United States Air and Radiation EPA420-P-98-019
Environmental Protection March 1999
Agency M6.FLT.006
&EPA Soak Length Activity
Factors for Diurnal
Emissions
DRAFT
> Printed on Recycled Paper
-------
EPA420-P-98-019
March 1999
for
M6.FLT.006
Assessment and Modeling Division
Office of Mobile Sources
U.S. Environmental Protection Agency
NOTICE
This technical report does not necessarily represent final EPA decisions or positions.
It is intended to present technical analysis of issues using data which are currently available.
The purpose in the release of such reports is to facilitate the exchange of
technical information and to inform the public of technical developments which
may form the basis for a final EPA decision, position, or regulatory action.
-------
- Draft -
Soak Length Activity Factors for Diurnal Emissions
Report Number M6.FLT.006
Last Revised on March 1, 1999
Edward L. Glover
U.S.EPA Assessment and Modeling Division
m6flt006.wpd DRAFT 1 Mar 1, 1999
-------
1.0 INTRODUCTION
MOBILE6 will compute and report (as a user option) hourly emission factors for
start, running, and evaporative emissions. These will be in addition to the standard daily
emission estimates which are currently calculated by MOBILES. The hourly emission
factors will allow the MOBILE6 model to provide more precise output that accounts for the
time of day that vehicle emissions occur. The temporal distribution of emissions is an
important factor in the formation of diurnal evaporative emissions.
The hourly emission estimates require considerable vehicle activity information and
analysis. The term "activity" refers to the vehicle's operating mode such as running, idling,
parked (soaking), etc. The specific activity information includes soak durations, time of
soak, trip lengths, time of trip, and other information. This document (M6.FLT.006)
discusses the issue of vehicle soak time as it pertains to diurnal emissions. Other activity
estimates needed to develop daily emission factors for hot soak, running loss, start and
running exhaust emissions, and resting loss emissions will be documented in other
MOBILE6 documents listed with the report numbers "M6.FLT.XXX". Also, for general
information on evaporative emissions and their modeling in the MOBILE6 model, the
reader is encouraged to review "M6.EVP.001" and "M6.EVP.002".
2.0 DATA SOURCES USED
The primary data source for this analysis is an EPA instrumented vehicle study
conducted in Baltimore, Spokane and Atlanta. In these studies, instrumentation to monitor
vehicle usage was installed with the motorists' permission on 168 randomly selected
vehicles while they were tested at an Inspection / Maintenance (I/M) station. The motorists
returned one or two weeks later to have the instrumentation removed. Information from
more than 8,500 vehicle-trips was recorded. The raw data collected from the study were
processed by the Radian Corporation under EPA contract to create a "trip characteristics"
file. This processed file was used to develop the hourly soak time distributions. For more
details regarding the instrumented vehicle study and the data processing, please refer to the
document "Travel Trip Characteristics Analysis" Final Report under EPA Contract 68-C1 -
0079 WA 2-05 with Sierra Research.
3.0 METHODOLOGY FOR DIURNAL EMISSION ACTIVITY
This section describes the basic methodology to develop the trip length activity
estimates used to calculate diurnal emissions. The process consisted of several steps.
These are discussed below.
m6flt006.wpd DRAFT 2 Mar 1, 1999
-------
3.1 Definitions
3.1.1 Diurnal Emissions
Diurnal emissions are evaporative emissions which may escape from a non-running
vehicle subsequent to its hot soak, and while it is exposed to a thermal gradient cycle of
sufficient magnitude. Emissions are produced primarily during and immediately following
the portion of the cycle when temperatures rise. Diurnal emissions are defined as excluding
emissions that occur during vehicle running, starting, or hot soak operation. They also
generally occur over a period of time of several hours, and must be apportioned to the
various hourly groups.
For purposes of the MOBILE6 model, three types of diurnals can occur. The first
type is the multi-day diurnal. This type occurs if a vehicle is operated, and then "soaks" (is
parked) for two or more days, and experiences two or more cycles of sufficiently large
thermal gradients during the multi-day soak period to raise fuel tank temperatures past a
threshold value. The second type is the full or one-day diurnal. This type of diurnal needs
to begin prior to the beginning of the temperature rise (i.e., prior to 6AM), and can last for
up to 24 hours. The third type is the interrupted diurnal. This type is similar to the
previous ones, except that the soak periods range from a minimum of one hour up to 24
hours, and they start later in the day (i.e., the vehicle is operated during the morning so that
the early morning heat build (begins at 6 AM) is interrupted). The diurnals which range
from 25 hours to 48 hours are a combination of a one day diurnal and an interrupted diurnal
or multi-day depending on when they start.
Depending on the length of the soak, the full diurnal and the interrupted diurnal may
represent only a fraction of the thermal cycle. Also, in terms of activity parameters, the
MOBILE6 model diurnal activity parameters will reflect vehicles which experienced more
than one interrupted diurnal during the day. Their contribution is a function of the length
and timing of vehicle trips during the day. For example, a vehicle that makes a quick trip
from 10:00 to 10:20 AM, and is parked until sundown will be reflected in the hourly
activity parameters as part of a full diurnal (from 6 AM to 10 AM), and an interrupted
diurnal from 12:20 AM until sundown. Duringthe two hour period between 10:20 AM and
12:20 AM the vehicle is in hot soak mode.
3.1.2 Hourly Diurnal Activities
For this analysis of vehicle activity, hourly diurnal activity estimates have a
different basis than hourly running emission activity estimates. Where the hourly running
emissions are based on the number and/or length of trips in the given hour, the hourly
diurnal activity estimates are based on whether a multi-day, full day, or interrupted diurnal
occurred during that hourly group. For example, a vehicle can have several trips during a
m6flt006.wpd DRAFT 3 Mar 1, 1999
-------
given hourly group interval; however, a vehicle is either experiencing a diurnal during a
given hourly group interval, or not experiencing it during the given hourly interval. In other
words the analysis assumed that each vehicle in the database could have either one or no
diurnals during a given hour on a given day.
3.2 Hourly Intervals
The 24 hour day was divided into 14 different hourly groups. Thirteen of these
groups have a duration of one hour. These start at 6:OOAM and run through 6:59:59PM.
The fourteenth group contains the remaining nighttime hours as one interval. Collapsing
these hours into one group was done for three reasons: (1) the emissions contributed during
the night have a relatively smaller impact on daily ozone than those contributed during the
morning or day, (2) there were relatively little data for these time periods, and (3) what data
were available produced results which showed very little hour to hour variance. The hourly
intervals are shown in Table 1. In addition to its use for diurnal activity estimates, the same
hourly group structure is used in the calculation of activity estimates for start emissions,
running emissions, hot soak emissions, resting loss emissions, and running loss emissions.
m6flt006.wpd DRAFT 4 Mar 1, 1999
-------
Table 1
Hourly Ranges
Group Name
6
7
8
9
10
11
12
13
14
15
16
17
18
24
Hourly Range
6-7
7-8
8-9
9- 10
10- 11
11 - 12
12- 13
13- 14
14- 15
15- 16
16- 17
17- 18
18- 19
19 -24 and 24 -5
Time
6 am to 7 am
7 am to 8 am
8 am to 9 am
9 am to 10 am
10 am to 11 am
1 1 am to noon
noon to 1 pm
1 pm to 2 pm
2 pm to 3 pm
3 pm to 4 pm
4 pm to 5 pm
5 pm to 6 pm
6 pm to 7 pm
7 pm to 6 am
3.3 Identifying the Hourly Diurnals in the Database
The instrumented vehicle database consists of 'trip' type data. This means that the
initial vehicle start time and stopping times were recorded while the vehicles were
operating. The database was based on more than 8,500 vehicle trips, and the corresponding
number of prior and subsequent soaks. For this analysis, the individual trip data were not
used directly since the activity information was needed on an hourly basis rather than on
a per trip basis. In essence, it was necessary to know for a given hourly group interval the
number of vehicles which were experiencing a diurnal, and the number of vehicles which
were in running mode or in hot soak mode.
Examination of the roughly 8,500 vehicle trips showed 168 vehicles, and 1,425
vehicle-days of data in the sample. The concept of vehicle-days was used instead of trips
m6flt006.wpd DRAFT
Mar 1, 1999
-------
because diurnals can occur over several hours, and each vehicle can experience either one
or zero diurnals in a given hour of a day. For example, at 6 to 7 AM, a vehicle is either
experiencing a diurnal or running / hot soaking (not experiencing a diurnal). The 1,425
vehicle-days were reduced down to 1,257 valid vehicle-days by omitting each vehicle's first
day of operation in the program. The omission of each vehicle's first test day was
necessary because the length of the soak prior to the installation of the instrumentation was
not known. For example, if vehicle #10 were operated for 14 days in the program, it would
account for 13 valid vehicle-days.
Creating the 13 hourly groups (nighttime group 14 was omitted) for diurnal
activities was necessary because diurnal emissions are a function of time of day and
duration of soak. For example, on a day with a typical temperature cycle, the diurnal
emission effect on the 11 to 12 AM interval after an eight hour soak will likely be larger
than the diurnal emissions on the 6 to 7 AM interval after an eight hour soak.
Once the database was organized into 1,257 valid vehicle-days, it was necessary to
determine for each of the 13 hourly groups whether a diurnal had occurred on each of the
1,257 valid vehicle-days, and the soak duration prior to the beginning of the hourly group.
By definition, a diurnal occurred if the vehicle was NOT operating, or had NOT operated
in the previous two hours. In practice this will be slightly more than two hours because the
data could not be analyzed on a basis any finer than by hour. For example, if a vehicle
started at 7:35 AM, it was assumed to start at 7:00 AM. Also, by the definition, it was
assumed that diurnal emissions would be very small in magnitude during the nighttime
hours of 7PM to 6 AM (group 14), although a vehicle's soak period could encompass part
or all of this time interval. As a result, the activity distribution for the 6 PM to 7 PM group
will be used for the nighttime hours.
For each hour, the duration of the preceding soaks were grouped in 12 intervals.
These intervals are shown in the first column of Table 2. They range from a soak duration
of 1 to 2 hours up to a multi-day soak of 72 hours or more. The 12th row (labeled "running
/ hot soak") contains the percentage of vehicles which were not experiencing a diurnal
during a given hourly group. For example, the fifth column of the fourth row of numbers
in Table 2 says that 1.67 percent of the valid vehicle-days had soaks which started between
four and five hours prior to 10 AM, and did not include any vehicle operation between 10
and 11 AM.
For illustration, the hourly diurnal activity results are shown in Table 2 for all of the
hourly groups and for a range of soak hours. The percentages in each column add to 100
percent and include vehicles which are not having a multi-day, full, or an interrupted
diurnal (See Section 4.3 for definition of diurnal types). The percentage of vehicles in a
given hourly group which are likely experiencing a multi-day diurnal are those in the rows
labeled as 48 to 71 and 72+. The percentage of vehicles which are likely (there are a few
exceptions depending on when the soak began) experiencing a full diurnal plus an
m6flt006.wpd DRAFT 6 Mar 1, 1999
-------
interrupted diurnal are those in the row labeled 24 to 47. The percentage of vehicles which
are experiencing an interrupted diurnal are in the rows labeled 1 to 2 through 8 to 23. For
example, the table shows that at the 6 to 7 AM interval, 61.02% of the vehicles are
experiencing a soak which has lasted between 8 and 24 hours. For comparison, by 3 to 4
PM, the percentage of vehicles in this category (8 to 24) has dropped to less than 6 percent.
3.4 Hourly Diurnal Activity Results
To apply the activity distribution to all soak lengths and to smooth the distribution
curve, MOBILE6 will apply activity values using a distribution curve. The actual diurnal
activity distribution parameters which will be used in MOBILE6 are shown in Table 3. A
set of four parameters (A, B, C, and D) is shown for each of the 13 hourly groups. The 6 -
7 group is the distribution for the 6 to 7 AM group, the 18+ group is the distribution for all
the evening and nighttime hours between 7 PM and 6 AM. These parameters are the result
of fitting Weibull equations (non-linear regression) to the diurnal activity results. Fitting
the Weibull equations to the activity results produces a smoother activity distribution for
the individual hourly groups than is shown in Table 2. The smoothness of the curve is
reflected in the high r-square values (> 0.97) of the fit. Using the fitted value for the
distribution was done in-order to conform to the structure of the diurnal emission factors
being built into MOBILE6. These require some fairly small time intervals. If the unfitted
data were used directly, some of these intervals would be represented by no data or only a
very small amount of data.
The Weibull function fit is of the form:
Y = A - B * exp( -C * Soaklength**D) Eqn 1
where A, B, C, and D are regression coefficients (listed in Table 3), and soaklength in hours
(1 to 72+) is the independent variable. The variable Y is the cumulative distribution in
percent.
The Weibull function fit is a cumulative distribution of the soaks which are
diurnals. It does not include the portion of the fleet which is in hot soak or running mode.
For example, substituting the coefficients for Hourly Group 6-7 from Table 3 and a soak
length of 72 hours into Equation 1 produces a maximum cumulative distribution of 85.0
percent. The remaining 15 percent of the fleet is in hot soak or running mode.
0.850 = 0.8502 - 0.8427 * exp(-0.001616 * 72 ** 2.6440) Eqn 2
m6flt006.wpd DRAFT 7 Mar 1, 1999
-------
In the MOBILE6 model it will be necessary to calculate the percentage of soaks in
a given hourly group which are X hours in duration. This is done by transforming the
cumulative Weibull distribution into a non-cumulative distribution using Equation 3.
D(i) = Y(i) - Y(i-l) Eqn 3
where D(i) is the distribution for the interval from t-1 to t, Y is the Weibull function
from Eqn 1.
For example, for Group 6-7 the percentage of soaks which are 5-6 hours is shown in
Equations 4 and 5.
0.09822= 0.8502 - 0.8427 * exp(-0.001616 * 5 ** 2.6440) Eqn 4
0.05906= 0.8502 - 0.8427 * exp(-0.001616 * 4 ** 2.6440) Eqn 5
0.0392 = 0.09822-0.05906 Eqn 6
The value is 0.0392 or 3.92 percent of the fleet has a preceding 5 to 6 hour soak
during the 6 to 7 AM period.
The results in Table 2 and Table 3 represent both weekdays and weekends. The
combination of the two types was necessary because of a lack of data on weekends (there
were insufficient numbers of weekend vehicle-days). Inclusion of these data may bias the
weekday results slightly; however, no analysis was done to quantify this potential bias. To
overcome this potential problem, the MOBILE6 input structure will allow for optional user
input of separate weekend/weekday parameters should they become available.
m6flt006.wpd DRAFT 8 Mar 1, 1999
-------
SOAK Mrs
1 to 2
2 to 3
3 to 4
4 to 5
5 to 6
6 to 7
7 to 8
8 to 23
24 to 47
48 to 71
72+
running /hot soak
TOTAL
6 -7 AM
1 .27%
0.72%
1 .27%
3.02%
3.50%
4.85%
5.81%
61 .02%
4.93%
0.88%
0.48%
12.25%
100.00%
7 -8 AM
2.23%
1 .27%
0.64%
0.95%
2.86%
2.94%
3.98%
47.81 %
4.06%
0.72%
0.32%
32.22%
100.00%
TABLE 2
Diurnal Activity Parameters bv Hourly Group
8 -9 AM
5.25%
2.15%
1.19%
0.64%
0.95%
2.70%
2.63%
36.20%
3.26%
0.64%
0.32%
44.07%
100.00%
9 -10 AM
12.97%
5.09%
1.91%
1.11%
0.56%
0.72%
2.55%
29.04%
2.39%
0.56%
0.16%
42.96%
100.00%
10-11 AM
14.88%
12.09%
4.93%
1.67%
0.95%
0.40%
0.56%
23.71 %
2.07%
0.48%
0.08%
38.19%
100.00%
11 -12AM
8.99%
13.13%
10.10%
4.77%
1.43%
0.80%
0.40%
17.90%
1.91%
0.48%
0.08%
40.02%
100.00%
12-1 PM
7.48%
6.92%
1 1 .30%
7.80%
4.30%
1.27%
0.64%
13.37%
1.27%
0.48%
0.08%
45.11%
100.00%
1 -2PM
7.80%
5.41%
5.57%
10.26%
7.40%
3.90%
1.11%
9.63%
1.27%
0.40%
0.08%
47.18%
100.00%
2 -3PM
11.14%
5.73%
4.30%
5.09%
9.39%
6.76%
3.26%
7.40%
0.95%
0.32%
0.08%
45.58%
100.00%
3 -4PM
8.99%
8.75%
4.85%
3.98%
3.82%
8.19%
4.93%
5.89%
1 .03%
0.24%
0.08%
49.24%
100.00%
4 -5PM
10.18%
6.44%
6.52%
4.14%
3.58%
3.74%
6.05%
6.60%
0.72%
0.24%
0.08%
51 .71 %
100.00%
5 -6PM
1 1 .93%
8.27%
4.85%
5.33%
3.34%
3.34%
3.10%
7.40%
0.48%
0.16%
0.08%
51 .71 %
100.00%
6 -7PM
13.84%
9.71%
7.08%
4.38%
4.85%
3.34%
3.10%
7.72%
0.48%
0.16%
0.08%
45.27%
100.00%
m6flt006.wpd DRAFT
Mar 1, 1999
-------
Table 3
Weibull Distribution Coefficients for Diurnal Activities
Hourly
Group
6-7
7-8
8-9
9- 10
10- 11
11 - 12
12- 13
13- 14
14- 15
15- 16
16- 17
17- 18
18 +
Coefficient
A
0.8502
0.6559
0.5418
0.5525
0.5987
0.5950
0.5313
0.4995
0.5039
0.4630
0.4361
0.4400
0.5025
Coefficient
B
.8427
0.6342
0.4793
0.3867
0.3971
0.8389
0.9246
0.6927
0.5180
0.4793
0.3870
0.3881
0.4509
Coefficient
C
0.001616
0.001473
0.001880
0.002715
0.038477
0.5479
0.6797
0.4367
0.2147
0.2227
0.1233
0.1723
0.1967
Coefficient
D
2.6440
2.5928
2.4486
2.2846
1.3344
0.5086
0.4943
0.6834
1.0038
1.0182
1.3237
1.1886
1.1494
Regression
R2 Value
0.995
0.996
0.995
0.994
0.973
0.974
0.981
0.979
0.976
0.981
0.989
0.996
0.998
m6flt006.wpd DRAFT
10
Mar 1, 1999
-------
4.0 USING THE HOURLY DIURNAL ACTIVITY INFORMATION
4.1 Overview
To use the diurnal activity information in the MOBILE6 model requires that the
activity information be integrated with the diurnal emission parameters. This is not a trivial
task since both the activity information and the diurnal emission information are both
functions of time of day and soak length. Also, the task is made more difficult since several
types of diurnal emissions exist, and some of these are a function of both the time of day
being evaluated and the time that the soak began.
4.2 Diurnal Types
Four different diurnal types are present in MOBILE6. These are Interrupted, Full,
Two-day, and Three-day. In addition, there are periods where there is no diurnal because
the vehicle is running or under a hot soak. More complete information on diurnal types and
diurnal emissions can be found in EPA documents EPA-420-P-98-011 "Modeling Hourly
Diurnal Emissions and Interrupted Diurnal Emissions Based on Real-Time Diurnal Data"
-M6.EVP.OO 1, and M6.EVP.003 "Evaluating Multiple Day Diurnal Evaporative Emissions
Using RTD Tests".
The emission effects of the Null diurnal (a running or non diurnal mode),
Interrupted diurnal, Full diurnal, and two and three day diurnals are all assumed to start
prior to 6 am. The interrupted diurnals are a function of the time of day at which the
diurnal started, ranging from 7am to 2pm. Each has a different emission effect as explained
in M6.EVP.002. These are labeled: "I - 7 am", "I - Sam", "I - 9am", "I - 10am", "I -1 lam",
"I - 12 am", "I - 1pm", and "I - 2pm". Several of these are illustrated in the sample
calculation.
4.3 Rules for Determining Diurnal Type
The MOBILE6 model uses rules in the hourly diurnal emissions calculation to
determine how to allocate vehicle activity to each diurnal emission type. The rules are
based on which hour of the day is being modeled, the soak duration, and in the case of
interrupted diurnals, the time of day the soak began. For example, for Hour X, the activity
data will indicate what percentage of the fleet has been soaking for what period of time.
The model must allocate these activity fractions to the appropriate type of diurnal.
Knowing how much activity data to allocate to interrupted, full, two-day, and three-day
diurnal types allows MOBILE6 to properly weight each type together for each hour of the
day.
m6flt006.wpd DRAFT 11 Mar 1, 1999
-------
Rules One through Four point to a case where there is no diurnal. These are the
Null diurnal cases. The vehicle is either in hot soak mode or resting loss mode. Rule Five
applies to an interrupted diurnal, and rules Six through Eight apply to full or multi-day
diurnals. Soak time refers to the length of the soak in hours, and hour refers to the hour of
the day on a 24 hour clock (i.e., 5 = 5 AM).
Table 4
Determining the Type of Diurnal
Rule#
Rulel
Rule 2
Rule3
Rule 4
RuleS
Rule 6
Rule?
RuleS
Rule
Soaktime < 1
Soaktime >= 1 and
Soaktime < 2
Hour<=5 (SAM)
Soaktime <= Hour - 13
Soaktime > Hour - 13 and
Soaktime <= Hour - 5
Soaktime > Hour - 5 and
Soaktime <= Hour +17
Soaktime > Hour +17 and
Soaktime <= Hour + 41
Soaktime > Hour + 41
Diurnal Type
Hot Soak
Resting Loss
Resting Loss
Resting Loss
Interrupted
Full
Two-Day
Three-Day
Explanation
Hot Soak Period - No diurnal
Vehicle Equilibrating to Ambient
- No diurnal
Assume no diurnal emission from
midnight to 5 am
No diurnal beginning in
Afternoon assume no temp rise
after 3 pm.
Hot Soak ended after 6 am causes
interrupted diurnal.
Diurnal begins at 6 am. No
significant diurnal on previous
day.
Diurnal begins at 6 am.
Significant diurnal previous day.
Diurnal begins at 6 am.
Significant diurnal previous two
or more days.
Application of the "diurnal rules" is shown graphically in the attached Excel
spreadsheet "Diur_Rule.xls". It requires Excel 97, and should be viewed in color. The
overall spreadsheet contains two worksheets (Tables A-l and A-2). The first worksheet is
a matrix with hour of the day as columns and Soaktime as rows. It shows the entire range.
The second worksheet shows only the interrupted diurnals. The reader should note that
there are several different types of interrupted diurnals based on the time of day the soak
period started. The overall worksheet matrix is color coded to represent each type of
diurnal / no diurnal. The color pink represents the periods of the day / soak combinations
when no diurnal occurs (hotsoak periods). The yellow color also represents the periods
when a diurnal does not occur. This is typically during the nighttime hours when only
m6flt006.wpd DRAFT
12
Mar 1, 1999
-------
resting loss emissions occur. The darker blue periods near the top of the figure represent
the interrupted diurnal periods. The green portion the figure indicates when a full day
diurnal can occur. The lighter blue section of the figure with white letters represents the
two day diurnal, and the red section at the bottom represents the three day diurnal periods.
4.4 Example Calculation
An example calculation is provided in the accompanying linked (attached)
spreadsheet. This calculation is fairly lengthy and involves three separate spreadsheets
linked as document EVP_DIU2.XLS. It is attached and discussed in this section to provide
a sample calculation of the method of combining the activities with the diurnal emissions.
The sample calculation provides an estimate for diurnal emissions in grams per hour per
vehicle and grams per hour per day per vehicle for the case of 1990-93 PFI vehicles which
did not fail the purge or pressure I/M test. Other 1981 and later model year groups and
evaporative I/M test status cases would use similar calculations.
The calculations in the spreadsheet are based on incorporating the methodology
described in document EPA420-P-98-011 (or M6.EVP.002) "Modeling Hourly Diurnal
Emissions and Interrupted Diurnal Emissions Based on Real-Time Diurnal Data", and the
diurnal activity estimates described in the current document.
The text of this example calculation is written to coincide with the Excel
spreadsheet EVP_DIU2.XLS. This spreadsheet consists of three tables. These are: FDD
"Non Gross", "Gross Temps", and "Hourly". The "Non Gross" table provides an example
calculation of the average daily diurnal emission estimate for a vehicle which is not a gross
liquid leaker. The "Gross Temps" table is a calculation of the daily temperature
distribution used to calculate the diurnal emissions of the gross liquid leakers. The
"Hourly" table is the primary table. It calculates the individual hourly diurnal emissions
for each soak length of non-gross liquid leakers and gross liquid leakers, and sums them
into a daily total. The gross liquid leakers and the non-gross liquid leakers are discussed
separately in document EPA420-P-98-011 (or M6.EVP.002), and in the MOBILE6 model
because gross liquid leakers are a small portion of the vehicle fleet, but nevertheless have
a large impact on the total diurnal emissions because of their tremendous emission rates.
4.4.1 Hourly Diurnal Emissions of Non-Gross Liquid Leakers
The "FDD Non Gross" table calculates the full day diurnal (FDD) for a vehicle that
is not a gross liquid leaker (NGLL). The formulas are shown at the top of the sheet and the
result (FDD) is given at the bottom of the sheet. The value is 2.6832 grams of HC for this
example.
The value of FDD is used in the "Hourly" spreadsheet in the columns Non Gross -
m6flt006.wpd DRAFT 13 Mar 1, 1999
-------
Diurnal grams (Worksheet "Hourly" Cells CC through CU) to calculate the hourly diurnal
emissions for each hourly group (i) at each soaktime (j). The formula which is used in each
cell of the matrix is:
Hourly Diurnal(i,j) = FDD * Diurnal Fract(ij) * Activity Fract(ij) Eqn 7
where hourly group i ranges from 6-7am to 11-12pm (18 groups)
where soaktime = j ranges from 1 to 72 hours
where Hourly Diurnal is the average g/vehicle for that hour, including both soaking and
non-soaking vehicles.
The 'Diurnal Fract(ij)' parameter is shown in worksheet "Hourly" Cells BI-CA for
each hourly group and soaklength. The 'rules' used to develop the Diurnal Fract parameter
are shown in worksheet "Hourly" Cells AO-BG. The value in each of the Diurnal Fract
cells is the fraction of a daily diurnal that cell or hourly group / soak length represents. As
reported in M6.EVP.002.
The "Activity Fract(ij)" parameter used in Eqn 7 is shown in worksheet "Hourly"
Cells U-AM. These are the fraction of an hourly group's activity represented by one of the
soaklengths. Each column normalizes to unity (i.e., Column V sums to 1.0). The activity
values shown in worksheet "Hourly" Cells U-AM were calculated from the cumulative
activity values shown in worksheet "Hourly" Cells A-S.
The final average daily diurnal for a non-gross liquid leaker is shown in worksheet
"Hourly" cell CU. This reflects the impact of the activity distribution and the interrupted
diurnals. The value for this example of vehicles that pass both purge and pressure tests is
1.271 grams / day - non-gross liquid leaker. In comparison, the value would be 2.6832
grams /day - nongross liquid leaker if all of the fleet received a full day diurnal.
4.4.2 Hourly Diurnals of Gross Liquid Leakers
Equation 8 was used to calculate the hourly diurnal emissions of gross liquid leakers
(HDGLL). It is a function of the hourly temperature difference (difference within an hour),
and the daily temperature difference (difference between the current hour and the
temperature prior to 6:OOAM). It is weighted by the activity distribution in Table 2.
HDGLL = 100.29*(0.008958 + (0.007383*Hourly DTemp) + (0.003053*Daily DTemp)) * Activity Eqn 8
The hourly and daily temperature differences are shown in worksheet "Hourly"
(Cells CX-DP). The calculation in Equation 8 excluding the activity weighting was done
in worksheet "Hourly" (Cells DR-EJ), and the activity weighting was done in worksheet
m6flt006.wpd DRAFT 14 Mar 1, 1999
-------
"Hourly" (Cells EL-FD).
The daily temperature difference for each hourly group and soak time is based on
the daily temperature profile found in the spreadsheet table "Gross Temps". This table
takes the standardized temperature profile found in the Code of Federal Regulations -
40CFR86, and creates a new standardized temperature profile based on the daily minimum
and maximum temperatures. In this case, the daily minimum and maximum temperatures
were 82 F to 106F.
For comparison, the final average diurnal emission result for liquid leakers for this
example is shown in Cell CU. It is 63.326 grams / day- gross liquid leaker. This is a very
large emission result, especially in comparison to the value of 1.271 grams / day - nongross
liquid leaker. However, the high value for the gross liquid leaker is an extreme (not close
to the average vehicle diurnal emission). Gross liquid leakers are (fortunately) relatively
rare in the fleet, and typically make up less than 1 percent. They reflect vehicles with
serious fuel system problems where liquid fuel/vapor (not just vapor) is escaping from the
system.
m6flt006.wpd DRAFT 15 Mar 1, 1999
-------
5.0 COMMENTS
Comments on this report and its proposed use in MOBILE6 should be sent to the attention
of the author, and submitted electronically to mobile@epa.gov, or by fax to (734)
214-493 9, or by mail to MOBILE6 Review Comments, US EPA Assessment and Modeling
Division, 2000 Traverwood Drive, Ann Arbor MI 48105. Electronic submission of
comments is preferred, since we will make any comments available on our web site. In
your comments, please note clearly the document that you are commenting on including the
report title and the code number listed. Please be sure to include your name, address,
affiliation, and any other pertinent information.
m6flt006.wpd DRAFT 16 Mar 1, 1999
-------
-------
P
I
O
o
CM
CM
g
CM
O
O
o
CM
E
Q.
1*- O
SO
00
0.
(0
I
o
o
to
1
o
o
I
C 0
O 0
0 CM
c *~
g
o
o
o
o
o
en
1
o
o
r-
= ^
Soak Time
hours
Prior
Diurnal Time
hours
o
-
1
a
I
1
I
-
1 (10 am|
I
O)
I (8 am)
I
CM
O
a
1
!
1(11 am|
I
-
I (9 am|
I
I (7 am|
„
-
i
i.
§
i
1 (10 am|
I
-
I
2
I
-
CM
a
1
!
fe
-
1 (10 am)
I
£.
I
-
I
IO
„
1
a
I
1
I
fe
0
-
I
01
1(8 am)
I
-
u>
-
a
1
!
i
~
1 (10 am|
i
I
I (7 am|
-
IO
1
a
§
I
I
0
~
1
CJ
i
I
CO
(O
a
1
!
fe
-
1 (10 am)
I
£.
I
i
O)
-
1
a
I
i
I
fe
0
-
I
01
I
2-
I
o
CO
E
a
I
!
i
-
1 (10 am|
i
I
I
-
en
E
a
§
i
I
I
0
-
I
01
i
I
CM
O
!
fe
-
I
f
01
I
i
5
-
i
0
-
I
O)
I
CO
I
5
CM
I
E
01
I
I
IO
5
I
OJ
E
CO
I
(O
5
I
i
t
IO
I
co en
(O h-
------- |