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 resultant 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 Assessment Group (OTAG) in 1995. This was a cooperative regional program
whose objectives were to foster thoughtful assessment of, and develop consensus solutions to the
problem of regional scale ozone and its transport to non-attainment areas1. 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.
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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 resultant 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
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.
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.10,11 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 model12. 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 and time. The particles were released at the surface and
tracked 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 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.
2
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.
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.
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Ozone Data
The ozone data used in this analysis came from the North American Integrated Ozone Daily
Maximum Data Set, 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 1). All but seven of the Eastern Canada sites were within 200 km of the USCanadian border. This data set was created by integrating ozone data from 7 networks including
EPA's Aerometric Information Retrieval System (AIRS) and Canada's National Air Pollution
Surveillance Network (NAPS). The data set is an update of the OTAG daily maximum ozone
data set which is described by Husar and Husar13.
The daily maximum one hour average ozone concentrations were then used to identify locally
and regionally high and low ozone days, during the five summers June – August, 1991 – 1995.
The locally high and low 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
ozone concentrations for each source region 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 weighted square Falke and Husar14. The regional daily
ozone concentrations were calculated by averaging the daily maximum ozone over all stations in
the OTAG domain (Figure 3). The regionally high and low ozone days were defined as days
with regional ozone concentrations above the 90th percentile and below the 10th percentile,
respectively.
Figure 3. The ozone monitoring site locations in the Eastern US with some valid data from 1991 through 1995 from
the AIRS, CASTNet, and other smaller monitoring networks. The OTAG domain is identified by the box.
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
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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.
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
5
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 5 m/s at Atlanta compared to 6 m/s at St. Louis.
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. The magnitude of the vector is
proportional to the distance between the source and center of mass (Figure 5). The transport
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wind vector indicates the general speed and direction of transport from the source. It can be
thought of as an average wind vector, but it incorporates the variations of wind speed and
direction with height, along the path 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 direction and speed 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
resultant 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
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 transport accounting for the combined influence of transport speed and direction
in the lowest few kilometers of the atmosphere. Also, the change in the transport condition due
to different pollutant lifetimes is taken into account. Last, the transport vectors are easily
understandable immediately conveying the direction and magnitude of the resulting 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.
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RESULTS
Transport Condition for 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.
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.
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 Conditions for High and Low Local Ozone Days
Transport conditions during locally 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.
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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. These transport conditions are indicative of poor airmass
dispersion resulting in transport of mass about half the distance in a given period of time
compared to an average day. Although the transport wind vectors throughout the Central East
are short, a common pattern is that along the border of this region, (e.g. northern Ohio, southeast
Missouri, Tennessee, and West Virginia) the transport wind vectors point outward from this
region. Therefore, the highest ozone concentrations around the Central East have resultant mass
transport from the Central East domain. The resultant transport directions shift from the Central
East as one moves away from its boundary, except along the east-northeast and southwestern
portions of the boundary.
In the west and northern parts of the domain in Figure 8A all 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 . These transport conditions are indicative of more persistent ventilating transport from a
given direction capable of transporting mass longer distances than in the southern and central
part of the Eastern US.
During the low ozone days (Figure 8B), the size of the SRIs are about equal to those during the
average ozone days. However, all along the borders of the US at Canada, Atlantic Ocean, and
the Gulf of Mexico the transport wind vectors are pointing into the US from the outside and are
longer than average. These conditions indicate that the locally low ozone days over the Eastern
US are associated with higher speed persistent transport that is predominately from outside the
US.
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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 Conditions for High and Low Regional Ozone Days
Transport conditions 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 conditions 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. During the regionally low ozone
days (Figure 9B), the Southeast is ventilated by strong westerly - southwesterly flow which bring
in air from the Gulf of Mexico. This 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 flow. This northerly flow converges with the south-southwesterly flow over
Kentucky and Tennessee producing westerly flow. In New England, substantial transport occurs
in all directions as shown by the New York SRI. However, the resultant transport is to the eastnortheast. The flow west of the Mississippi is characterized by clockwise transport centered over
eastern South Dakota, Nebraska, and western Iowa.
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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.
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SUMMARY AND CONCLUSIONS
The ozone transport climatology over the Eastern North America was established by relating
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 were
established by determining regional source impact areas from forward airmass histories for ~320
sources. The source impact area was for a one day pollutant lifetime. Qualitative estimates of
airmass transport directions and speeds were then derived based upon the size and shape of the
source impact areas. The transport conditions were 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 regional scale airmass dispersion was poor due to airmass stagnation or recirculation in the
Central East from eastern Missouri to West Virginia and the Southeast from east Texas to South
Carolina. However, the remainder of the domain was associated with stronger more persistent
southerly flow from west Texas to Minnesota and southwesterly flow from Wisconsin to
Connecticut, southeastern Canada and along the Atlantic coast to North Carolina. 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 addition, all along the boundary of the Central East domain,
the resultant mass transport was from the Central East. The resultant transport directions shifted
from the Central East as one moved away from its boundary, except along the east-northeast and
southwestern portions of the boundary.
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. The Regionally low ozone days had northerly flow from Wisconsin and Michigan which
converged over Kentucky and Tennessee with swift westerly-southwesterly flow in the
Southeast. In New England, substantial transport occurred in all directions with resultant mass
transport to the east. The flow west of the Mississippi was characterized by clockwise transport
centered over eastern South Dakota, Nebraska, and western Iowa.
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