9B.13
NORTH AMERICAN QUARTERLY SOURCE TO RECEPTOR TRANSPORT PARAMETERS:
TRANSPORT SPEED, HEIGHT, AND DIRECTION FREQUENCY
Bret Schichtel* and Rudolf Husar
Washington University St. Louis Missouri
1.0 INTRODUCTION
The relative source contribution of mass to
a receptor, source receptor relationship (SRR), is
due to a combination of transport and kinetic
processes. The specific processes controlling the
source contribution vary in space and time, and
can only be determined by breaking the SRR into
characteristic transport and kinetic parameters. In
this work, we focus on the transport components
deriving parameters characterizing the speed of
transport, vertical dilution, and transport direction
frequency from regional scale source to receptor
transport calculations. These transport
parameters are presented as North American
quarterly averages for 1991 – 1995. This
extended abstract further defines the relevant
transport parameters and calculation
methodology, and illustrates them using third
quarter 1995 results.
America. A source plume can be viewed as a
direct simulation of a smoke stack’s plume of inert
material. The plumes are calculated by continually
releasing three tracer particles from each source
every two hours, and tracking their movement for
12, 24, 48, or 96 hours (Figure 1). At any instance
in time, the plume identifies the downwind three
dimensional location of particles that were
previously released from the source. Thus, the
relative source particle contribution to a receptor is
simply the sum of the plume’s particles in the
receptor volume divided by the total number of
plume particles.
2.0 METEOROLOGICAL DATA AND MODELED
SOURCE PLUMES
The transport parameters are calculated
using synoptic scale meteorological fields from the
National Meteorological Center’s Nested grid
Model (NGM) (Rolph, 1992). The NGM database
contains three dimensional wind vectors with a
horizontal resolution of ~160 km and ten vertical
layers up to seven kilometers at a two hour time
step. The lowest layer has a height of ~160
meters.
The NGM data is used to drive a Monte
Carlo particle dispersion model (Schichtel and
Husar 1996). The model simulates atmospheric
transport and diffusion by tracking the movement
of multiple “particles” released from sources. The
NGM wind fields are used to advect the particles
in three dimensional space, while the intense
vertical mixing that takes place within the
atmospheric boundary layer is simulated using a
Monte Carlo technique.
The model is used to generate 12, 24, 48,
and 96 hour source plumes every two hours from
504 sources evenly distributed over most of North
*Corresponding
author address: Bret A. Schichtel,
Washington Univ., CAPITA, St. Louis, MO
63130-4899; e-mail: bret@mecf.wustl.edu
Figure 1. A noon St. Louis plume on August 16,
1992.
3.0 TRANSIT PROBABILITY DENSITY
FUNCTION, Pt
The source to receptor transport is
quantified via the transit probability density
function (pdf), Pt, the relative source contribution
of an inert species to surrounding receptors per
unit receptor volume (Lamb and Seinfeld 1973;
Schichtel and Husar 1996). Surface transit pdfs
are created for each source plume by calculating
the plume’s relative source contribution to the first
layer of NGM grid cells and normalizing by the grid
cell volume. Column transit functions are also
calculated from the plume’s relative source
contribution to each NGM grid column and
Figure 2a. St. Louis 2 PM column transit pdf
during quarter 3 1995. The transit pdf was
calculated using 2 day plumes.
normalized by the grid cell area. The semi-hourly
transit functions are then aggregated together over
specified time ranges. Figure 2 presents the
average 2 PM St. Louis column and surface transit
pdfs during the third quarter (July, August,
September) of 1995 and a maximum plume
lifetime of two days.
4.0 CHARACTERISTIC TRANSPORT
PARAMETERS
The magnitude of the transit pdf is
inversely dependent on speed of transport to the
receptor and the degree of vertical dilution of
particles at the receptor, and it is directly
dependent on the frequency mass is transport in
the direction of the receptor. The transit pdf is
also inversely dependent on the source to receptor
distance. This dependence is not associated with
transport, but is due to the geometry of the
problem. All three transport parameters vary with
each source receptor pair. However, single
characteristic transport speeds and heights are
derived for the speed and vertical dilution
parameters. The transport parameters are derived
such that the transit pdf can be reconstructed from
them via the equation:
Pt ij = Fij / (Uj * Hj * rij)
(1)
where Pt Surface transit pdf
F Transport direction frequency
U Characteristic transport speed
H Characteristic transport distance
r Source to receptor distance
Figure 2b. St. Louis 2 PM surface transit pdf
during quarter 3 1995. The transit pdf was
calculated using 2 day plumes.
i
j
Receptor index
Source index
4.1 Characteristic Transport Speed, U
The transport speed is a measure of the horizontal
dilution of mass from a source, and the extent of
horizontal transport of the mass in a fixed amount
of time. The transport speed for each source to
receptor pair is estimated from the source to
receptor distance divided by the average particle
age at the receptor. A single characteristic source
transport speed is calculated by averaging
together all source to receptor transport speeds
weighted by the number of particles at the
receptors.
The influence of the transport speed on
the transit pdf is evident by the outer boundary of
the two day column transit function in Figure 2a.
This boundary marks the maximum distance the
plume particles travel in two days. If the transport
speeds were larger, then the transit pdf would
extend beyond this boundary spreading the plume
particles over a larger area. This horizontal
dilution reduces the overall transit pdf.
The third quarter 1995 North American
transport speed fields calculated from 2 PM
plumes for each modeled source are presented in
Figure 3. As shown, the lowest speeds (4-5 m/s)
are throughout the south from Southern California
to South Carolina. In this region a band through
Texas and Oklahoma have speeds greater than 5
m/s. The highest transport speeds are in northern
US and southern Canada where they are between
6-7 m/s.
Figure 4. North American 2 PM characteristic
transport height during the third quarter of 1995,
assuming a pollutant lifetime of 2 days.
4.3 Transport Direction Frequency, F
Figure 3. North American transport 2 PM speeds
during the third quarter of 1995, assuming a
pollutant lifetime of 2 days.
4.2 Characteristic Transport Height
As a plume is transported from the source
to the receptor the particles are vertically diluted
via intense afternoon mixing. The greater the
extent of vertical dilution the fewer particles at the
surface, and the lower the transit pdf. The extent
of the vertical dilution can be quantified by way of
a characteristic transport height, H, which is the
average plume particle height. Figure 4 presents
the third quarter 1995 North American
characteristic transport heights calculated from 2
PM plumes for each modeled source. The largest
transport heights are in the Southwest where they
can exceed 2000 m in Utah and Arizona. The
transport heights are lower and more uniform
throughout the Eastern US, 1200 – 1600 m.
The transport direction frequency, F, is the
probability that the plume particles will be
transported in a given direction. Plume particles
transported away from a source normally do not
follow straight trajectories. Therefore, the
transport direction frequency is dependent on the
direction and radial distance from the source, and
will vary for each source receptor pair.
The transport direction frequency is
calculated by solving equation 1 for Fij:
Fij = Pt ij * Uj * Hj * rij
(2)
In addition to the directional frequency of
transport, Fij encompasses differences between
the characteristic transport speed and height and
the transport speed and height for each source
receptor pair.
The 2 PM transport direction frequency,
during the third quarter of 1995 and a maximum
plume lifetime of two days, is presented in Figure
5. As shown, the transport from St. Louis occurs
most frequently to the north – northeast.
5.0 Future uses
The database of transport parameters
constitutes a resource that is ideally suited for
investigating transport causes of modeled source
contributions. Also, this database can be used to
investigate the relationship between transport and
air quality. For example, by sorting and
aggregating the transport parameters based upon
the air quality at a source region questions such
as “from what direction does transport occur and
at what speed when the ozone concentrations are
high?” can be asked. The answers to such
question can provide insights into the likely
directions and upwind distances that source
regions can contribute to the air quality.
The source contribution of mass to a
receptor is due to a combination of transport
processes carrying the source mass to the
receptor and kinetic processes chemically
transforming and physically removing the mass
during transport. If it is assumed that meteorology
is not significantly influenced by trace constituents
in the atmosphere, then transport is taken as
being independent of the kinetic processes.
By breaking the source receptor
relationship into characteristic transport and kinetic
parameters it is possible to identify causal factors
for modeled source contributions, as well as
identifying potential receptor regions of a source’s
emissions. In this work, we focus on the transport
component of the source receptor relationship,
and deriving transport speed, transport direction,
and vertical dilution parameters from source to
receptor transport . These transport parameters
are presented as North American seasonal
averages during 1991 – 1995.
If it is assumed that meteorology is not
significantly influenced by trace constituents in the
atmosphere, then transport can be taken as being
independent of the kinetic processes
with equal probability of being transported
in any direction and remain at the surface, then
the transit pdf to a receptor at distance r from the
source is proportional to the ratio of the receptor
length, l, and the circumference of a circle with
radius equal to the source to receptor distance, r,
The chance of any randomly selected plume
impacting a receptor will decrease as the receptor
distance increases.
4.1 Source to Receptor Distance
The inverse dependence on source to
receptor distance is due to the geometry of the
problem. All plumes must impact the receptor cell
the source is located in. It must also impact at
least one of eight cells immediately surrounding
the source, at least one of the 16 grid cells
immediately surrounding the eight grid cells etc.
In an ideal case where all particle trajectories are
straight and remain at the surface, it can be shown
from mass conservation that the transit probability
is inversely proportional to source to receptor
distance. The decrease in transit probability with
distance is clearly seen in Figure 2.
The transport speed of interests is
dependent on the rate mass is transport away
from the source to the receptor. Therefore, the
transport speed is defined as the source to
receptor distance divided by the average particle
age at each receptor. The transport speed varies
with each source receptor pair. These multiple
speeds were reduced to a single characteristic
transport speed by averaging together all transport
speeds to each receptor weighted by the number
of particles at the receptors. The influence of the
transport speed on the transit functions is evident
by the outer boundary of the two day column
transit function in Figure 2. This boundary marks
the extent the plume particles travel in two days. If
the transport speeds were larger, then the transit
pdf would extend beyond this boundary spreading
the plume particles over a larger area. This
horizontally dilution would reduce the overall
transit pdf.