alysis of
Tianjia Tang, Mike Roberts, and Cecilia Ho
INTRODUCTION
INTRODUCTION
O
n January 29, 2002, the US Environmental
Protection Agency (EPA) officially released the
latest motor vehicle emission factor model -
MOBILE6. This release represents significant
achievements in understanding both motor
vehicle performance and driver behavior when
estimating motor vehicle emissions. The model
also establishes routines to compute and analyze
fuel and vehicle certification standards, state
programs and different highway facilities as
related to vehicle emission factors. Significant
efforts by US EPA were also carried out
to establish a national default database used by
the computer model. Along with the MOBILE6
release, more than 48 technical papers related
to MOBILE6 development were also released.
Overall, MOBILE6 model predicts higher
emission rates in near future years and
lower emission rates in out years when
compared to MOBILE5 series models.
MOBILE6 is the approved US EPA motor
vehicle emission factor model for estimating
volatile organic compounds (VOC), nitrogen oxides
(NOx), and carbon monoxide (CO) from different
vehicles. State and local air quality and trans-
portation agencies outside of California are
required to use MOBILE6 in State Implementation
Plan (SIP) development, and transportation
conformity determination. The official release of
MOBILE6 on January 29, 2002 started the 2-year
grace period before MOBILE6 is required for
new conformity determinations in most areas.
As the end of the grace period approaches,
transportation as well as air quality agencies
are gaining an understanding of the model
behavior, especially in impacts of using localized
data as compared to EPA's national default data.
US Department
of Transportation
Federal Highway
Administration
Understanding the behavior of the model under various
conditions becomes more critical as the tasks of
collecting local data is time and resources intensive. The
purpose of this paper is to provide a basic evaluation of
the MOBILE6 model behavior under various conditions.
Through the understanding of the model's behavior, state
and local agencies can prioritize costly data collecting
efforts and ultimately initiate emission control strategies
according to parameter sensitivity.
METHODOLOGY
The MOBILE6 model utilized in this sensitivity
analysis was the official version released by the EPA on
January 29, 2002. All modeling runs were conducted on a
Gateway X86-based PC with a GenuineIntel 600 Mhz
microprocessor under the Windows 2000 Professional
operating system.
During the sensitivity analysis, input parameters
other than those being tested, were based on EPA national
default data. For all scenarios, the calendar year used for
modeling was 2005. For both VOC and NOx emission
factor modeling, the evaluation month was July. For CO
emission factor modeling, the analysis month was January.
Temperatures used in all NOx and VOC analyses, unless
otherwise noted, were 72.0
o
F and 98.0
o
F for the daily
minimum and maximum. For analysis of CO emission, the
daily minimum and maximum temperatures used were
20.0
o
F and 60.0
o
F. Fuel Reid Vapor Pressure (RVP) was
8.5 psi throughout all runs except for fuel RVP testing.
Emission factors obtained and cited in this analysis are
composite vehicle emission factors (VOC: start + running+
evaporative; NOx and CO: start + running) unless
otherwise noted.
The least significant digit of the MOBILE6
emission factor output is the thousandth digit. When the
emission factor is multiplied by vehicle miles traveled,
which is typically in millions, the thousandth digit of the
emission factor becomes very significant. In this
sensitivity analysis, when an emission factor experiences a
change in the least significant digit (the thousandth digit)
as a result of one unit change in the parameter being MOBILE6
tested, the term "sensitive" is used to describe the effect of
the parameter on the emission factor.
PARAMETERS EVALUATED
Ramp Vehicle Miles Traveled
Since freeway ramps are directly associated with
freeway mainlines, two different cases were created here
for evaluation. The first case simulates a freeway mainline
with an average speed of 55 miles per hour (mph). The
second case simulates a freeway mainline with an average
speed of 30 mph. A total percentage of vehicle miles
traveled (VMT) between a freeway mainline and ramp is
100% for both cases. The effects of ramp vehicle miles
traveled on vehicle emission rates are evaluated through
the average speed command.
As shown in Fig. 1, the VOC emission rate is very
sensitive to the ramp VMT percentage in the 55 mph main-
line average speed case. The VOC emission rate is not sen-
sitive to the ramp VMT percentage at the 30 mph mainline
average speed. The average increase of VOC emission rate
is 0.0028 g/mi per 1% ramp VMT percentage increase at
the 55 mph mainline average speed case.
NOx emission rates are inversely related to ramp
VMT percentages. In the case of 55 mph mainline freeway
average speed, the ramp VMT reduces the NOx emission
rate at an average of 0.0036 g/mi per one percent of ramp
VMT percentage increase. For the 30 mph case, NOx emis-
sion rates are not sensitive to ramp VMT percentages (Fig. 2).
CO emission rates are exceedingly sensitive to
ramp VMT percentages in both cases. The average
increases in CO emission rates are 0.0489 g/mi and 0.0731
g/mi per 1% increase of ramp VMT percentage with a
mainline freeway speed of 55 mph and 30 mph (Fig. 3).
A highway ramp speed of 34.6 mph is coded in
MOBILE6 by US EPA as the default. Other than driving
cycle differences between a freeway mainline and ramp,
the facility speed is the dominant factor in explaining
effects of ramp VMT percentage on all emission rates.
Vehicle Miles Traveled
By Facility
Two different cases were evaluated here for
vehicle miles traveled among freeway, arterial and local
facilities. The first case is to assign 100% VMT between a
mainline and an arterial/collector. The second case is to
assign 100% VMT between an arterial and a local facility.
The objective is to evaluate effects from roadway facility
types on vehicle emission rates.
As indicated in Fig. 4, as the VMT ratio between a
freeway mainline and arterial changes from 20/80 to 80/20,
changes in VOC emission rates are minimal. The average
2 MOBILE6
sensitive to changes in the VMT ratio. For every 1%
increase in local facility VMT ratio, emission rates
decrease 0.0069 g/mi for VOC and increase 0.0015 g/mi
for CO. NOx emission rates are not sensitive to VMT ratio
between arterial and local facilities. The average decrease
in NOx emission is less than 0.0008g/mi per 1% increase
in local VMT ratio.
Based on driving cycles used for freeway, arterial
and local roadway facilities in the MOBILE6 model, one
of the most significant differences among all driving
characteristics is speed. The effects of VMT allocation
among roadway types on emission rates can be attributed
principally to speed differences.
Roadway Facility Speed
The average speed command was used to test
roadway speeds in two different VMT cases. The first case
assigns 100% VMT to a freeway mainline. The second
case assigns 100% VMT to an arterial/collector roadway.
As indicated in Figs. 10 (a & b), 11 (a & b), and
12 (a & b), emission rates for all three pollutants are
exceedingly sensitive to speed changes. As speed increas-
es, VOC emission rate decreases rapidly in a linear fashion
within the range of 2.5 mph and 7.5 mph. Between 10.0
mph and 65.0 mph, VOC emission rates decrease in a
pseudo-linear trend. When speed reaches 30 mph and
higher, VOC emission rates are the same for freeway
mainline and arterial facilities.
3
decrease in VOC emission rate is less than 0.00026 g/mi
per 1% increase in the freeway VMT percentage . It is
concluded that VOC emission rates are not sensitive
toVMT ratios between a freeway mainline and an arterial.
NOx emission rates that are affected by the VMT
ratio between a freeway mainline and an arterial is shown
in Fig. 5. Unlike VOC emission rates, NOx emission rates
are sensitive to VMT ratio. As the freeway mainline/
arterial VMT ratio increases, NOx emission rates
increase dramatically, with an average increase of 0.0043
g/mi per 1% increase of VMT ratio.
The CO emission rate, as affected by the VMT
ratio, exhibits the same trend as the NOx. The average
increase in the CO emission rate is 0.0228 g/mi per 1%
increase in the VMT ratio (Fig. 6).
Figs. 7, 8, and 9 depict the effect of arterial and
local facility VMT ratio on VOC, NOx, and CO emission
rates, respectively. Both VOC and CO emission rates are MOBILE6
4
NOx and CO emission rates exhibit a third order
polynomial relationship with facility speeds. Between 10
mph and 35 mph, the NOx emission rate decreases as speed
increases. Within the 35 mph to 65 mph range, the NOx
emission rate increases as speed increases. CO emission
rates decrease as speed increases in the range of 2.5 to 25
mph and increase in the range of 25 to 65 mph. Within the
Humidity
The absolute humidity used in the testing ranged
from 20 grains/lb to 118.0 grains/lb. At a temperature of
75.0
o
F, the 118.0 grains/lb represents 100% relative
humidity. As indicated in Fig. 13, as humidity increases,
VOC emission rates increase. However, the emission rate
range of 2.5 mph to 10 mph (Figs. 10b and 12b), both CO
and VOC emission rates increase dramatically as the
facility speed drops to