Eastern North America Transport Climatology During
Average, High and Low Ozone Days
Bret A. Schichtel and Rudolf B. Husar
Center for Air Pollution Impact and Trend Analysis, Washington University, One Brookings
Drive, Campus Box 1124, St Louis, Missouri 63130 bret@mecf.wustl.edu
ABSTRACT
The ozone transport climatology over Eastern North America, the Eastern US and Southeastern
Canada, was established by relating high and low ozone concentrations to their respective
regional scale transport conditions during five summers (June, July, and August) from 19911995. Qualitative estimates of airmass transport directions and speeds were derived from
regional source impact areas. The airmass transport was established for both locally and
regionally high (90th percentile) and low (10th percentile) daily maximum ozone concentrations.
The local regions were defined based on a ~160 km grid over the Eastern US and the regional
area was the OTAG domain. The locally high ozone days were associated with transport from
within the Eastern US. In contrast, locally low ozone days were associated with transport
predominately from outside the Eastern US, e.g. Canada, Atlantic Ocean, and Gulf of Mexico.
During the locally high-ozone days dispersion in the Southeast, east Texas to South Carolina and
the Central East, eastern Missouri to West Virginia, was typically poor due to stagnating or
recirculating air masses. However, the western and northern sections of the domain experienced
stronger and more persistent southerly and southwesterly transport, respectively. Also, along the
boarders of the Central East average mass transport was from the Central East, indicating this as
a common transport pathway for elevated ozone. The high regional ozone days were associated
with slow meandering transport over Kentucky, Tennessee, and West Virginia with strong
clockwise transport around this region of stagnation. The low regional ozone days had higher
speed southwesterly transport from the Gulf of Mexico and northwesterly transport from Canada
into the Eastern US.
INTRODUCTION
Regional scale transport of ozone and its precursors has been an area of considerable interest in
recent years. In the Northeastern US, there is concern that ozone originating from distant
upwind sources significantly contributes to their non-attainment areas preventing them from
reaching attainment using only local controls. These concerns led to the establishment of the
Ozone Transport Region in the 1990 Clean Air Act comprising of states from Maine to Virginia.
This new provision called for VOC and NOx controls throughout the region in order to reduce
ozone transport to downwind areas (Novello, 1992). In 1995 the region of concern was
expanded to the entire Eastern US with 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 directions 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 airmass histories using synoptic scale winds (Ludwig et al., 1977; Chung,
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1977; Wolf et al., 1977??check; Samson and Shi, 1988; Brankov et al., 1998; Poirot and
Wishinski, 1998; Wishinski and Poirot, 1998). However, recently several analyses have
examined the transport throughout the lowest few kilometers of the atmosphere using surface
and upper air measurements (Lyons et al., 1995; Blumenthal et al., 1998; Moore and
Blumenthal, 1998; Ray et al., 1998; St. John and Chameides 1997??). A novel approach was
employed by Clark and Clarke (1984) were they used a constant-level balloon, i.e. tetroon, and
aircraft sampling to track airmass transport and ozone concentrations along the Northeast
seaboard. With the exception of Husar et al., 1999. All of these analyses to date have focus on
individual sites or regions or select sites throughout the Eastern North America no analysis has
examined the ozone transport association over the entire Eastern North America.
These studies have generally examined only one meteorological scale of transport, such as near
surface (surface wind analysis) or synoptic scale transport (airmass history analyses). However,
examination of the three dimensional wind fields have shown dramatically different transport
directions and speed with elevation (Husar et al., 1978; McNight??(nocturnal jet) Blumenthal et
al., 1998 and Moore and Blumenthal, 1998, Ray et al., 1998). Analysis of three dimensional
meteorology in the Northeastern US during the summer of 1995 (Blumenthal et al., 1998)
revealed that long range transport of pollutants was associated with synoptic scale meteorology.
However, channeled flows and nocturnal jets, which occur below the synoptic scale, also
influenced long range transport, and local flow were typically associated with near surface
transport.
Points
1) regional analysis
2) synoptic winds (this is actually a good thing) The focus of this analysis was to identify
evidence of regional scale transport therefore the synoptic scale was the appropriate
meteorological scale to examine.
3) Looking at the tails.
4) I am unique inthat I use a new texhinge for characterizint the tranport based upon forward
airmass histories. Also the analsis examines the ozone - airmass transport over the entire
Eastern North America as opposed to select sites.
This paper presents an ozone transport climatology over Eastern North America, the Eastern US
and Southeastern Canada conducted for the OTAG AQA. The ozone transport climatology
relates the average, high and low ozone concentrations to their respective regional scale transport
conditions during five summers (June, July, and August) from 1991-1995. The transport
conditions are estimated from regional source impact areas computed from forward airmass
histories assuming a finite pollutant lifetime. Qualitative estimates of the airmass transport
directions and speeds are then derived from the source impact areas and presented as a source
region of influence, a boundary encompassing the source impact, and as transport wind vectors,
vectors of the average airmass transport direction and speed from the source. The climatology is
established for both locally and regionally high and low ozone concentrations.
This analysis addresses ozone transport in two ways. First, from the transport climatology it is
possible to identify regions where the transport conditions are conducive to the accumulation of
ozone from local or sub-regional sources, as well as regions which may be influence by regional
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scale transport. Second, by contrasting the transport conditions during average, high and low
ozone concentrations unique transport pathways for a given region, as well as common pathways
for multiple regions can be identified.
While these techniques and meteorological data drivers are able to characterize regional scale
transport, they are poorly suited for, and not intended for identification of local flows or near-by
source influences.
This analysis was originally conducted for the OTAG AQA Workgroup and was supported by a
number of other analyses including…………….
This report was prepared to support the deliberations of the OTAG Air Quality Analysis
workgroup. The current analysis can be viewed as a complement to the OTAG ozone transport
studies using back trajectory analysis (Poirot and Wishinski, 1996), forward trajectory and
regional impact analysis (Schichtel and Husar, 1996) and analysis of aircraft and surface
observations in the Northeast (Blumenthal et al., 1997).
A number of technical analyses were conducted in support of OTAG. These included intensive
photochemical grid modeling of four regional episodes by the Regional and Urban Scale
Modeling Workgroup (RUSM) and the analysis of air quality and meteorological data by the Air
Quality Analysis Workgroup (AQA). The AQA studies included analyses using back
trajectories2,3,4, surface wind speed and direction5, analysis of aircraft and surface observations in
the Northeast6, as well as ozone spatial and temporal pattern analysis7,8. The consensus
conclusions from the multiple studies have been summarized in the OTAG Air Quality Analysis
Workgroup Executive Summary (Volume I)9
DATA SOURCES AND PROCESSING
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
stereographic grid covering most of North America with a grid size of approximately 160 km at
35 degrees latitudes (Figure 1). The upper air data are positioned on ten terrain following sigma
surfaces, from approximately 150 m to 7000 m.
The NGM data were used to drive the CAPITA Monte Carlo model (Schichtel and Husar 1996,
Schichtel and Husar 1998??). 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 1) 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 transport off of the NGM
grid. At any instance in time, a plume identifies the downwind three dimensional location of
particles that were previously released from the source. For example, Figure 2A, presents a five
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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.
Figure 1. The NGM grid. 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.
A
B
Figure 2. A) The July 5, 1995 noon five day St. Louis plume. B) The July 5, 1995 daily five day St. Louis plume.
The daily plume is comprised of the 12 plumes from 2 AM to midnight on July 5, 1995.
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The twelve forward plumes in each day were grouped together creating daily five day forward
plumes (Figure 2B). The resulting database of daily plumes contained the raw airmass transport
information that was filtered and aggregated together to create the transport climatologies.
Ozone Data
The ozone data used in this analysis came from the North American Integrated Ozone Daily
Maximum Data Set (Schichtel and Husar 1998 reference web doc), 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 Eastern US and Canada (Figure 3). All but seven of the
Eastern Canada sites were within 200 km of the US-Canadian border. This data set was created
by integrating ozone data from 7 networks including EPA's Aerometric Information Retrieval
System (AIRS) (ref??), CASTNet (ref??) and Canada's National Air Pollution Surveillance
Network (NAPS) (ref??). 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 local and
regional high and low ozone days, during the five summers June – August, 1991 – 1995. The
high and low local ozone days were days with the daily maximum ozone at each source region,
defined in Figure 1, above and below the 90th and 10th percentiles respectively. The source
region daily maximum ozone was calculated by spatially interpolating ozone concentrations to a
40 km grid which were then averaged over each NGM grid cell. The spatial interpolation used
an inverse distance square weighted technique (Falke and Husar, 1996). The regional daily
maximum ozone concentrations were calculated by averaging the daily maximum ozone over all
stations in the OTAG domain (Figure 3). The high and low regional ozone days were defined as
days with regional ozone concentrations above the 90th percentile and below the 10th percentile,
respectively.
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Figure 3. The ozone monitoring site locations in the Eastern US with some valid data from 1991 through 1995 from
the AIRS, NAPS, CASTNet, and other smaller monitoring networks. The box identifies the boundaries of the
OTAG domain.
METHODOLOGY
Airmass transport climatologies were calculated by filtering and aggregating the daily plumes to
derive estimates of the regional source impact. In order to account for different pollutant
lifetimes, simple decay kinetics were incorporated by weighting each particle by e-k where k is
the rate of decay with units inverse time and  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 1/k, 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
colored based upon their percent remaining mass. During this time period, particles from the
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 St. Louis
source. Also, both the particle density and percent remaining mass decreases with increasing
distances from the source. The decrease in particle density is primarily due to the fact that the
further distance away from the source, the less likely the wind will blow in that direction. That
is, the angle a receptor of a fixed size makes in relation to the source decreases as the receptor is
moved further away from the source. Therefore, as the angle decreases transport within this
angle occurs less frequently and less mass will impact the receptor. The percent remaining mass
decreases due to increased travel time with distance from the source which allows for more mass
to be removed via the decay processes. Therefore, while the St. Louis emissions can impact
Maine the largest impact is nearer to the source.
6
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.
Source Regions of Influence
The transport information contained in the distribution of the particles in Figure 4 can more easily
be seen by defining a boundary around the source encompassing 1/e of the ambient mass emitted
by the source, i.e. approximately 63% of the mass. The boundary encompasses the smallest area
possible, so the columnar concentration along this boundary is constant. This boundary is
referred to as the source region of influence or SRI, and is denoted by the white line in Figure 4.
The selection of encompassing 1/e of the ambient mass within the SRI has the benefit that the
average time of transport to the SRI is approximately the pollutant lifetime. Thus, the SRI marks
the average distance traveled by the source mass before being removed. This is strictly true only
under the ideal conditions of straight particle trajectories with constant speeds.
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, at the St. Louis source the region of influence extends
to central Indiana for a one day lifetime, but for a two day lifetime it extends to 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 transport speeds for the Atlanta source. Slower Atlanta wind speeds are also seen in the
NGM data where during this time period the average wind speed within the first kilometer of the
atmosphere is 18 km/hr at Atlanta compared to 21.5 km/hr at St. Louis.
7
A
B
Figure 5. St. Louis, MO (A) and Atlanta Georgia (B) source regions of influence during June – August, 1991 – 1995
for a pollutant with one and two day lifetimes.
The St. Louis SRIs in Figure 5 are elongated to the northeast. This elongation is due to a higher
fraction of the emitted mass being transported further away from the source in the northeast
direction then say in the southwest direction. This primarily results from higher frequency of
transport in the direction of the elongation. The effects of wind speed and decay tend to offset
each other, since high wind speeds horizontally dilute the plume decreasing the concentrations,
but at a given distance the time for decay is less, increasing the concentrations.
Transport Wind Vectors
An alternative means of representing the source impact is as a transport wind vector, a vector in
the direction from the source to the center of the ambient mass where the vector magnitude is
proportional to the distance between the source and center of mass (Figure 5). Thus, the
transport wind vector identifies the direction and magnitude of the average mass transport. The
transport wind vector is related to an average wind vector that incorporates the variations of wind
speed and direction with height along the paths of transport, as well as with time. A short vector
indicates that the source emissions impact almost equally in all directions around the source,
while a long vector shows that the transport directions and speeds are balanced such that the
source emissions have a larger impact in the direction of the vector.
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 Figure 6, where a source with two
very different SRI’s has equivalent transport wind vectors
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Equal Transport Vectors
High speed transport
occurring in all directions
Low speed transport
predominately to the east
Figure 6. A high speed and low speed transport condition producing equal transport vector.
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 a measure 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 condition 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. Last, the transport vectors are easily understandable immediately conveying the
direction and magnitude of the average mass transport.
The primary drawback of this methodology is that it does not adequately account for vertical
dilution. Thus, it is essentially a two dimensional analysis. Also, the technique is qualitative. It
is not possible to quantify the airmass transport speeds and direction, but it is necessary to
interpret the SRIs and transport wind vectors at one location and time period as compared to
another. 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.
RESULTS AND DISCUSSION
Transport Climatology During Average Ozone Days
The transport wind vectors for the average summer ozone concentrations from 1991– 1995 for a
pollutant with a one day lifetime are presented in Figure 7. In this figure, SRIs for five urban
and industrial regions are overlaid the transport wind vectors. As shown by the SRIs, substantial
transport occurs in all directions throughout the Eastern US, but there is a prevailing transport
direction in all regions. 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
more eastward, and after the Mississippi River it is primarily to the east. However, along the
Atlantic coast the prevailing transport direction shifts to north-northeast.
9
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 at Chicago. This is an indication
that the average transport speeds are lower in the South than the rest of the East.
As one would hope this pattern mimics 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) where it has been
shown that the synoptic scale transport over the US is dominated by the Pacific and Atlantic
permanent high pressure systems and the Arctic airstream.
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 regions to Atlanta Georgia, Houston, TX, Chicago, IL, Ohio
River Valley, and New York, NY.
Transport Climatology During High and Low Local Ozone Days
The transport climatology during local high and low ozone days were constructed based upon the
highest and lowest 10% of the daily maximum ozone concentrations at each source region
(Figure 8). At each source region the highest and lowest ozone daily maximums usually
occurred on different days. Therefore, the transport conditions at each source region represent
transport over different time periods.
During the high ozone days (Figure 8A), the size of all of the SRIs decreased, compared to the
average conditions in Figure 7, indicating slower average transport. This decrease is largest in
the South (east Texas to South Carolina) and Central East (eastern Missouri to West Virginia),
where the SRIs decreased by ~50%. The small SRIs in these regions are associated with short or
meandering transport wind vectors. In the west and northern parts of the domain in Figure 8A all
10
of the transport wind vectors are aligned and longer than for the 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
to the east-northeast. Also, the SRIs at the two northern sites, Chicago and New York, are about
two times larger than those in the South. Around the border of the Central East (e.g. northern
Ohio, southeast Missouri, Tennessee, and West Virginia) all of the transport wind vectors point
outward from this region.
During the low ozone days (Figure 8B), the size of the SRIs are larger than average along the
border of the Eastern US and about equal to those during the average ozone days in the interior.
All 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. In
the interior, Illinois to Pennsylvania the average transport is to the east.
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 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.
Similar findings have been found from other climatological and episode studies. The association
of high ozone with poor dilution in the central and southeastern areas has been observed in a
number of other studies (Ludwig et al., 1977; Samson and Shi 1988; Chameides and Cowling
1995; St. John and Chameides 1997; Husar and Renard, 1998). The association of elevated
ozone and higher speed transport out side of the south and central east were also found from
airmass history analyses (Brankov et al., 1998; Poirot and Wishinski, 1998; Samson and Shi,
1988) and analysis of surface winds (Ludwig et al., 1977; Husar and Renard 1988; Mukammal et
al., 1982). Blumenthal et al., (1998) and Moore and Blumenthal, (1998) examined three
dimension airmass transport and ozone measurement during the summer of 1995 along the
Eastern Seaboard. They found that elevated ozone (>800 m agl) was associated with swift
synoptic scale transport from the west. However, slow transport along the eastern seaboard
occurred along the surface. Long range transport of elevated ozone concentrations along the
Northeast Corridor was also noted by Clark and Clarke (1984)
Contrasting the transport during the 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 US, while day with high local ozone has either stagnant or transport occurring from
within the Eastern US. Therefore, elevated ozone in the Eastern US is associated with airmasses
that primarily resided within the Eastern US domain. Similar results were found from the
regional scale analysis of Poirot, Husar and Renard (1988) and analysis of surface winds at
Ontario (Mukammal et al., 1982). Similarly, on a smaller scale, the Central Eastern US also
appears to be a common transport pathway for its neighboring regions during high ozone days.
The association of elevated ozone with transport from the Central East was first noted by Poirot
and Wishinski (1998) and was also found in the analysis by Husar and Renard (1988).
11
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 regions to Atlanta Georgia, Houston,
TX, Chicago, IL, Ohio River Valley, and New York, NY.
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Transport Climatology During High and Low Regional Ozone Days
Transport climatology during the regionally high and low ozone days were constructed for each
source from 10% of the days with the highest and lowest daily maximum ozone concentrations
averaged over the OTAG domain (Figure 9). The transport climatology at each source region
were constructed from the same subset of days.
The regionally high ozone episodes, presented in 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 high pressure system over the Eastern US. It has been shown by numerous
episodes and climatological studies that regional ozone episodes are associated with migratory
high pressure systems most of which move out of west-central Canada into the Central Eastern
US which then either become quasi stationary or move eastward to the Atlantic (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 dominate by northerly transport. In New England, substantial
transport occurs in all directions as shown by the New York SRI. However, the average
transport is to the east-northeast. The flow west of the Mississippi is characterized by clockwise
transport centered over eastern South Dakota, Nebraska, and western Iowa.
13
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 regions to Atlanta Georgia, Houston,
TX, Chicago, IL, Ohio River Valley, and New York, NY.
14
CONCLUSIONS
Examination of dispersion conditions during locally high-ozone days showed that dispersion in
the Central Southeast 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. These results support the notion that ozone
exceedances in the Central and Southeastern areas are predominately “homegrown” 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 domain. In addition, all along the boundary of the Central
East domain, the average mass transport was from the Central East.
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 that of a large high pressure system over the Eastern
US.
ACKNOWLEDGMENT
This work was support by the EPA grant……………… The AIRS data, CASTNet data, NAPS
data…………………..
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