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FHWA Resource Center
AIR QUALITY TEAM
Publications
Tianjia Tang, Ph.D.,
PE Corresponding author.
Contact Information:
Tianjia Tang
10 South Howard Street, Suite 4000, FHWA, Resource Center,
Baltimore, MD 21201
Phone: 410-962-2177
Tianjia.Tang@FHWA.DOT.GOV
Michael Claggett, Ph.D., Joon Byun, Ph.D., Mike Roberts,
Jessica Granell, Ph.D. are all Federal Highway Administration personnel.
Dale Aspy is from US EPA Region 4, Atlanta Georgia
ABSTRACT
The newly released
MOBILE6.2 has incorporated both air toxic and particulate matter emission
factor modeling functions. A series of test runs were performed to gain
a better understanding of the air toxic modeling function and overall
model behavior. These test runs/scenarios evaluated the changes in emission
factors of all six built in air toxic compounds as affected by vehicle
activities, fuel physical properties, fuel chemical compositions, oxygenated
fuel additives, and environmental conditions. Based on results obtained,
both exhaust and evaporative emission factors for acrolein, acetaldehyde,
benzene, 1,3 butadiene, formaldehyde, methyl tertiary butyl ether are
inversely proportional to both freeway and arterial vehicle speeds. This
phenomenon follows the trend of total organic gas emission factors. Effects
from roadway facility differences indicated that the higher the percentage
of vehicle miles traveled on a freeway, the lower the air toxic emission
factors on a per vehicle mile traveled basis. Both exhaust and evaporative
air toxic emission factors increase when fuel RVP value and sulfur content
increase. It is interesting to note that diesel sulfur content has no
effect on the six toxic compound emission factors. Effects from fuel chemical
compositions on all emission factors varied. However, chemical compositions
do have significant effects on all air toxic compound emission factors.
On the same note, both min/max temperatures and humidity affect all air
toxic emissions significantly. The time series evaluation shows that all
six tested air toxic compound emissions decrease linearly from year 2002
to 2020.
INTRODUCTION
Since the official
release of the MOBILE6.0-motor vehicle emission factor modeling program
by the US EPA on January 29, 2002, two updated versions known as MOBILE6.2.01
and MOBILE6.2.03 have been released. MOBILE6.2 has functions of estimating
both air toxic and particulate emission factors in addition to all functions
contained in MOBILE6.0. According to EPA's technical guidance, MOBILE6.2's
air toxic function is a consolidation of an earlier hazardous air pollutant
estimation model known as MOBTOX to MOBILE6.0. With the robust capability
of the MOBILE6 model, it appears that this new function can evaluate effects
of more specific vehicle activities, environmental conditions, fuel properties,
and other parameters on air toxic emissions. The objective of this analysis
is to explore the effects of various parameters and conditions on emission
factors of compounds built into the air toxic module of the MOBILE6.2
model. These six built-in air toxic compounds are acrolein, acetaldehyde,
benzene, 1,3 butadiene, formaldehyde, and MTBE. Evaluation of diesel particulate
matter (DPM) emission behavior as affected by various vehicle and environmental
conditions through MOBILE6.2 is reported separately from this study.
METHODOLOGY
For all modeling
runs, optional data (not associated with required MOBILE6 Commands) other
than those being tested were based on MOBILE6 national defaults. For all
scenarios, the modeling year (calendar year) was 2005. Fuel Reid Vapor
Pressure (RVP) used for all runs except in RVP testing was 8.5 pounds
per square inch (psi). Min/Max temperatures were 88.0 degrees Fahrenheit
and 100.0 degrees Fahrenheit except in the case of min/max temperature
testing. Other conditions as listed below were also applied to all scenarios
when appropriate. Volume based gas aromatic, olefin, and benzene contents
(v/v) were 15.0% (v/v), 15.0% (v/v), and 1.5% (v/v), respectively; E200
was 50.0% (v/v) while E300 was100.0% (v/v). Oxygenated fuel consisted
of 8.0% (v/v) MTBE with a 1.0% market share, 10% (v/v) ethyl tertiary
butyl ether (ETBE) with a 1.0% market share, 5.0% ethanol (ETOH) (v/v)
with a 1.0% market share, and 1.0% tertiary amyl methyl ether (TAME) (v/v)
with a 1.0% market share.
Emission factor data
for all the six air toxic compounds were plotted on a single graph for
a given tested parameter in order to conduct trend analysis. Raw data
for one or more emission factors on the graph were scaled either up or
down. For example, benzene data in a given graph were scaled up by multiplying
all data points by three. In this case, the legend for benzene in the
graph would be benzene*3. The true emission factor for benzene in this
graph is equal to the graph value divided by three. By conducting the
above scale up or down operations, trend deviation and pattern development
are more decipherable. Units for all air toxic compounds are milligram
per vehicle miles traveled (mg/mi). The unit for Total Organic Gas (TOG)
emission factor is gram per mile (g/mi).
Tested parameters
are categorized into the following groups: 1) vehicle activities such
as vehicle speed and VMT ratio, 2) fuel physical properties such as Reid
Vapor Pressure (RVP), 3) fuel chemical compositions such as benzene content,
aromatic content, olefin content, sulfur content, E200, and E300 contents,
4) other chemical additives used in oxygenated fuel, 5) environmental
factors such as min/max temperatures, and 6) time series data for calendar
years between 2002 and 2020.
RESULTS AND DISCUSSION
Vehicle Activity
Vehicle Speed
The "AVERAGE SPEED" command was used to evaluate both arterial
and freeway mainline speed effects. Speeds ranging from 2.5 to 65.0 miles
per hour (mph) with a 2.5 mph increment were tested.
It is clear from
emission data that emission factors for all six air toxic compounds are
following the trend of TOG emission. Between 2.5 and 22.5 mph, the relationship
can be approximated by an exponential function. Emission factors decrease
rapidly as vehicle speeds approach 22.5 mph. Between 22.5 and 65.0 mph,
all exhaust air toxic emission factors decrease in a flat linear fashion.
The higher the speed, the lower the exhaust emission factor.
Unlike the exhaust
emission trend, the initial rapid decrease of evaporative emission stops
at 5.0 mph. The second rapid decrease occurs between 5.0 and 12.5 mph.
Between 12.5 and 65.0 mph, the relationship between evaporative emission
and vehicle speed can be described through a linear model. Again, the
higher the speed becomes, the lower the emission factor.
The relationship between exhaust emission factor and arterial roadway
is essentially a duplicate of the emission factor/freeway facility correlation.
It clearly shows that roadway facility speed is the major factor leading
to exhaust air toxic emission factor differences.
VMT Ratio
Effects of vehicle miles traveled (VMT) ratios between different roadway
facilities were evaluated by using the command of "VMT BY FACILITY"
and appropriate external VMT files. Tested VMT ratios between freeway
mainline/arterial and arterial/local were from 0.0% to 100.0%. The objective
of this test is to understand how critical it is to accurately allocate
VMT among all roadway types during transportation demand modeling and
forecasting.
Relationships between
VMT ratios and emission factors are linear for all exhaust emissions.
The higher the percentage of the VMT on a local road, the higher the exhaust
air toxic emission factor. Effects of VMT ratio between a freeway mainline
and an arterial facility on air toxic emissions are similar in trend but
less pronounced in magnitude as compared with the arterial/local VMT ratio
effects. Vehicles traveling a freeway produce lower air toxic emission
factors than vehicles traveling an arterial facility on a per mile traveled
basis.
Evaporative benzene
and MTBE emission factors are also linearly related to VMT ratios for
both cases.
Fuel Physical Property
Fuel Reid Vapor Pressure
Fuel Reid Vapor Pressure (RVP) is one of the few required inputs to run
MOBILE6. While it is known that a high RVP fuel produces high amounts
of evaporative and exhaust VOC emissions, it is not clear how RVP affects
air toxic emissions. The objective for the RVP test is to understand how
air toxic emissions are influenced by fuel RVP changes. Effects from fuel
RVP ranging from 6.5 to 15.5 psi were evaluated.
As fuel RVP increases, TOG emission factors increase with different rates
depending on the range of RVP data. Unlike the TOG emission trend, the
air toxic compound exhaust emissions exhibit three distinctively different
trends. Between 6.5 and 8.5 psi, air toxic compound emission factors decease
in a linear fashion as fuel RVP increases. Between 8.5 and 12.5 psi, the
air toxic compound emission factors increase with increasing fuel RVP
value. Once the fuel RVP value exceeds 12.5 psi, benzene, formaldehyde,
and 1,3 butadiene emission factors start to decrease again as fuel RVP
increases. The acetaldehyde, acrolein, and MTBE emission factors remain
unaffected while the TOG emission factor increases at a lower factor.
As fuel RVP increases,
both benzene and MTBE emission factors increase although MTBE does not
increase as rapidly as benzene
Fuel Chemical Composition
Gasoline Sulfur Effects
The MOBILE6 command "FUEL PROGRAM" was used to evaluate effects
of gasoline sulfur (ranging from 32 to 92 ppm) on exhaust air toxic emission
factors.
Based on results obtained, all exhaust toxic air compound emission factors
increase linearly as gasoline sulfur increases. It appears that all air
toxic compound emissions follow the TOG emission trend.
Gasoline sulfur content
has no effect on evaporative air toxic compound emissions.
Diesel Sulfur Effects
Diesel sulfur content has no effect on emissions of all six air toxic
compound emissions tested here.
E200 Effects
E200 is a gasoline volatility measurement. It is the volume percentage
of a given amount of gasoline that evaporates at 200 degrees Fahrenheit
under one atmospheric pressure. The range of E200 tested was between 30.0%
and 70.0%
According to test
results, exhaust TOG, MTBE and acrolein emissions are not affected by
E200 contents. Exhaust benzene emissions increase slightly when E200 increases
from 35.0% to 45.0%. Between 45.0% and 70.0% E200 contents, exhaust benzene
emissions decrease rapidly as E200 content increases. Emission factors
for both formaldehyde and acetaldehyde increase as the E200 content increases.
One of the most dramatic
phenomena is the decreasing of 1, 3-butadiene emission as the content
of E200 increases.
E200 content has
no effects on evaporative benzene and MTBE emissions.
E300 Effects
E300 is another measurement of gasoline volatility. It is the volume percentage
of a given amount of gasoline evaporated at 300 degrees Fahrenheit under
one atmospheric pressure. The range of E300 tested was from 70.0% to 100.0%.
Exhaust benzene
emission increases rapidly with increasing E300 content. This observation
is just the opposite of the E200 result.
Formaldehyde, acetaldehyde and 1,3 butadiene exhaust emissions increase
as E300 increases from 70.0% to 80.0%. Between 80.0% and 100.0%, the emission
factors for these three compounds decrease as E300 increases.
Acrolein, MTBE and
TOG emission factors are not affected by E300 contents. This observation
is consistent with the E200 data.
Evaporative emission
factors for both MTBE and benzene are not affected by E300 changes. This
result is the same as the E200 results.
Aromatic Effects
Aromatic compounds are any compounds that possess the six-carbon ring
(benzene) structure. Effects of aromatic content on air toxic compound
production were evaluated by using the command of "Gas Aromatic%."
Aromatic contents ranging from 10.0% to 55.0% were evaluated while benzene
content was fixed at 1.5%.
Exhaust emission
factors for TOG, acrolein, and MTBE are not affected by the contents of
aromatic compounds. However, exhaust benzene emission factor increase
rapidly in a virtually linear mode as the aromatic content increases.
While exhaust benzene
emission factors increase with increasing aromatic contents, butadiene,
formaldehyde, and acetaldehyde emissions decrease in a linear fashion.
According to test
result, evaporative emission factors for both benzene and MTBE are not
affected by the amounts of aromatic compounds in fuel.
Benzene Effects
While benzene is one of many compounds referred to as aromatic, it is
the only reported air toxic chemical among all aromatic compounds. Effects
of benzene ranging from 0.0% to 5.0% were analyzed.
As benzene content
increases from 0.0% to 5.0%, exhaust benzene emissions increase linearly.
This same linear response is also observed with the evaporative benzene
emission factors.
No other air toxic
compound emission factors are affected by benzene content.
Olefin Effects
Olefin refers to a class of compounds containing either double or triple
bonds. Effects of olefin content ranging from 0.0% to 30.0% on the production
of air toxic compounds were analyzed.
While 1,3 butadiene
emission factor increases rapidly with increasing olefin content, acetaldehyde
and benzene emission factors increases are less obvious. On the other
hand, formaldehyde emission factor decreases as olefin content increases.
Both MTBE and benzene
evaporative emissions are not affected by olefin content.
Oxygenated Fuel Effects
To evaluate effects from fuel additives used in oxygenated fuel blending,
the MOBILE6 command "OXYGENATE" was used. For each additive,
a 70.0% market share was used.
Ethanol Effects
The ethanol contents ranging from 0.0% to 10.0% volume (v/v) were analyzed
for their effects on air toxic emissions.
Exhaust emission
factors for all air toxic compounds except acetaldehyde decrease as ethanol
content increases. This decreasing trend is also true with TOG emissions.
This phenomenon may be contributed to the direct oxidation of ethanol
to acetaldehyde during the combustion process.
According to test
result, as more ethanol is blended in the fuel, evaporative emissions
of both benzene and MTBE decrease. This phenomenon is especially true
for evaporative benzene emissions.
MTBE Effects
The blending volume percentages for MTBE ranging from 0.0% to 14.5% were
used in analyzing MTBE effects on air toxic emission. Exhaust emission
factors for all toxic compounds except formaldehyde decrease linearly
as MTBE content increases.
Evaporative benzene
emission factors decrease as MTBE content increases.
ETBE Effects
The blending volume percentages for ETBE ranging from 0.0% to 16.0% were
used to evaluate ETBE effects on air toxic emission. Similar patterns
as ethanol affecting air toxic emissions are observed. As ETBE content
increases, all air toxic emissions except acetaldehyde decrease. This
is a very interesting discovery since ETBE belongs to ether family and
not the alcohol.
For both evaporative
and exhaust emissions, similar patterns as EOTH affecting air toxic emissions
are observed.
TAME Effects
The blending volume percentages for TAME ranging from 0.0% to 18.0% were
analyzed for their impacts on air toxic emissions. While effects from
ETBE resemble the ethanol effect, effects from TAME on all air toxic emissions
resemble the MTBE impact. Formaldehyde emission factors increase in a
linear fashion as TAME content increases while all other air toxic emission
factors decrease.
Environmental Factors
Effects from environmental factors including minimum/maximum daily temperatures
and humidity were evaluated for their impacts on air toxic emissions.
While testing minimum temperature effects, maximum daily temperature was
held at 70 degrees Fahrenheit. The minimum daily temperature was held
at 80 degrees Fahrenheit, while daily maximum temperature effects were
evaluated.
All exhaust air toxic
compounds and TOG emissions increase as daily minimum temperature decreases.
All emission factors are virtually parallel to each other. As the daily
maximum temperature increases, all exhaust emissions increase. However,
all air toxic compound emissions are at a much lower rate than the TOG
emission factor.
On other hand, evaporative
emissions of both benzene and MTBE decrease as daily minimum temperature
decreases. Evaporative emissions of both benzene and MTBE increase as
daily maximum temperature increases.
Humidity impacts
on air toxic emissions showed that as humidity increases, all exhaust
emissions including all air toxic compounds, MTBE and TOG increase at
a similar factor. Humidity has no effect on evaporative emissions.
Time Series Analysis
By using default MOBILE6 data, a trend evaluation was carried out for
calendar years between 2002 and 2020.
Exhaust emissions
for all air toxic compounds and TOG are declining as time moves forward.
This declining trend is essentially parallel to the TOG trend.
Evaporative emission
trends for both benzene and MTBE are also declining as time moves forward.
However, evaporative benzene emission factors decline at a much faster
rate than MTBE.
SUMMARY
Effects of various
parameters including vehicle, roadway facility, environment, and time
on six different air toxic compound emissions and TOG were evaluated.
While some parameters affected certain air toxic compounds significantly,
others appeared to have no impacts.
ACKNOWLEDGEMENTS
The authors would
like to thank Robert Kafalenos and Kevin N. Black, Federal Highway Administration-
Air Quality, Washington, DC for their helpful review and comments.
REFERENCES
Technical Description
of the Toxics Module for MOBILE6.2 and Guidance on Its use for Emission
Inventory Preparation. United States Environmental protection Agency,
EPA420-R-02-029, November 2002.
User's Guide
to MOBILE6.1 and MOBILE6.2: Mobile Source Emission factor Model. United
states Environmental protection Agency, EPA420-R-02-028, October 2002.
User's Guide to
MOBILE6.1 and MOBILE6.2: Mobile Source Emission factor Model. United states
Environmental protection Agency, EPA420-R-03-010, August 2003.
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