- Email: [email protected]
Commuter Exposure Modeling Part II: Emissions and Dispersion Modules and Generation of Exposure Statistics
AtmosphericPollution 1980, Proceedingsof the 14th International Colloquium,Paris, France, May 5-8,1980, M.M. Benarie (Ed.), Studies in Environmental Science,Volume 8 0 Elsevier Scientific Publishing Company,Amsterdam - Printed in The Netherlands
13
COMMUTER EXPOSURE MODELING PART 11: EMISSIONS AND DISPERSION MODULES AND GENERATION OF EXPOSURE STATISTICS P. B. SIMMON and R. M. PATTERSON SRI International, Menlo Park, CA 94025 (U.S.A.) and W. B. PETERSEN U.S. Environmental Protection Agency, Research Triangle Park, NC
2 7 7 1 1 (U.S.A.)
ABSTRACT A model methodology has been designed to compute commuter exposure statistics
through simulation of the traffic, vehicular emissions, and atmospheric dispersion of roadway-related air pollutants. A detailed description of the emissions and dispersion elements of the commuter exposure modeling methodology and a discussion of the results generated by the model are presented in this paper; the traffic
element of the methodology is discussed in a companion paper. The commuter exposure model's emissions module incorporates two emissions methodologies: a treatment based on average route speed, and one that considers the effects of driving mode changes. Dispersion of pathway emissions is simulated by two separate dispersion treatments, while dispersion from nonpathway sources is computed with the simple Hanna-Gifford model.
The model produces a number of statistics describing annual
average or short-term worst-case exposure. INTRODUCTION This is the second (Paper 11) of two papers describing the development of a computer model of commuter exposure to air pollution. The modeling methodology requires the model user to define the major commute corridors or "pathways" in a metropolitan area. The computer model reads traffic and roadway characteristics of the pathways, general information regarding traffic and emissions on the remainder of the roadway network, and various meteorological data. This information is used by the three major modules of the model to simulate traffic flow, compute vehicular emissions, and calculate the atmospheric dispersion of roadway-related air pollutants. Various statistics describing pathway exposures (concentration integrated with time over the length of the pathway) are the output of the model.
Paper I gives an overview
of the model methodology and describes the computer model's traffic flow computa-
tion module.
This paper continues the discussion with a description of the emission
14 rate computation module, the dispersion module, and the methodology used to generate exposure statistics. EMISSION RATE COMPUTATIONS Emission rates for each pathway segment and grid square are computed using one of two types of emission treatments:
a treatment based on the average route speed
and one that considers the effects of driving mode changes on emissions. The first treatment is the EPA methodology based on the Federal Test Procedure (FTP) (ref. 1). The second treatment is the "Automobile Exhaust Emission Modal Analysis Model," (ref. 2) or modal model.
Emissions modeling along pathways requires both treat-
ments; FTP-based emissions estimates are suitable for nonpathway sources. Emission rates (Q) for pathway segments that are freeways, expressways, or arterials outside the central business district (CBD) are found by multiplying an emission density, E, computed using the FTP methodology and chosen according to the average route speed on the segment, by the demand volume (V) on the segment. To compute the average emission rate over a segment having congested flow, it is convenient to break the emissions into two components:
those occurring during
normal flow, E, and the excess over normal flow emissions that occur during congested flow, E'-E,
where E' represents the emissions during congested flow. The
component of total emissions due to uncongested flow is given by Q, with E chosen according to the average route speed on the uncongested portion of the segment. The average emission rate over the pathway segment is the sum of the components due to uncongested (Q) and congested (Q')
flow, as given by:
where E' corresponds to an average route speed of 20 mi/h,
i is the number of
vehicles affected by the backup, L = segment length (m), Lq = backup length (m), and
+
is the duration (seconds) of the backup.
Travel on arterials within the CBD is characterized by interrupted flow and low speeds, and a modal emissions treatment should be used.
This is available
through an adaptation of EPA's modal model. The modal model determines an instantaneous emission rate, e(t>, which is a function of vehicle speed, v, and acceleration, a. Acceleration to or from a given speed is assumed to be a perturbation to the steady-state emission rate. For the commuter exposure model, simulation of the effects of modal emissions is facilitated through the introduction of the concept of excess modal emissions. Excess modal emissions are those that occur over and above those that would have
15 occurred had the vehicle not stopped. The total excess emissions, EE(g/m/s),
the
sum of idle emissions, and the acceleration and deceleration parts of the excess emission, Em, may be expressed as
Where N = number of vehicles in the backup; b10 = constant (g/s), D = delay
(s),
P = proportion of vehicles stopped for the signal, and Cy = signal cycle length
(s)
The cruise emissions that would have occurred had the vehicle not stopped are found by integrating the modal model expression for the steady-speed emission rate
(G
)
over 4 T. The cruise emissions component may be calculated by E = C
6 V/3600 v 1609.344/3600
(3)
where 1609.344 is the number of meters in a mile. Once emissions are calculated with the modal treatment, they are averaged for each pathway segment. DISPERSION MODELING Dispersion of Pathway Emissions For the purpose of computing the dispersion of vehicular emissions on commute pathways, limited-access and nonlimited-access pathway segments are distinguished from segments located in a street canyon. The dispersion of pollutants emitted by vehicles on both limited-access and nonlimited-access roadways is computed using a technique taken from the CALINE 2 (ref. 3) dispersion model. The model formulation is based on the treatment of pollution dispersion as the vector sum of two components: dispersion occurring along the horizontal wind component oriented perpendicular to the roadway, and dispersion along the horizontal wind component parallel to the roadway. The parallel wind model assumes that the roadway is divided into a series of square area sources as wide as the roadway. The concentration downwind of the area source is computed as if the emissions originated from a virtual source located upwind of the area source, at a distance that forces the model to assume a uniform concentration within a mixing cell over the roadway. The equation used to compute the concentration from each area source is:
3 where X = concentration from parallel dispersion (g/m ) , U = wind speed (m/s), P Q k = line-source emission rate (g/m/s), W = roadway width (m), 8 = angle between
16
wind direction and roadway, y = perpendicular distance between receptor and roadway edge plus an initial dispersion parameter (m), z above grade-level (m), dispersion function (m).
0
Y
=
height of the receptor
= horizontal dispersion function (m), and o
= vertical
The concentrations from each area source are summed to give the parallel component, which is then corrected by a stability-dependent factor. The cross-wind or normal component of concentration is given by
xn
=
O
iexp [- (31I $
U
(5)
3
where X = concentration from normal dispersion (g/m ) n
.
The parallel and crosswind components are summed to give the total concentration when
€Iis
non-zero.
Dispersion on a roadway with tall buildings on both sides is greatly influenced by the presence of the buildings.
The street-canyon dispersion treatment used in
the commuter exposure model is based on the empirical street-canyon model developed by Johnson et al. (ref. 4 ) and modified by Ludwig et al. (ref. 5 ) .
For the commuter
exposure model, the concentration on the roadway (X) was assumed to be the average of the expressions for the concentrations on the windward and leeward sides of the street, as given by
x=
KQk
2(U+0.5)
where Q,
=
(h
+
$)
line-source emission rate (g/m/s), K
=
empirically derived nondimensional
constant =2 m, and W = street width (m). Segment pollutant concentrations are summed for each pathway and integrated over travel time to yield pathway exposures. Dispersion of Nonpathway Source Emissions It is expected that the portion of the total pathway integrated concentration (exposure) that results from nonpathway sources will be small in comparison to the portion resulting from traffic on the commuter pathways themselves. Therefore, a very simple emissions and dispersion treatment is used. The line-source emissions on nonpathways have been aggregated into area-source emissions from grid squares. For each pathway segment a concentration is computed, according to the so-called Hanna-Gifford (ref. 6 ) dispersion treatment, at receptors
17 located at the endpoints and the midpoint of the segment. These concentrations
(X) are given by
where C
=
$ $&
;
Qo= emission rate of the grid square in which the receptor is located (g/m2 /s),
U
=
wind speed (m/s), D
=
city size, and a and b are stability-dependent constants.
The normalized integrated concentration over the segment (the exposure E) is found by performing a stepwise integration over travel time from one endpoint to the midpoint and the midpoint to the other endpoint of the segment. The model computes exposures for all segments of a pathway and sums the results to yield the pathway exposure resulting from nonpathway sources. On-Roadway/In-Vehicle Concentration Relationship The little information that exists about the relationship between the concentration on the roadway and the concentration within a vehicle indicates that concentrations of CO inside a vehicle are about equal to that on the outside. Therefore, the commuter exposure model assumes the concentrations at the two locations are identical. Generation of Commuter Exposure Statistics Exposures are computed for each pathway according to the meteorological conditions of the mode of model operation chosen by the model user.
If the short-term
mode is chosen, one exposure is computed for each pathway, for the input meteorological conditions and traffic information. If the model is operating in the annual mode, morning and evening exposures are computed for each pathway for 576 sets of meteorological conditions (each combination of 6 wind speeds, 16 wind directions, and 6 atmospheric stability classes).
These exposures are weighted according to
the frequency of occurrence of each set of conditions and summed for each pathway. I n ad-ditinr!to the exposures, the model stores the total travel time or times on
each pathway and the average number of commuters using the pathway. When the model is run in the short-term mode, the output includes a list of the exposure on each pathway for the input worst-case meteorological and traffic conditions and the average and standard deviation of pathway exposure. When the model is in the annual mode it lists the annual average exposure on each pathway and over the modeled region. For either mode, the model produces data for two histograms. For the short-term mode the data pertain to a single commute; for the annual mode, the data are representative of annual variations. First, the range of exposures
18 found on all pathways is divided into several classes. Then for each class, two parameters are listed:
the percentage of the commuting population (commuting
vehicles multiplied by the average number of commuters per vehicle) treated by the model that experience exposure levels in the class, and the probability of experiencing the exposure levels in the class (i.e., the percentage of time commuters are exposed to the levels of the exposure class). may be output at the user's discretion.
Subsets of these statistics
Finally, the model user may call for
model output in grahical form. For a summary of model output, the reader is referred to Part I, Overview. ACKNOWLEDGEMENTS This work was supported by the U . S .
Environmental Protection Agency under
Contract No. 68-02-2754. REFERENCES 1 Mobile Source Emission Factors, Final Document, Environmental Protection Agency, Office of Transportation and Land Use Policy, Washington, D.C., 1978. 2 Modal Program Guide, an update to Automobile Exhaust Emission Modal Analysis Model, U.S. EPA Report No. EPA-46013-74-005, 1974. 3 K.E. Noll, T.L. Miller, and M. Claggett, A Comparative Analysis of EPA HIWAY, California, and CALINE 2 Line-Source Dispersion Models, submitted to Transportation Research Board, Washington, D.C., 1976. 4 W.B. Johnson, W.F. Dabberdt, F.L. Ludwig, and R.J. Allen, Field Study for Initial Evaluation of an Urban Diffusion Model for Carbon Monoxide, Comprehensive Report, CRC and Environmental Protection Agency (EPA) Contract CAF'A-3-68 (1-69), SRI International, Menlo Park, CA, 1971. 5 F.L. Ludwig and W.F. Dabberdt, Evaluation of the APRAC-1A Urban Diffusion Model for Carbon Monoxide, Final Report, CRC and EPA Contract CAPA-3-68 (1-69), SRI International, Menlo Park, CA, 1972. 6 S.R. Hanna, A Simple Method of Calculating Dispersion from Urban Area Sources, J. Air Pollution Control Assoc. 21, pp. 774-777, 1971.
13
COMMUTER EXPOSURE MODELING PART 11: EMISSIONS AND DISPERSION MODULES AND GENERATION OF EXPOSURE STATISTICS P. B. SIMMON and R. M. PATTERSON SRI International, Menlo Park, CA 94025 (U.S.A.) and W. B. PETERSEN U.S. Environmental Protection Agency, Research Triangle Park, NC
2 7 7 1 1 (U.S.A.)
ABSTRACT A model methodology has been designed to compute commuter exposure statistics
through simulation of the traffic, vehicular emissions, and atmospheric dispersion of roadway-related air pollutants. A detailed description of the emissions and dispersion elements of the commuter exposure modeling methodology and a discussion of the results generated by the model are presented in this paper; the traffic
element of the methodology is discussed in a companion paper. The commuter exposure model's emissions module incorporates two emissions methodologies: a treatment based on average route speed, and one that considers the effects of driving mode changes. Dispersion of pathway emissions is simulated by two separate dispersion treatments, while dispersion from nonpathway sources is computed with the simple Hanna-Gifford model.
The model produces a number of statistics describing annual
average or short-term worst-case exposure. INTRODUCTION This is the second (Paper 11) of two papers describing the development of a computer model of commuter exposure to air pollution. The modeling methodology requires the model user to define the major commute corridors or "pathways" in a metropolitan area. The computer model reads traffic and roadway characteristics of the pathways, general information regarding traffic and emissions on the remainder of the roadway network, and various meteorological data. This information is used by the three major modules of the model to simulate traffic flow, compute vehicular emissions, and calculate the atmospheric dispersion of roadway-related air pollutants. Various statistics describing pathway exposures (concentration integrated with time over the length of the pathway) are the output of the model.
Paper I gives an overview
of the model methodology and describes the computer model's traffic flow computa-
tion module.
This paper continues the discussion with a description of the emission
14 rate computation module, the dispersion module, and the methodology used to generate exposure statistics. EMISSION RATE COMPUTATIONS Emission rates for each pathway segment and grid square are computed using one of two types of emission treatments:
a treatment based on the average route speed
and one that considers the effects of driving mode changes on emissions. The first treatment is the EPA methodology based on the Federal Test Procedure (FTP) (ref. 1). The second treatment is the "Automobile Exhaust Emission Modal Analysis Model," (ref. 2) or modal model.
Emissions modeling along pathways requires both treat-
ments; FTP-based emissions estimates are suitable for nonpathway sources. Emission rates (Q) for pathway segments that are freeways, expressways, or arterials outside the central business district (CBD) are found by multiplying an emission density, E, computed using the FTP methodology and chosen according to the average route speed on the segment, by the demand volume (V) on the segment. To compute the average emission rate over a segment having congested flow, it is convenient to break the emissions into two components:
those occurring during
normal flow, E, and the excess over normal flow emissions that occur during congested flow, E'-E,
where E' represents the emissions during congested flow. The
component of total emissions due to uncongested flow is given by Q, with E chosen according to the average route speed on the uncongested portion of the segment. The average emission rate over the pathway segment is the sum of the components due to uncongested (Q) and congested (Q')
flow, as given by:
where E' corresponds to an average route speed of 20 mi/h,
i is the number of
vehicles affected by the backup, L = segment length (m), Lq = backup length (m), and
+
is the duration (seconds) of the backup.
Travel on arterials within the CBD is characterized by interrupted flow and low speeds, and a modal emissions treatment should be used.
This is available
through an adaptation of EPA's modal model. The modal model determines an instantaneous emission rate, e(t>, which is a function of vehicle speed, v, and acceleration, a. Acceleration to or from a given speed is assumed to be a perturbation to the steady-state emission rate. For the commuter exposure model, simulation of the effects of modal emissions is facilitated through the introduction of the concept of excess modal emissions. Excess modal emissions are those that occur over and above those that would have
15 occurred had the vehicle not stopped. The total excess emissions, EE(g/m/s),
the
sum of idle emissions, and the acceleration and deceleration parts of the excess emission, Em, may be expressed as
Where N = number of vehicles in the backup; b10 = constant (g/s), D = delay
(s),
P = proportion of vehicles stopped for the signal, and Cy = signal cycle length
(s)
The cruise emissions that would have occurred had the vehicle not stopped are found by integrating the modal model expression for the steady-speed emission rate
(G
)
over 4 T. The cruise emissions component may be calculated by E = C
6 V/3600 v 1609.344/3600
(3)
where 1609.344 is the number of meters in a mile. Once emissions are calculated with the modal treatment, they are averaged for each pathway segment. DISPERSION MODELING Dispersion of Pathway Emissions For the purpose of computing the dispersion of vehicular emissions on commute pathways, limited-access and nonlimited-access pathway segments are distinguished from segments located in a street canyon. The dispersion of pollutants emitted by vehicles on both limited-access and nonlimited-access roadways is computed using a technique taken from the CALINE 2 (ref. 3) dispersion model. The model formulation is based on the treatment of pollution dispersion as the vector sum of two components: dispersion occurring along the horizontal wind component oriented perpendicular to the roadway, and dispersion along the horizontal wind component parallel to the roadway. The parallel wind model assumes that the roadway is divided into a series of square area sources as wide as the roadway. The concentration downwind of the area source is computed as if the emissions originated from a virtual source located upwind of the area source, at a distance that forces the model to assume a uniform concentration within a mixing cell over the roadway. The equation used to compute the concentration from each area source is:
3 where X = concentration from parallel dispersion (g/m ) , U = wind speed (m/s), P Q k = line-source emission rate (g/m/s), W = roadway width (m), 8 = angle between
16
wind direction and roadway, y = perpendicular distance between receptor and roadway edge plus an initial dispersion parameter (m), z above grade-level (m), dispersion function (m).
0
Y
=
height of the receptor
= horizontal dispersion function (m), and o
= vertical
The concentrations from each area source are summed to give the parallel component, which is then corrected by a stability-dependent factor. The cross-wind or normal component of concentration is given by
xn
=
O
iexp [- (31I $
U
(5)
3
where X = concentration from normal dispersion (g/m ) n
.
The parallel and crosswind components are summed to give the total concentration when
€Iis
non-zero.
Dispersion on a roadway with tall buildings on both sides is greatly influenced by the presence of the buildings.
The street-canyon dispersion treatment used in
the commuter exposure model is based on the empirical street-canyon model developed by Johnson et al. (ref. 4 ) and modified by Ludwig et al. (ref. 5 ) .
For the commuter
exposure model, the concentration on the roadway (X) was assumed to be the average of the expressions for the concentrations on the windward and leeward sides of the street, as given by
x=
KQk
2(U+0.5)
where Q,
=
(h
+
$)
line-source emission rate (g/m/s), K
=
empirically derived nondimensional
constant =2 m, and W = street width (m). Segment pollutant concentrations are summed for each pathway and integrated over travel time to yield pathway exposures. Dispersion of Nonpathway Source Emissions It is expected that the portion of the total pathway integrated concentration (exposure) that results from nonpathway sources will be small in comparison to the portion resulting from traffic on the commuter pathways themselves. Therefore, a very simple emissions and dispersion treatment is used. The line-source emissions on nonpathways have been aggregated into area-source emissions from grid squares. For each pathway segment a concentration is computed, according to the so-called Hanna-Gifford (ref. 6 ) dispersion treatment, at receptors
17 located at the endpoints and the midpoint of the segment. These concentrations
(X) are given by
where C
=
$ $&
;
Qo= emission rate of the grid square in which the receptor is located (g/m2 /s),
U
=
wind speed (m/s), D
=
city size, and a and b are stability-dependent constants.
The normalized integrated concentration over the segment (the exposure E) is found by performing a stepwise integration over travel time from one endpoint to the midpoint and the midpoint to the other endpoint of the segment. The model computes exposures for all segments of a pathway and sums the results to yield the pathway exposure resulting from nonpathway sources. On-Roadway/In-Vehicle Concentration Relationship The little information that exists about the relationship between the concentration on the roadway and the concentration within a vehicle indicates that concentrations of CO inside a vehicle are about equal to that on the outside. Therefore, the commuter exposure model assumes the concentrations at the two locations are identical. Generation of Commuter Exposure Statistics Exposures are computed for each pathway according to the meteorological conditions of the mode of model operation chosen by the model user.
If the short-term
mode is chosen, one exposure is computed for each pathway, for the input meteorological conditions and traffic information. If the model is operating in the annual mode, morning and evening exposures are computed for each pathway for 576 sets of meteorological conditions (each combination of 6 wind speeds, 16 wind directions, and 6 atmospheric stability classes).
These exposures are weighted according to
the frequency of occurrence of each set of conditions and summed for each pathway. I n ad-ditinr!to the exposures, the model stores the total travel time or times on
each pathway and the average number of commuters using the pathway. When the model is run in the short-term mode, the output includes a list of the exposure on each pathway for the input worst-case meteorological and traffic conditions and the average and standard deviation of pathway exposure. When the model is in the annual mode it lists the annual average exposure on each pathway and over the modeled region. For either mode, the model produces data for two histograms. For the short-term mode the data pertain to a single commute; for the annual mode, the data are representative of annual variations. First, the range of exposures
18 found on all pathways is divided into several classes. Then for each class, two parameters are listed:
the percentage of the commuting population (commuting
vehicles multiplied by the average number of commuters per vehicle) treated by the model that experience exposure levels in the class, and the probability of experiencing the exposure levels in the class (i.e., the percentage of time commuters are exposed to the levels of the exposure class). may be output at the user's discretion.
Subsets of these statistics
Finally, the model user may call for
model output in grahical form. For a summary of model output, the reader is referred to Part I, Overview. ACKNOWLEDGEMENTS This work was supported by the U . S .
Environmental Protection Agency under
Contract No. 68-02-2754. REFERENCES 1 Mobile Source Emission Factors, Final Document, Environmental Protection Agency, Office of Transportation and Land Use Policy, Washington, D.C., 1978. 2 Modal Program Guide, an update to Automobile Exhaust Emission Modal Analysis Model, U.S. EPA Report No. EPA-46013-74-005, 1974. 3 K.E. Noll, T.L. Miller, and M. Claggett, A Comparative Analysis of EPA HIWAY, California, and CALINE 2 Line-Source Dispersion Models, submitted to Transportation Research Board, Washington, D.C., 1976. 4 W.B. Johnson, W.F. Dabberdt, F.L. Ludwig, and R.J. Allen, Field Study for Initial Evaluation of an Urban Diffusion Model for Carbon Monoxide, Comprehensive Report, CRC and Environmental Protection Agency (EPA) Contract CAF'A-3-68 (1-69), SRI International, Menlo Park, CA, 1971. 5 F.L. Ludwig and W.F. Dabberdt, Evaluation of the APRAC-1A Urban Diffusion Model for Carbon Monoxide, Final Report, CRC and EPA Contract CAPA-3-68 (1-69), SRI International, Menlo Park, CA, 1972. 6 S.R. Hanna, A Simple Method of Calculating Dispersion from Urban Area Sources, J. Air Pollution Control Assoc. 21, pp. 774-777, 1971.