 0% 0% 0%
Biodiesel B2 1% 2% 1% 0%
Biodiesel B10 2% 4% 11% 5% 1%
Biodiesel B20 4% 10% 12% 10% 1%
Biodiesel B100 10% 37% 69% 40% 5%
Compressed Natural Gas 50% 95% 50%
Emulsion 15% 20%
Source: U.S. EPA Diesel Emission Quantifier.
*Values shown indicate the percentage reductions in emissions relative to conventional diesel. Negative values
indicate increases in emissions relative to diesel. Missing values are not known and treated as 0%.
**HC = hydrocarbons.
Source: Resource Systems Group, Transit Air Benefits Calculator
Figure A-4. Instructions and list of fuel types.
Passenger vehicle emissions are based on emissions from passenger cars and light-duty gaso-
line trucks, weighted by the actual VMT mix for the chosen city.
4. Enter the transit service parameters--the block of cells in grey under Transit Service Parame-
ters allows the user to input up to four different types of vehicles in the bus fleet. For example,
if the fleet were made up of 75% conventional buses and 25% compressed-natural-gas (CNG)
buses, these two would be entered in separate rows with the VMT allocated proportionally.
a. Enter the type of buses in the fleet--the bus choices available and the associated emissions
reductions are based on the U.S. EPA's Diesel Emission Quantifier.9 The quantifier is an
online calculator for estimating the reduction in a fleet of diesel equipment. The quantifier
should not be used to calculate emissions reductions to be incorporated into a SIP without
first consulting the U.S. EPA Regional Office and SIP guidance documents. The types of
transit vehicles modeled are shown in Table A-1. The numbers after "Biodiesel" indicate
the percentage of biodiesel in the fuel--for example, B20 is 20% biodiesel and 80% con-
ventional diesel. The list can be accessed by clicking on the cell, then clicking on the arrow
to the right of the cell (see Figure A-4). Instructions automatically appear when the cell is
9 cfpub.epa.gov/quantifier/view/index.cfm as of October 2007.
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82 Planning for Offsite Airport Terminals
Table A-2. Emission-control technologies with emissions reductions
in the calculator.
Emission Control Technology NOx PM HC CO CO2
Diesel Oxidation Catalyst 20% 50% 30%
Diesel Oxidation Catalyst + Closed Crankcase Ventilation 25% 40% 30%
Diesel Particulate Filter 85% 90% 90%
Diesel Particulate Filter + Closed Crankcase Ventilation 95% 90% 90%
Diesel Particulate Filter + Lean NOx Catalyst 25% 90% 75% 75%
Diesel Particulate Filter + Hybrid Electric 50% 95% 90%
Diesel Particulate Filter + Exhaust Gas Recirculation 40% 90% 90% 90%
Partial Flow Filter 50% 75% 75%
Recalibration 25%
Selective Catalytic Reduction 70% 40% 70% 90%
Source: U.S. EPA Diesel Emission Quantifier
clicked (left); the list of fuel types are shown for a given selection when the drop-down
arrow to the right of the cell is clicked.
b. Enter the emissions control technology for the bus--as above, the emission-control tech-
nologies are those found in the U.S. EPA's Diesel Emissions Quantifier. The list with their
associated emissions reductions is shown in Table A-2. When the grayed cell is clicked,
instructions pop up, and when the arrow to the right is clicked, the list of emission controls
to select pops up (see Figure A-5). Emission-control technologies instructions appear when
the cell is clicked (left); the list of emission controls are shown for selection when the drop-
down arrow to the right of the cell is clicked (right).
c. Enter the idling-control strategy for the bus--as above, the idling strategies are those
found in the U.S. EPA's Diesel Emissions Quantifier. The list with their associated emis-
sions reductions are shown in Table A-3. If no control strategies are used, the cell can be
left blank. The cell instructions and drop-down list are shown in Figure A-6. Idle Control
Strategies instructions appear when the cell is clicked (left); the list of strategies is shown
for a selection when the drop-down arrow to the right of the cell is clicked (right).
d. Enter the VMT for the bus type--enter the VMT of the particular bus type into Column F
(see Figure A-7 for instructions). A simplified method for calculating the VMT is available
Source: Resource Systems Group, Transit Air Benefits Calculator
Figure A-5. Instructions and emission-control technologies.
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Appendix 83
Table A-3. Idle-control strategies with emissions reductions
in the calculator.
Idling-control Strategies NOx PM HC CO CO2
Direct-fired Heater 98% 99% 95%
Auxiliary Power Unit 94% 81% 73%
Engine Shutdown 100% 100% 100%
Plug-in 95% 100% 100%
Source: U.S. EPA Diesel Emission Quantifier
Source: Resource Systems Group, Transit Air Benefits Calculator
Figure A-6. Instructions and idle-control strategies.
by clicking on the calculator button to the right of the word "VMT." The calculator, shown
in Figure A-8, calculates Annual VMT as follows:
Daily VMT = (Number of Buses per Day)
× (Average Number of Round Trips per Bus)
× (Miles per Round Trip);
Weekly VMT = (Weekday VMT) × 5 + (Weekend VMT) × 2; and
Annual VMT = (Weekly VMT) × 52.
The Bus VMT Calculator estimates annual VMT based on buses per day, round trips
per bus, and miles per round trip.
e. Enter the idling time for the bus type--the idling time of the particular bus types
should be entered in Column F (see Figure A-9 for instructions). However, a simpli-
fied method is also available by clicking on the calculator button in the column header.
The calculator, shown in Figure A-10, calculates idle time as follows:
(Number of Buses per Day) × (Number of Round Trips per Bus)
× (Stops per Round Trip per Bus) × (Minutes per Stop)
+ (Minutes of Idle during Warm-up and Shutdown)
Daily Idle Time in Hours =
(60 minutes/hour);
rs = (Weekday Idle) × 5 + (Weekend Idle) × 2; and
Weekly Idle Hour
Annual Idle Hours = (Weekly Idle) × 52.
The idle-time calculator estimates annual idling hours based on the buses per day, round
trips per bus, number of stops, idle time per stop, and idling at the garage.
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84 Planning for Offsite Airport Terminals
Source: Resource Systems Group, Transit Air Benefits Calculator
Source: Resource Systems Group, Transit Air
Benefits Calculator Figure A-8. The VMT calculator.
Figure A-7. On-screen
instructions for entering
bus VMT.
As inputs are entered, the results are automatically calculated. A screenshot of the results is
shown in Figure A-11. The first two rows of the table show the total bus-operating and bus-idle
emissions in tons for NOx, PM10, VOC, CO, and CO2, with the third row showing the sum of
the first two rows. The next row shows the tons of these pollutants reduced from the displaced
passenger vehicles (cars, pickups, and SUVs). These emissions are based on runs of the U.S.
EPA's NMIM model and are based on all roadway types and a default mix of vehicle speeds. Note
that emissions improvements due to reduced levels of congestion resulting from diversions to
the transit mode aren't taken into account. This can be calculated using other models, but is
likely a secondary effect with less impact on emissions than from VMT reductions.
The net change in emissions is shown next. A negative value indicates a reduction in regional
emissions and, thus, a net improvement in air quality for that pollutant.
Source: Resource Systems Group, Transit Air
Benefits Calculator
Figure A-9. On-screen
Source: Resource Systems Group, Transit Air Benefits Calculator
instructions for entering
bus-idling time. Figure A-10. Idle-time calculator.
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Appendix 85
Net Change in Emissions Back to Main
NOx PM10 VOC CO CO2
Bus running emissions 44.0 0.4 0.4 3.1 7252.8 tons
Bus idling emissions 0.1 0.0 0.0 0.0 20.6 tons
TOTAL Bus Emissions 44.1 0.4 0.4 3.1 7273.4 tons
minus
Passenger Vehicle Emissions Offset 57.4 1.4 65.2 928.5 21399.2 tons
Net Change in Emissions -13.3 -1 -64.8 -925.4 -14125.8 tons
Net change in greenhouse gas emissions equivalent to CO2 emitted by -1,443,587 gallons of gasoline per year
Note: A negative change indicates a net reduction in air emissions.
Source: Resource Systems Group, Transit Air Benefits Calculator
Figure A-11. Main results table in the calculator.
At the present time, the calculator only estimates one greenhouse gas--CO2. The total gaso-
line equivalent shown in Figure A-11 is equal to the net change in CO2 multiplied by 102.195 gal-
lons of gasoline combusted per ton of CO2 emitted.
NMIM calculates emissions for a total of 39 pollutants including criteria pollutants, HAPs, and
greenhouse gases. The change in emissions of all these pollutants is shown in the "Results" tab of
the workbook. The output for the sample problem shown above is illustrated in Table A-4.
Outside of the calculator, the user can use the results and weight the emissions of each pollu-
tant to create a single non-cancer and cancer health impact score. There are several weighting
schemes available.10
10For weighting schemes, see "Human Toxicity Potentials for Life-Cycle Assessment and Toxics Release Inventory Risk Screen-
ing" in Environmental Toxicology and Chemistry by E.G. Hertwich, S.F. Mateles, W.S. Pease, and T.E McKone; SETAC 2001;
20(4): 92839.
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86 Planning for Offsite Airport Terminals
Table A-4. Change in running emissions for all pollutants
calculated by NMIM.
Net change in running emissions for all pollutants
Passenger
Bus Running Vehicle Net Change
Emissions Offset in Emissions
Pollutant (tons) (tons) (tons)
Ethyl benzene 1.12E-02 1.45E-03 9.78E-03
Styrene 1.95E-03 1.52E-03 4.29E-04
1,3 Butadiene 2.87E-03 4.65E-03 -1.78E-03
Acrolein 3.19E-04 2.67E-03 -2.35E-03
Toluene 7.46E-02 2.32E-03 7.23E-02
Hexane 1.25E-02 3.99E-03 8.52E-03
Anthracene 2.78E-06 2.53E-05 -2.26E-05
Propionaldehyde 3.44E-04 4.42E-03 -4.08E-03
Pyrene 4.08E-06 2.67E-05 -2.26E-05
Xylene 4.17E-02 3.48E-03 3.82E-02
Chromium6 1.46E-06 5.28E-07 9.33E-07
Benzo(g,h,i)perylene 8.41E-07 6.16E-06 -5.32E-06
Indeno(1,2,3,c,d)pyrene 2.52E-07 6.85E-07 -4.33E-07
Benzo(b)fluoranthene 4.00E-07 7.53E-06 -7.13E-06
Fluoranthene 2.99E-06 1.51E-05 -1.21E-05
Benzo(k)fluoranthene 4.00E-07 7.53E-06 -7.13E-06
Acenaphthylene 1.35E-05 2.53E-05 -1.18E-05
Chrysene 3.36E-07 4.79E-06 -4.46E-06
Formaldehyde 5.56E-03 5.96E-02 -5.40E-02
Benzo(a)pyrene 3.36E-07 8.90E-06 -8.57E-06
Dibenzo(a,h)anthracene 0.00E+00 0.00E+00 0.00E+00
2,2,4-Trimehtylpentane 2.61E-02 4.78E-04 2.57E-02
Benz(a)anthracene 3.36E-07 2.74E-05 -2.71E-05
Benzene 2.63E-02 8.00E-03 1.83E-02
Manganese 1.23E-06 8.23E-07 4.02E-07
Nickel 2.66E-06 2.64E-06 1.67E-08
Chromium(Cr3+) 2.19E-06 7.91E-07 1.40E-06
Acetaldehyde 2.10E-03 2.19E-02 -1.98E-02
Acenaphthene 2.40E-06 1.64E-05 -1.40E-05
Phenanthrene 8.33E-06 3.83E-05 -3.00E-05
Fluorene 4.96E-06 3.36E-05 -2.86E-05
Naphthalene 4.43E-04 9.59E-04 -5.17E-04
CO 11.887 3.606 8.281
CO2 273.969 1914.908 -1640.939
Nox 0.735 12.254 -11.519
PM10 0.019 0.769 -0.750
PM2.5 0.009 0.694 -0.686
SO2 0.005 0.200 -0.195
VOC 0.835 0.725 0.110
Note: A negative change indicates a net reduction in air emissions.
