Paper to be submitted to Atmospheric Environment
January 3, 2000
Ozone Transport Climatology Over Eastern North American
Bret A. Schichtel and Rudolf B. Husar
Center for Air Pollution Impact and Trend Analysis (CAPITA)
Washington University, One Brookings Drive, St Louis, Missouri 63130-4899
Email: bret@mecf.wustl.edu; Fax: (314) 935-6145
ABSTRACT
The ozone transport climatology was established by relating high and low ozone
concentrations to their respective regional scale transport conditions during five summers from
1991-1995. The pattern of airmass transport was also established from characteristic source
influence areas over Eastern North America. Daily maximum ozone concentrations were used to
define locally and regionally high (90th percentile) and low (10th percentile) ozone days. The
local regions were defined based on a ~160 km grid over the Eastern US and the regional area
was the OTAG domain. Examination of transport during locally high-ozone days showed that
dispersion in the central part of the Eastern US, i.e., from Tennessee to Northern Indiana, is
typically poor due to stagnating or recirculating air masses. However, the western and northern
sections of the domain experience stronger and more persistent southerly and westerly winds,
respectively with the characteristic transport from Texas through Iowa to New York. These
results support the notion that ozone exceedances in the central and Southern Eastern US are
predominately “homegrown” while the western and northern section of the domain are more
influenced by regional transport. In contrast, on locally low-ozone days, the transport was
predominantly from outside (e.g., Canada and the Gulf of Mexico) into the Eastern US. In
addition, high local ozone concentration surrounding the center of the Eastern US were
1
associated with average transport from that region. Regionally high ozone days were associated
with slow meandering or recirculating transport over Kentucky, Tennessee, and West Virginia,
with strong clockwise transport around this region.
Keywords: Ozone; Long-range transport; Local source impact; Airmass history; Lagrangian
particle model;
INTRODUCTION
In the Northeastern US, there is concern that ozone originating from distant upwind
sources significantly contributes ambient concentrations preventing areas from reaching
attainment of the National Ambient Air Quality Standard (NAAQS) using only local controls.
These concerns led to the establishment of the Ozone Transport Region (OTR) in the 1990 Clean
Air Act comprised of eleven states from Maine to Virginia. This provision called for VOC and
NOx controls throughout the region in order to reduce ozone transport to downwind areas
(Novello, 1992). By 1995 it was recognized that significant inter-state ozone transport takes
place over other regions and the region of concern was expanded to the entire Eastern US. This
lead to the establishment of the Ozone Transport Assessment Group (OTAG) whose purpose
was to examine the contribution of transported ozone to non attainment areas throughout the
Eastern US (OTAG, 1997).
A common technique for investigating the role of ozone transport has been to examine
the association between ozone concentrations and airmass transport direction and speed. The
airmass transport has typically been estimated from surface winds (Ludwig et al., 1977;
Mukammal et al., 1982; Vukovich 1995; Flaum et al, 1996; St. John and Chameides 1997;
Husar et al,. 1999) or synoptic scale airmass histories (Ludwig et al., 1977; Chung, 1977; Wolf
et al., 1977; Samson and Shi, 1988; Brankov et al., 1998; Poirot and Wishinski, 1998; Wishinski
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and Poirot, 1998). Clark and Clarke (1984) used a constant-level balloon, i.e. tetroon, and
aircraft sampling to track airmass transport and ozone concentrations along the Northeastern
seaboard.
A common finding among these studies is that high ozone in the central and southeastern
US are association with poor dilution (Ludwig et al., 1977; St. John and Chameides 1997; Husar
and Renard, 1998). Also, elevated ozone concentrations outside of these regions are associated
with higher speed transport (Brankov et al., 1998; Poirot and Wishinski, 1998; Samson and Shi,
1988; Ludwig et al., 1977; Husar and Renard 1988; Mukammal et al., 1982). In the episode
studied by Clark and Clarke (1984) long range transport of elevated ozone concentrations
occurred along the Northeast Corridor.
The above studies examined only one meteorological scale of transport, i.e. near surface
(surface wind analysis) and synoptic scale transport (airmass history analyses). However,
analyses examining three dimensional transport throughout the first few kilometers of the
atmosphere using upper air measurements and models (Husar et al., 1978; Lyons et al., 1995;
Blumenthal et al., 1998; McNider et al., 1998; Moore and Blumenthal, 1998) have shown
dramatically different transport directions and speeds with elevation. For example, analysis of
three dimensional meteorology in the Northeastern US during the summer of 1995 (Blumenthal
et al., 1998) revealed that elevated ozone above 800 m was associated with swift synoptic scale
transport from the west. However, slow transport along the eastern seaboard occurred along the
surface.
This paper presents an ozone transport climatology over an eastern North American
domain, consisting of the US east of the Rocky Mountains and Southeastern Canada. The ozone
transport climatology relates the high and low ozone concentrations to their respective synoptic
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scale transport conditions during five summers (June, July, and August) from 1991-1995. The
airmass transport is derived from regional source influence areas computed from forward airmass
histories, assuming a fixed pollutant exponential decay of ~ 2%/hr, i.e. a lifetime of one day.
The climatology is established for both locally and regionally high and low ozone
concentrations. While ozone concentrations are the result of the interaction between all
meteorological scales the focus was on synoptic scale transport because this analysis was to
identify evidence of transport on the scale of 1000 km.
This analysis addresses ozone transport in two ways. First, from the transport
climatology regions can be identified where the transport conditions are conducive to the
accumulation of ozone from local sources and other regions that may be influenced primarily by
regional scale transport. Second, unique transport pathways to a given region as well as common
pathways to multiple regions can be identified.
This analysis was originally conducted to support the deliberations of the OTAG Air
Quality Analysis workgroup. It complements other OTAG ozone transport studies using back
trajectory analysis (Brankov et al., 1998; Poirot and Wishinski, 1998; Wishinski and Poirot,
1998), surface wind speed and direction (Husar et al.. 1999), and the analysis of aircraft and
surface observations in the Northeast (Blumenthal et al., 1998 and Moore and Blumenthal, 1998,
Ray et al., 1998). The OTAG Air Quality Analysis Workgroup consensus conclusions from the
multiple studies were summarized in the Executive Summary (Guinnup and Collom, 1998).
DATA SOURCES AND PROCESSING
Ozone Data
The ozone data came from the North American Integrated Daily Maximum Ozone Data
Set (Schichtel and Husar 1998), which contains the daily maximum ozone for the entire U.S.
(1415 sites) and Canada (167 sites) from 1986 – 1996. Almost 700 sites are located in the
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Eastern US and Canada (Figure 3). All but seven of the eastern Canadian sites were within 200
km of the US-Canadian border. This data set was created by integrating ozone data from 7
networks. The majority of the data came from EPA's Aerometric Information Retrieval System
(AIRS), the Clean Air Status and Trends Network (CASTNet) and Canada's National Air
Pollution Surveillance Network (NAPS). The data set is an extension of the OTAG daily
maximum ozone data set (Husar and Husar 1998) used extensively in the OTAG air quality
analysis and model evaluation studies.
The daily maximum one hour average ozone concentrations were used to identify locally
and regionally high and low ozone days during the five summers (June – August) from 1991 –
1995. The high and low local ozone days were defined as days when the daily maximum ozone
over a source region was above and below the 90th and 10th percentiles respectively. The source
region's daily maximum ozone was calculated by spatially interpolating the ozone concentrations
to a 40 km grid which were then averaged over a 160 km grid defined in Figure 2. The spatial
interpolation used an inverse distance square weighted technique. The regional daily maximum
ozone concentrations were calculated by averaging the daily maximum ozone over all stations in
the OTAG domain (Figure 3) for a given day. Regionally high and low ozone days were defined
as days with regional ozone concentrations above the 90th percentile and below the 10th
percentile, respectively.
Meteorological Data
The transport climatology was generated from meteorological data from the National
Meteorological Center's Nested Grid Model (NGM) archived at the National Climatic Data
Center (Rolph and Draxler 1990; Rolph, 1992). This database contains three dimensional wind
fields, temperature and specific humidity as well as a number of surface variables, including the
mixing height. The data have a time resolution of 2 hours and are spatially configured on a polar
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stereographic grid covering most of North America with a grid size of approximately 160 km at
35 degrees latitude (Figure 2). The upper air data are positioned on ten terrain-following sigma
surfaces, from approximately 150 m to 7000 m.
Airmass Transport Calculations
The NGM data were used to drive the CAPITA Monte Carlo model (Schichtel and Husar
1996, Schichtel and Husar 1997). This model simulates airmass transport and diffusion by
tracking the movement of multiple particles released from a source. 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
which evenly distributes the particles between the surface and the mixing height.
The model was used to generate five day forward plumes from 506 sources evenly
distributed over most of North America (Figure 2) every two hours from 1991 through 1995.
The plumes were calculated by continually releasing three tracer particles from each source
every two hours and tracking their movement in space for five days or until they were
transported off of the NGM grid. At any instant in time, a plume identifies the downwind three
dimensional location of particles that were previously released from the source. For example,
Figure 3A presents a five day St. Louis, Missouri plume at noon on July 5 1995 impacting a
region from St. Louis to Minnesota and part of southern Ontario. The particles were released
from the source between 2 hours (black) and five days (light gray) prior to July 5 1995 at noon.
The twelve forward plumes in each day were grouped together creating daily five day forward
plumes (Figure 3B).
These daily plumes were aggregated together to create the airmass transport
climatologies. In order to account for different pollutant lifetimes, simple decay kinetics were
incorporated by weighting each particle by e-kt where k is the rate of decay with units inverse
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time and t is the particle age. This weighting process assumes that each particle starts with the
same initial mass which is then removed via transformation and deposition processes at a
constant rate of k. Under these ideal conditions, the average residence time of the emitted mass
is k-1, which will be referred to as the pollutant lifetime.
The aggregated kinetic plumes for the 1995 summer (June, July, and August) assuming a
pollutant lifetime of two days, i.e. k ~ 2%/hr, is presented in Figure 4. The particles have been
shaded based upon their percent remaining mass. During this time period, particles from the St.
Louis source impact virtually the entire Eastern US at one time or another. However, it is
evident that mass is preferentially transported to the east–northeast and south–southwest of the
source. Also, both the particle density and percent remaining mass decrease with increasing
distance from the source. The decrease in particle density is due to increased dilution, and the
percent remaining mass decreases due to increased decay with travel time. Due to the
combination of these two effects, the largest impact clearly occurs near the source.
Source Region of Influence
The transport information contained in the distribution of the particles in Figure 4 can
more easily be visualized by defining a boundary around the source encompassing the fraction 1/e
(~63%) of the ambient mass emitted by the source. The boundary encompasses the smallest area
possible, so the columnar concentration along this boundary is constant. This boundary is a
measure of the average distance traveled by the source mass before being removed and will be
referred to as the source region of influence or SRI.
The SRI was calculated by first summing the mass of each particle within the column of
each NGM grid cell. The largest grid cell masses were then summed together until 63% of the
mass was accounted for. The smallest grid mass value was taken as the concentration along the
SRI and an isopleth line was drawn on the grid of masses with this constant value.
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The size and shape of the SRI is due to a combination of the pollutant lifetime and mass
transport speed and direction. The influence of these factors is evident in the St. Louis, MO and
Atlanta, GA SRIs for the five summers 1991 – 1995 in Figure 5. As shown, the size of the SRI
increases with the pollutant lifetime. For example, the St. Louis source region of influence
extends to central Indiana for a one day lifetime, but for a two day lifetime it extends to western
Pennsylvania. The area encompassed by the SRI for a given pollutant lifetime is primarily
dependent on the speed mass is transported away from the source with larger transport speeds
resulting in larger SRI areas. The one day Atlanta SRI is about 40% smaller than St. Louis’
(Figure 5) indicating slower net transport speeds for the Atlanta source.
The St. Louis SRIs in Figure 5 are elongated to the northeast due to a higher fraction of
the emitted mass being transported in the northeast direction. This primarily results from higher
frequency of transport in the direction of the elongation. Changes in wind speed with transport
direction were found to be minimal, since the effects of wind speed and decay tend to offset.
High wind speeds horizontally dilute a plume decreasing the concentrations, but the transport
speeds reduce the time for decay, increasing the concentrations.
Transport Wind Vectors
An alternative means of representing the directionality of the net airmass transport is as a
transport wind vector. It is a vector whose magnitude is proportional to the length of the line
connecting the location of the source and the center of mass of all particles (Figure 5). Thus, the
transport wind vector identifies the direction and magnitude of the average mass transport
incorporating the variation of wind speed and direction with height and along the path of
transport. A short vector indicates that the source emissions impact almost equally in all
directions around the source, while a long vector results from preferential transport in a given
direction.
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The transport wind vector is a convenient means of simplifying the information contained
within the SRI. However, lost in this representation is any indication of the overall size of the
SRI, thus the speed of transport, and the fact that the source can and does impact regions not in
the direction of the transport wind vector. This can be seen in the schematic in Figure 6, where a
source with two very different SRIs has equivalent transport wind vectors
Benefits and Drawbacks
This methodology for characterizing airmass transport has several features and benefits.
First, it is source oriented, which allows for assessing the potential transport of mass from a
source to multiple down wind receptors. Second, the SRIs and transport wind vectors are
measures of the total horizontal dilution accounting for the combined influence of transport
speed and direction in the lowest few kilometers of the atmosphere. For example, poor dilution
conditions, such as recirculation and flow reversals, properly result in small SRIs and transport
vectors. Ventilating conditions such as high speed flow and persistent transport directions result
in larger SRIs and transport vectors. Also, changes in transport due to different pollutant
lifetimes is taken into account. Finally, the transport vectors easily convey the direction and
magnitude of the average mass displacement.
The primary drawback of this methodology is that it is essentially a two dimensional
analysis since it does not adequately account for vertical dilution. Also, the transport directions
and speeds derived from this technique are qualitative. Last, the meteorological data drivers are
able to characterize the regional scale transport, but they are poorly suited for the identification
of local flows dependent on complex surface meteorology.
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RESULTS AND DISCUSSION
Average Summertime Transport Climatology
The transport wind vectors and SRIs for five urban and industrial regions for the average
summer day from 1991– 1995 are presented in Figure 7. A one day pollutant lifetime was used,
since the lifetime of ozone peak values is about one to two days (Neu et al., 1994). As shown by
the SRIs, transport occurs in all directions throughout the Eastern US, but there is a prevailing
transport direction in all regions of the eastern North American domain. The prevailing transport
direction in the western part of the domain, Texas to Nebraska, is to the north-northeast. East of
this region, the prevailing transport shifts eastward, and beyond the Mississippi River it is
primarily to the east. Along the Atlantic coast the prevailing transport direction shifts from the
southwest.
The source region of influences increase in size from south to north. For example, the
Atlanta Georgia source region of influence is about 50% smaller than Chicago's. This is an
indication that the average transport speeds are lower in the South than the rest of the East. This
transport pattern is similar to the average transport patterns found in other climatological studies
of surface pressure and surface and upper air measured winds (Holzworth, 1972, Bryson and
Hare, 1974; Wendland and Bryson, 1981; Husar, 1985).
Transport Climatology During High and Low Local Ozone Days
The transport climatology during local high and low ozone days are presented in Figure
8. During the high ozone days (Figure 8A), the size of all of the SRIs decreased compared to the
average summertime conditions (Figure 7) indicating slower average transport. This decrease is
largest in the South (east Texas to South Carolina) and the center of the domain from Tennessee
to North Indiana, where the SRIs decreased by ~50%. The small SRIs in these regions are
associated with short and/or variable transport wind vectors. In the western and northern parts of
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the domain, the transport wind vectors are aligned and are longer than those during average
conditions. In the Great Planes, the airmass transport is to the north, while from the Great Lake
States to New England, southeastern Canada and along the Atlantic coast to North Carolina the
transport is persistently from the west-southwest. Also, the SRIs at the two northern sites,
Chicago and New York, are about twice as large as those in the South. Interestingly,
surrounding the center of the domain, northern Ohio, southeast Missouri, Tennessee, and West
Virginia, all of the transport wind vectors point outward from this region. In other words, on
average high local ozone concentration in those states occur when the winds are from this central
region.
During the low ozone days (Figure 8B), the size of the SRIs are larger than on average
along the border of the Eastern US and about equal in the interior to those during an average
summer day (Figure 7). Along the borders of the US at Canada, Atlantic Ocean, and the Gulf of
Mexico the transport wind vectors are aligned pointing into the US from the outside and are
longer than average. Hence, low ozone concentrations in these areas occur during swift winds
from outside the Eastern US. In the interior, Illinois to Pennsylvania, the average transport is
from the west.
The transport climatology on high ozone days are indicative of poor airmass dispersion in
the central and southern regions of the Eastern US resulting in transport of mass about half to
two thirds the distance compared to an average day. In the other parts of the domain, more
persistent ventilating transport from within the Eastern US domain occurs that is capable of
transporting mass longer distances. These results are consistent with the notion that elevated
ozone in central and southeastern areas are predominantly "homegrown" while the other parts of
the domain are also influenced by regional transport.
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Contrasting the transport during high and low ozone days, it is evident that the low ozone
days over the Eastern US are associated with swift persistent transport predominately from
outside the Eastern US, while days with high local ozone have either stagnant transport or
transport occurring from within the Eastern US. Therefore, elevated ozone in the Eastern US is
associated with airmasses that primarily reside within the domain. On a smaller scale, the central
part of the domain also appears to be a common transport pathway for its neighboring regions
during high ozone days. Similar results were found from regional scale analysis (Poirot and
Wishinski, 1998; Husar and Renard, 1998) and analysis of surface winds at Ontario (Mukammal
et al., 1982).
Transport Climatology During Regionally High and Low Ozone Days
The regionally high ozone episodes (Figure 9A) are characterized by slow meandering or
recirculating transport over Kentucky, Tennessee, and West Virginia, with a strong clockwise
transport around this region. This flow pattern is consistent with the flow pattern of a large
stagnating high pressure system over the Eastern US. It has been shown by numerous studies
that Eastern US regional ozone episodes are associated with high pressure systems (Stasuik and
Coffey, 1974; Wolff et al., 1977; Vukovich et al., 1977; King and Vukovich, 1982; Ludwig et
al., 1977; Husar et al., 1977; Vukovich, 1979; Vukovich and Fishman, 1986; Vukovich, 1995).
During the regionally low ozone days (Figure 9B), the Southeast is ventilated by strong
westerly - southwesterly flow which brings in air from the Gulf of Mexico. The flow pattern
turns to more southwesterly flow along the Atlantic coast. The north central part of the domain
over Wisconsin and Michigan is dominated by northerly transport. In New England, substantial
transport occurs in all directions as shown by the New York SRI, but the average transport is
from the west - southwest. The flow west of the Mississippi is characterized by clockwise
transport centered over eastern South Dakota, Nebraska, and western Iowa.
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CONCLUSIONS
Examination of transport conditions during locally high-ozone days showed that transport
in the central part of the Eastern US is typically poor due to stagnating or recirculating air
masses. However, the western and northern sections of the Eastern US experience stronger and
more persistent southerly and westerly winds, respectively. These results support the notion that
ozone exceedances in the Central and Southeastern areas are predominately “homegrown” due to
local sources, while the western and northern section of the domain are more influenced by
regional transport. In contrast, on low-ozone days, the transport was predominantly from outside
(e.g., Canada and the Gulf of Mexico) into the Eastern US. In addition, high local ozone
concentrations surrounding the center of the Eastern US were associated with average transport
from that region.
Regionally high ozone days were associated with slow meandering or recirculating
transport over Kentucky, Tennessee, and West Virginia, with strong clockwise transport around
this region. This flow pattern is consistent with the circulation pattern of a large high pressure
systems over the Eastern US.
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FIGURE CAPTIONS
Figure 1. The ozone monitoring site locations in the Eastern US from the AIRS, NAPS, CASTNet, and other
smaller monitoring networks.
Figure 2. The grid used by the NGM meteorological data set. The grid lines cross at the center of each grid
cell. The squares identify the location of the 506 sources from which plumes were calculated.
Figure 3. A) The St. Louis five day plume for noon July 5, 1995. B) The St. Louis daily five day plume for
July 5, 1995. The daily plume was created by combining the 12 five day plumes generated every two hours
from 2 AM to midnight on July 5, 1995.
Figure 4. The merging of the five day forward plumes from St. Louis during June, July, and August 1995.
The plume particles have been colored based upon their percent remaining mass assuming a two day lifetime,
i.e. a constant decay rate of 2.1 %/hr. The white line marks the boundary of the source region of influence.
Figure 5. St. Louis, MO (A) and Atlanta GA (B) source regions of influence during June – August, 1991 –
1995 for a pollutant with one and two day lifetimes.
Figure 6. High and low speed transport condition producing equal transport vector.
Figure 7. Transport wind vectors and source regions of influence for June – August, 1991 – 1995. The source
regions of influence are for the nearest modeled source to Atlanta, GA, Houston, TX, Chicago, IL, Ohio River
Valley, and New York, NY.
Figure 8. Transport wind vectors and source regions of influence for the highest (A) and lowest (B) 10% of
local ozone days during June – August, 1991 - 1995. Local ozone is the daily maximum ozone averaged over
each source region. The source regions of influence are for the nearest modeled source to Atlanta, GA,
Houston, TX, Chicago, IL, Ohio River Valley, and New York, NY.
Figure 9. Transport wind vectors and source regions of influence for the highest (A) and lowest (B) 10% of
regional ozone days during June – August, 1991 - 1995. Regional ozone is the average daily maximum over
the OTAG domain. The source regions of influence are for the nearest modeled source to Atlanta, GA,
Houston, TX, Chicago, IL, Ohio River Valley, and New York, NY.
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Figure 1. The ozone monitoring site locations in the Eastern US from the AIRS, NAPS, CASTNet, and other
smaller monitoring networks.
Figure 2. The grid used by the NGM meteorological data set. The grid lines cross at the center of each grid
cell. The squares identify the location of the 506 sources from which plumes were calculated.
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Figure 3. A) The St. Louis five day plume for noon July 5, 1995. B) The St. Louis daily five day plume for
July 5, 1995. The daily plume was created by combining the 12 five day plumes generated every two hours
from 2 AM to midnight on July 5, 1995.
Figure 4. The merging of the five day forward plumes from St. Louis during June, July, and August 1995.
The plume particles have been colored based upon their percent remaining mass assuming a two day lifetime,
i.e. a constant decay rate of 2.1 %/hr. The white line marks the boundary of the source region of influence.
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A
B
Figure 5. St. Louis, MO (A) and Atlanta GA (B) source regions of influence during June – August, 1991 –
1995 for a pollutant with one and two day lifetimes.
Equal Transport Vectors
High speed transport
occurring in all directions
Low speed transport
predominately to the east
Figure 6. High and low speed transport condition producing equal transport vector.
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Figure 7. Transport wind vectors and source regions of influence for June – August, 1991 – 1995. The source
regions of influence are for the nearest modeled source to Atlanta, GA, Houston, TX, Chicago, IL, Ohio River
Valley, and New York, NY.
20
A
B
Figure 8. Transport wind vectors and source regions of influence for the highest (A) and lowest (B) 10% of
local ozone days during June – August, 1991 - 1995. Local ozone is the daily maximum ozone averaged over
each source region. The source regions of influence are for the nearest modeled source to Atlanta, GA,
Houston, TX, Chicago, IL, Ohio River Valley, and New York, NY.
21
A
B
Figure 9. Transport wind vectors and source regions of influence for the highest (A) and lowest (B) 10% of
regional ozone days during June – August, 1991 - 1995. Regional ozone is the average daily maximum over
the OTAG domain. The source regions of influence are for the nearest modeled source to Atlanta, GA,
Houston, TX, Chicago, IL, Ohio River Valley, and New York, NY.
22