Ozone as a Function of Local Wind Speed and Direction:
Evidence of Local and Regional Transport
Rudolf B. Husar and Wandrille P. Renard
Center for Air Pollution Impact and Trend Analysis (CAPITA)
Washington University
St. Louis, MO 63130-4899
July 26, 1997
Introduction
There is considerable evidence that ozone has both local as well as regional impacts up to 5001000 km from the source of its precursors. However, a full quantification of the ozone sourcereceptor relationship in an unambiguous and robust way has so far eluded air quality analysts.
The reasons include:
(1) Like all pollutants, ozone transport is subject to the complexities of horizontal, vertical and
mixing motion of the atmosphere.
(2) Being a secondary pollutant, ozone is formed at some distance from the precursor emissions
which makes the point of origin ambiguous.
(3) The chemical reactions that generate ozone also destroy some ozone in the photochemical
process, which makes the end point of transport ambiguous.
(3) The ozone precursor gases include NOx and reactive volatile organic compounds (VOCs)
which generally arise from different sources, further complicating the point of ‘origin’.
The complexities of atmospheric transport along with the fuzzy definitions of the beginning and
end times for transport constitute major conceptual barriers. As a consequence, major questions,
such as ‘is there ozone transport’ and ‘what is the approximate range of transport’ are justifiably
subject to considerable debate. Photochemical ozone models are useful in delivering overall
source-receptor relationships and results of emission scenarios but by themselves complex
models are of marginal use in advancing the conceptual framework for ozone transport analysis.
This report does not address the complete ozone source-receptor relationship. Rather it is aimed
at answering, in a very crude way, the simple questions ‘is there ozone transport’ and ‘what is the
approximate range of transport’ and ‘what are the major source areas’ for ozone. The
dependence of ozone concentration on transport is analyzed by classifying the existing ozone
concentration data into wind direction and wind speed bins, followed by concentration averaging
in each bin. Given long enough sampling record, say, ten years, the dependence of ozone on wind
direction and wind speed can be extracted statistically from the climatological records.
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 backtrajectory analysis (Poirot and Wishinsky, 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).
OTAG Mission and Goals
The mission of OTAG is to identify control strategies and implementation options for the
reduction of regional ozone over the eastern US The operational goals of OTAG are stated as (1)
A general reduction in ozone and ozone precursors aloft throughout the OTAG region and (2) a
reduction of ozone and ozone precursors at the boundaries of nonattainment areas.
Policy-Relevant and Scientific Results
It is suggested that the directionally and wind speed sorted ozone data analysis reported here can
serve the OTAG policy deliberations in several ways:
1. Source identification. The location of an ozone source can be "triangulated" using ozone
data sorted by wind direction. If the O3 concentrations at downwind monitoring sites
surrounding a source "point" to a common location as a source of elevated ozone, then it
can be stated that the particular source "causes" the elevated concentrations in its vicinity.
2. Imported vs. "homegrown" ozone apportionment. The directionally sorted ozone
concentrations can be utilized as a rough estimate of the magnitude of the local ozone at a
given site by estimating the excess concentrations just "downwind" from a local source.
3. Transported ozone estimation. During high wind speeds elevated ozone concentrations
can only be attributed to transported ozone. Hence, the technique can provide an estimate
of transported ozone.
Data Sources and Processing
Data Sources and Quality Control
The ozone data used in this report were collected from multiple sources:
Data Set
Supplying Organization
Years
AIRS
CASTNet
EMEFS
SCION
LADCO
EPA
EPA
Eulerian Model Evaluation and Field Study
Southern Oxidant Study
Lake Michigan Air Directors Consortium
1991-1995
1991-1995
1988
1993, 1995
1991 (88, 93, 95)
GEORGIA
NORTH CAROLINA
State of Georgia
State of North Carolina
1988, 91, 93, 95
Data from each network were extracted and combined into a single integrated data set. The
details of the data sources and quality control procedures are discussed in the report "Preparation
of Ozone Files for Data Analysis".
The first examination of average daily maximum ozone maps has revealed anomalous ozone
"holes" and peaks at unexpected locations. For those sites the hourly and daily maximum ozone
values were re-examined for possible inconsistencies. Sudden systematic changes in the ozone
concentrations, as well as major deviation from neighboring sites were the main clues for
anomalous behavior. As a result of this quality control process, 6 out of 709 monitoring sites
were discarded. The remaining data were used in all the subsequent computations exactly as
submitted by the networks.
Data Processing Procedures
The data processing was conducted in the following major steps below:
1.
2.
3.
4.
Data from individual networks were quality controlled and formatted uniformly.
The hourly ozone data from all the networks were combined into a single database.
The daily maximum (1-hour average) ozone was extracted from the hourly data.
For each monitoring station the average, percentiles and exceedances of daily maximum
ozone was computed.
5. The results for all stations were contoured and plotted on maps and for easy presentation.
In this analysis the ozone data for the 1986-1995 were merged with wind direction and wind
speed data from meteorological monitoring sites. For every ozone monitoring site the nearest
meteorological monitoring site was identified and assigned to the ozone site. If the closest
meteorological site did not have direction and wind speed, then the next closest meteorological
site was selected.
The wind direction and wind speed was obtained from the National Weather Service synoptic
monitoring network, consisting of about 300 sites in the conterminous US. The wind direction
and wind speed represent the surface observations at local noon.
The ozone concentrations have been sorted and averaged for specific wind direction and speed
ranges for every monitoring site. The average ozone concentration was computed for each wind
direction range in 45 increments. The first directional wind was between 0-45, i.e. when the
wind blew from north or northeast. This resulted in 8 wind directional concentration bins. The
average concentrations for each directional bin was further classified by wind speed, ranging
between 0-2, 2-4, 4-6, 6-8 m/s increments. Thus, there were eight directional and four wind
speed bins, yielding a total of 32 concentration bins.
It needs to be recalled, that ten years, 1986-1995, of ozone and meteorological data were used in
the statistics. Nevertheless, for some classification bins, particularly at high wind speeds the
number of data points were limited. In order for a station to qualify, at least ten days of data was
required for a valid wind direction/wind speed bin.
Framework for Analysis
The analysis below is based on the notion that the ozone concentration at a given location and
time is composed of three components: (1) tropospheric background (biogenic and
stratospheric sources), (2) regional ozone (anthropogenic sources that are more than 100-200 km
from the receptor), and (3) local or "homegrown" ozone ( local sources that are <100-200 km
from the receptor.
O3 Tot = O3 Trop + O3 Reg +O3 Loc
The above division is suggested to support air quality management decisions rather than for
process-oriented scientific analysis. In reality, ozone is not additive in chemical kinetic sense.
Hence, the distinction among the three components needs to be based on somewhat arbitrary
definitions of background, regional and local ozone.
The magnitude of the tropospheric background ozone, O3 Trop, (30-40 ppb on the average) can be
established through monitoring data at locations removed from anthropogenic sources. Based on
the relative uniformity of tropospheric background ozone at remote locations, it is assumed that
the tropospheric background ozone is in the 30-40 ppb average range throughout the OTAG
domain. Clearly, this assumption requires further scrutiny and documentation.
The other reasonably well known quantity is the measured total ozone concentration, O3 Tot,
which is the sum of tropospheric, regional, and local contributions. The magnitude of the
regional and local ozone components is more difficult to establish. Most of the attention in this
report is devoted to the regional-local apportionment.
The second major issue pertains to the dimensionally of the analysis framework. Since O3
formation and destruction is governed by kinetic rate processes, the pertinent dimension for
analysis is over time. The spatial dimension is incidental. The relationship between the time and
the spatial dimension is given by the characteristic wind speed. For example, the daytime ozone
formation time in a puff of precursors is between 1-3 hours. During that time, the puff of
pollutants may be transported only 4-10 km at 1 m/s wind speed or up to 40-100 km at 10 m/s
wind speed.
While the range of formation times is rather narrow, the range of transport during the ozone
formation phase is at least one order of magnate depending on wind speed. For similar reasons,
the removal processes tend to limit the life time of ozone to 1-2 days but the transport distance
may vary by an order of magnitude due to the variations in characteristic wind speed during the
lifetime of ozone. For this reason, establishing the spatial scale of transport is much more
difficult and fuzzy than quantifying the temporal scales of ozone. Nevertheless, spatial analysis is
relevant since most air quality management decisions involve spatial rather than temporal scales.
Local and Regional Ozone Classification using Wind Speed and Direction
The analysis below is a crude attempt to estimate the role of regional and local ozone using the
ozone concentration dependency on wind speed and direction. The analysis is based on a simple
premise that if the measured ozone concentration declines steadily with increasing wind speed,
i.e. ventilation, then the ozone is largely "homegrown" contributed by local sources. On the other
hand, if the ozone concentration does not decline with wind speed, then the ozone is attributable
to distant sources, i.e. regional sources.
A simple transport model for local ozone
It is instructive to examine the wind speed dependence of ozone using a simple one dimensional
transport model (Figure 1a). The pollutant emissions are confined to a mixing height of H[m].
Within the mixing layer the unidirectional wind speed is U[m/s] and carries a background
concentration C0[g/m3] into a source area. The source area itself has an emission density of
Q[g/m2,s] as well as the source length in the direction of the wind vector, L[m]. Assuming that
the local emissions are mixed instantaneously, the concentration, C[g/m3], averaged over the
source region can be estimated by the expression:
C = C0 + QL/UH
The total concentration is the sum of background and the locally contributed values. The second
term on the right side represents the local contribution. It is proportional to the source strength
(QL) and it is inversely proportional to the ventilation coefficient (UH). The dependence of the
local contribution on wind speed is inverse, as illustrated in Figure 1b. The concentration, C, is
highest at low wind speeds because the pollutants accumulate due to poor ventilation. With
increasing wind speed, concentrations asymptotically approaches the regional background
concentrations due to the rapid dilution of the local contributions.
Fig 1a. Schematic illustration of a simple one-dimensional model
Fig 1b. Concentration as a function of wind speed at different local source strengths.
Inherently, the model is only applicable near the sources where the removal processes are not
significant. Also, the background concentration entering a source area, C0, represents the sum of
all ozone contributions from natural and anthropogenic sources. The role of variable regional
ozone concentration is not incorporated.
In the analysis below, the above simple model is used to interpret the measured ozone
concentrations as being of local or regional in origin. Strongly declining concentrations with
wind speed will be interpreted as evidence of local source contributions, since higher wind
speeds cause increasing dilution of local contributions. If the concentration is found to be
constant with wind speed, then it taken as evidence that the local contribution is not significant,
hence regional transport dominates. The actual origin of the regional ozone is identified only
vaguely through directional analysis.
A simple transport model for non-local (regional) ozone
[Note: This section is yet completed]
Fig 2a. Schematic illustration of a simple one-dimensional model
Fig 2b. Concentration as a function of wind speed at different local source strengths.
Not a box but an integral. Has to include removal.
Results
The results of the analysis are presented in three different forms:
1. Maps and animations of average ozone concentrations for specific wind direction and
wind speed. These yield a spatial pattern of ozone in the OTAG domain for different
wind conditions.
2. Charts of average concentrations as a function of wind speed and wind direction at
specific locations. These charts are helpful in illustrating the role of transport.
3. Ozone roses, i.e. average ozone concentration as a function of wind direction for specific
subregions. These are useful "pointers" toward the ozone source areas.
Ozone as a Function of Surface Wind Direction and Wind Speed
The maps of average ozone concentration for four wind directional sectors at low (<3m/s),
medium (3-6m/s), and high (>6m/s) are shown in three sets of Figures 3, 4 and 5, respectively. In
each set, the first four figures (a,b,c,d) show the average concentration for wind directional
quadrants. The last figure, e, depicts the average concentration for all wind directions.
Figure 3a.
Figure 3b
Figure 3c
Figure 3d
Figure 3e
Figure 3. Maps of average ozone concentration at low ( <3 m/s) wind speed. a) 0-90
degrees. b) 90-180 degrees c) 180-270 degrees d) 270-360 degrees.
Figure 4a
Figure 4b
Figure 4c
Figure 4d
Figure 4e
Figure 4. Maps of average ozone concentration at intermediate ( 3-6 m/s) wind speed. a)
0-90 degrees. b) 90-180 degrees c) 180-270 degrees d) 270-360 degrees.
Figure 5a
Figure 5b
Figure 5c
Figure 5d
Figure 5e
Figure 5. Maps of average ozone concentration at high( >6 m/s) wind speed. a) 0-90
degrees. b) 90-180 degrees c) 180-270 degrees d) 270-360 degrees.
Figure 6a
Figure 6b
Figure 6c
Figure 6d
Figure 6e
Figure 6. Maps of average ozone concentration at all wind speeds. a) 0-90 degrees. b) 90-180
degrees c) 180-270 degrees d) 270-360 degrees.
At low wind speeds, (Fig 3e), the highest overall concentrations are found in the in the vicinity of
metropolitan-industrial areas including the northeastern urban corridor, Atlanta, Dallas, Houston,
St. Louis as well as over the Ohio River Valley. The directionally classified ozone concentrations
(Figures 3 a-d) indicate that the ozone concentration pattern do not vary substantially with wind
directional quadrant but tend to be somewhat higher just downwind of the urban areas compared
to the upwind levels.
At low wind speed, say 2 m/s, the transport distance of an air mass is about 200 km per day, as
indicated by the length of the arrows, placed over several OTAG locations. These arrows serve
merely as a guide to the eye. The time period of one day was chosen since it represents the
approximate time between the precursor emissions and the ozone removal.
The examination of the directionally sorted ozone concentration maps is particularly instructive
when viewed through animations. Each animation shows 36 frames, one for each 10 degrees of
wind direction. The contour map for each frame was constructed by averaging O3 values for a 90
degree window centered at the particular angle. The 90 wind direction window was necessary to
provide adequate number of data points for spatial mapping. The directionally animated maps for
low (<3 m/s) wind speeds show the circular meandering of high concentrations around the major
metropolitan areas. The high concentrations tend to be slightly downwind of the urban centers.
It is evident, that at low and meandering wind speeds of < 3m/s, the bulk of the ozone is
transported over short distances of < 200 km. Under these conditions, local emissions tend to
contribute significantly to the formation of locally generated or ‘homegrown’ ozone, particularly
over high emission areas.
At intermediate wind speeds (3-6 m/s), the directionally classified concentrations indicate
substantial differences between the ozone maps depending on wind direction (Figures 4 a-d). At
5 m/s wind speed, the transport distance of an air mass is about 500 km per day, as indicated by
the length of the arrows in Figures 4 a-d.
For northerly wind directions, 270 (Figs 4a and d), the concentrations are low throughout
the northern belt of OTAG. Evidently, northerly winds at moderate wind speeds bring low ozone
concentrations to Minnesota, Wisconsin, Michigan, and New England. On the other hand,
northerly winds are also associated with higher ozone concentrations in the southern belt of
OTAG domain from Texas through Georgia.
When the winds are from the southerly (90-270) quadrants (Figures 4 b and c), the northern belt
of OTAG, Minnesota to Maine is experiencing higher ozone concentrations, particularly in the
mid-section from Michigan through New York. At the same time, during the southerly winds the
ozone concentration in the southern belt from Texas to Georgia is low. Clearly, southerly winds
bring low ozone to the Gulf states. Within the states adjacent to the Ohio River Valley, northerly
winds, 270-90 ,are associated with higher ozone levels in Kentucky, Tennessee and West
Virginia, i.e. just south of the Ohio River Valley. On the other hand, southerly winds (90-270)
are associated with higher ozone concentrations in the belt stretching from Illinois through
Pennsylvania, just north of the Ohio Valley. Evidently, the Ohio River Valley is responsible for
elevated average O3 levels in the adjacent states at moderate wind speeds.
The directional animation at intermediate (3-6 m/s), wind speed indicate that in the vicinity of
metropolitan areas the upwind-downwind concentration difference is more pronounced. The
concentration pattern show features of plumes of ozone that appear to originate from specific
source areas, directed in downwind. The dependence of ozone concentrations on wind direction
in the vicinity of urban areas is more noticeable than at low wind speeds. Also, the distance of
urban influence is longer (up to 500 km) then at low wind speeds. Based on the directional
alignment of ozone, it is suggested that at moderate wind speeds of about 5 m/s, . O3 transport
from source areas occurs at regional distance scales of over 500 km which corresponds to about
one day of transport. Clearly, this is a very rough estimate.
At high wind speeds (>6 m/s), the directionally classified concentrations indicate major regional
concentration differences depending on wind direction (Figure 5 a-e). At wind speed of 8 m/s,
the transport distance of an air mass is about 800 km per day, which is comparable to the half
width of the OTAG domain.
During strong northerly winds the northern belt of OTAG has low ozone, while the southern belt
from Texas to Georgia has higher values. Conversely, during strong southerly winds (90-270),
the entire southern belt is virtually ozone free, while the northerly states from Illinois through
New England experience elevated concentrations.
The animations at high (>6 m/s) wind speeds indicate the ozone generated in the OTAG domain
is blown into the "downwind" quadrants of the OTAG domain while the other three quadrants
remain at low levels. At high speeds, the directional displacement of ozone is on the order of
1000 km which is again comparable to a one-day transport distance.
Another interesting feature of ozone maps at high wind speeds is that the concentrations in urban
areas do not differ significantly from the regional background. Notable exception is the
northeastern urban corridor, as well as Western Michigan where the near-urban concentrations
are also elevated, particularly during southwesterly winds.
The examination of ozone concentrations at high wind speeds appears to be of particular interest
to OTAG. At such wind speeds all the measured ozone concentrations could be called regional
(non-local or not “homegrown”) by the following reasoning. Even during the photochemically
active midday hours, it takes 1-2 hours of irradiation in order to create the bulk of ozone from
local precursor emissions. During the ozone formation time, the high speed transport moves the
emissions 50-100 km from their source. Conversely, any ozone that is measured at a location at
high speeds must have been emitted at least 50-100 km upwind. One could say, therefore, that
"homegrown" ozone is nonexistent at high speeds since the source is removed from the receptor.
Hence, virtually the entire ozone mass measured at a specific location is non-local or regionally
transported ozone.
The average ozone concentration for the four quadrants of wind direction regardless of the wind
speeds is shown in Figure 6a-e. The purpose of the animation is to illustrate the overall
diminishing and re-distribution of concentrations with increasing wind speed. These maps also
show the location of monitoring sites that have satisfied the conditions of acceptance. The spatial
patterns are very similar to the maps for 3-6 m/s, since the winds are in that range most of the
time. The directional animations at all wind speeds are also similar to the corresponding
animationas at intermediate wind speeds. An additional animation illustrates the ozone pattern
with the condition that the wind is blowing in the same direction two days in a row. Again the
pattern is similar to the one-day, intermediate wind speed pattern.
In summary, average ozone concentration maps at low wind speeds (<3 m/s) the level of ozone
throughout the OTAG domain. Ozone concentration hot-spots appear over the major
metropolitan areas and the Ohio River are Valley but the concentrations are virtually the same
regardless of the wind direction. At intermediate, 3-6 m/s wind speeds, the overall concentrations
are lower, and the urban-industrial influence appears to be displaced up to 500 km in downwind
direction. At high wind speeds over 6 m/s, the most metropolitan source areas do not cause
elevated ozone in their own vicinity. Rather, higher concentrations appear in the downwind
corners of the OTAG domain, up to 1000 km from the domain center. The ozone concentration
pattern at different wind directions and speeds are consistent with an atmospheric O3 life time of
about one day and a corresponding transport distance of 200, 500 and 800 km at 2, 5, and 8 m/s
respectively.
Ozone Concentration as a Function of Wind Speed
At low wind speeds ozone, as any other pollutant, tends to accumulate near source areas and
diminish with increasing wind speed due to higher ventilation. However, increasing wind speed
also causes ozone transport from one source region to another. This is best illustrated in the
animations of ozone maps as a function of wind speed. Each frame represents the ozone
concentration at increasing wind speeds. The maps are anchored at low(<3m/s), intermediate(36m/s) and high (>6 m/s) speeds and interpolated (6 additional frames) between these three
values.
At low wind speed, the outlines of high ozone source regions near the metropolitan areas and the
Ohio River Valley are clearly discernible through ozone peaks. As the wind speed increases, the
urban-industrial peaks diminish and a more regionally uniform pattern develops. The feature that
is most relevant to OTAG is the that elevated, nonurban ozone persists even at high wind speed
which is direct evidence of regional-scale transport.
It is also instructive to examine the wind speed/direction segregated ozone concentrations at
specific monitoring sites, Figure 7 a-m. Each figure shows the average ozone concentration at
five characteristic wind speeds, 1, 3, 5, and 7 m/s, aggregated from a speed range of 0-2, 2-4, 4-6
and 6-8 m/s respectively. The data are further stratified by 4 wind directional quadrants from 090, through 270-360 . A fifth line in each chart represents the wind speed dependence of ozone,
regardless of the wind direction, incorporating the directional frequency of ozone occurrences.
Each data point represents the average ozone concentration satisfying the above filtering
conditions but drawn from all the available sites in a 100x100 km rectangle surrounding each
metropolitan area.
Southwestern metropolitan areas. For Dallas-Ft. Worth (Figure 7a) the subregional average
ozone concentrations are about 80 ppb at 1 m/s, declining to about 55 ppb at 7m/s wind speed.
Assuming that the ozone at the high wind speed is all regional, Dallas metropolitan area
contributes about 25 ppb (80-55) to the regional background of 55 ppb. The wind speed
dependence of ozone at Dallas-Ft. Worth does not depend on wind direction since; the lines for
all four wind directional sectors overlap. In this regard, Dallas-Ft. Worth is unique among the
metropolitan areas.
Data for Houston, TX (Figure 7b) show a remarkably strong decline of ozone concentration with
wind speed. During stagnating wind conditions (0-2 m/s) the average concentration at the
subregional monitoring sites is 85 ppb, and there is a near linear decline to about 35 ppb at high,
7 m/s wind speeds. Taking the regional ozone entering the city as 35 ppb, the Houston
metropolitan area contributes 50 ppb (85-35) to the regional background. The wind directional
dependence of ozone clearly shows, that during northerly winds (270-90 ) the regional
background ozone levels (about 50 ppb) are about 20 ppb higher than during the southerly winds
(about 30 ppb) from the Gulf of Mexico. In fact, during strong northerly winds, the regional
ozone is comparable in magnitude to regional ozone estimate at Dallas-Ft. Worth, suggesting a
common origin of regional ozone over eastern Texas.
The wind speed/direction charts for the southwestern metropolitan areas show strongly declining
concentrations with increasing wind speeds. This is indicative that local sources are the dominant
contributors to the average ambient ozone concentrations .
South-central metropolitan areas. At Birmingham, AL (Figure 7c), low wind speeds are
associated with moderate ozone concentration of about 75 ppb, which decline to 50 ppb at 7 m/s.
At low wind speeds the concentrations are almost independent on wind direction. However, at
high wind speed, southerly winds are associated with 40 ppb, while northerly winds carry 60 ppb
of ozone. This pattern is similar to Houston and indicates that northerly winds carry regional
ozone that is about 20 ppb higher than the tropospheric background of 30-40 ppb.
Atlanta, GA (Figure 7d) also shows unique wind speed and direction dependence of ozone. The
average concentration at low wind speeds is about 80 ppb and declines by 25 ppb to about 55 ppb
at 7 m/s. The unique feature for Atlanta is that during southerly winds the concentration decline
is roughly linear with wind speed as expected from dilution by ventilation. However, winds from
the northern sectors show speed invariant ozone concentration up to 4 m/s, followed by decline at
higher speeds. High wind speeds from the northwest (270-360 ) bring ozone at 65 ppb, while
southeasterly winds form the Atlantic bring <50 ppb ozone. It is evident, that in Atlanta, both the
ozone concentration at high wind speeds, as well as the shape of wind speed dependence differs
for northerly and southerly winds. A full interpretation of such pattern will have to await further
analysis.
The ozone level at Charlotte, NC (Figure 7e) is about 75 ppb at low wind speeds, and about 55
ppb at high wind speeds. Overall, the concentration decline with wind speed is moderate, except
for strong southeasterly winds from the Atlantic when the average ozone concentration drops to
35 ppb. At Charlotte, NC the ozone levels are highest when the winds blow either from the
northwest or from the southeast.
The average ozone concentrations at Nashville, TN (Figure 7f) is about 65 ppb at low wind
speeds and declines to about 50 ppb at about 7 m/s, yielding a high-low speed difference of only
15 ppb. Evidently, the Nashville metropolitan area contributes less local ozone than the other
major southern metropolitan areas. At high wind speeds the ozone concentrations of northerly
winds are about 10 ppb higher than southerly winds.
In St. Louis, MO (Figure 7g), the average ozone level at low wind speed is 80 ppb, at high speed
is 55 ppb, with an excess of 25 (80-55) ppb ozone. At most urban areas, at low wind speeds the
ozone levels are similar at all directional sectors. The unique feature of St. Louis is that at low
wind speeds there is a remarkable directionality of ozone concentrations. At 0-2 m/s,
northeasterly winds (0-90 deg) are associated with 65 ppb, while southwesterly winds with 95
ppb ozone. At this time, no explanation is offered for the unique pattern.
The wind speed and directional filtering of ozone data show that over the Southeastern
metropolitan areas ozone declines with wind speed suggesting substantial local contributions.
However, there is also evidence that during winds from the center of the OTAG domain, the
concentration do not decline with wind speed, suggesting transported ozone.
North-central metropolitan areas. The selected north-central metropolitan areas under
consideration include Chicago, IL, and Detroit, MI.
The Chicago, IL (Figure 7h) subregion has remarkably low average ozone concentration of 62
ppb at 0-2 m/s, and declines to about 50 ppb at high speeds. The unique feature of Chicago is that
winds from the northerly sectors show substantial decrease of ozone with wind speed while
winds from the south (90-270 ) are associated with about 60 ppb of ozone, regardless of the
wind speed.
Detroit, MI (Figure 7i) exhibits an interesting pattern in that the average ozone concentration
changes only 7 ppb from low wind speeds (62 ppb) to high wind speed (55 ppb). Furthermore,
the weak speed-dependence of ozone arises from the compensating effects of different wind
directions. During southerly winds the ozone concentration increases slightly from 60 ppb at low
wind speeds to 65 ppb at high wind speeds. On the other hand, at northerly winds, the
concentrations decline from over 60 to 45 ppb.
The north-central metropolitan areas of Chicago and Detroit show a remarkable directionality of
the ozone-wind speed dependence. Wind directions from outside the OTAG domain are
associated with declining ozone with wind speed. This is interpreted as evidence of substantial
local contributions. On the other hand, the wind directions from the center of the OTAG domain
are associated with speed-independent ozone levels, implying transported ozone. Taking into
account all wind directions, the average ozone appears to be dominated by transported rather than
local ozone.
Northeastern corridor. The populated northeastern corridor is known for high density of ozone
exceedances, high emission density of ozone precursors, as well as evidence that regional scale
ozone transport into the region is significant.
Washington, DC (Figure 7j) shows moderate decline of 15 ppb (80-65) ozone concentration with
increasing wind speed. Southerly winds show somewhat higher concentration then northerly
directions. The lowest ozone levels are observed when the winds blow from the northwesterly
quadrant.
Philadelphia, PA (Figure 7k) and New York City (Figure 7l) metropolitan areas show
qualitatively similar ozone concentration dependence on wind speed and wind direction. The
average concentration at high and low wind speeds differs only by about 5 ppb (70-65). At low
wind speeds the concentrations are almost independent on wind speeds. However, at high wind
speeds southwesterly winds carry about 80 ppb of ozone, while swift northeasterly winds have
ozone concentrations near the tropospheric background.
The Boston, MA (Figure 7m) metropolitan area shows virtually no dependence of ozone
concentration on wind speed, except during northeasterly winds. Directionally, southwesterly
winds are associated with the highest (70 ppb) ozone while northeasterly transport brings lowest
(45 ppb) ozone concentrations to Boston.
The cities of the northeastern corridor show a common feature of a weak decline of average
ozone concentration on wind speed. It is evident, that regional ozone (65 ppb) is a large fraction
of the total average ozone (80 ppb) in the Washington metropolitan area. Evidently,
southwesterly winds transport ozone into these metropolitan areas with an average concentration
of 80 ppb into the Philadelphia-New York City corridor. During northeasterly winds local
contributions appear to be significant. The remarkable lack of wind speed dependence at Boston,
clearly indicates that the average ozone concentration is dominated by transported ozone.
The wind speed and directional dependence of ozone in the Philadelphia-New York corridor
deserves further consideration. During southwesterly winds, the ozone concentration actually
increases with increasing wind speed up to 5 m/s (500 km per day). A possible explanation of
ozone increase with wind speed is that ozone from the west-southwest is transported from major
distant sources such that ozone removal between the source and receptor is significant. At higher
wind speeds the transport time is reduced which can result in less removal and increased
concentration. However, this explanation is rather tenuous and demands further scrutiny.
The other issue with regard to the transport in the Northeast is that the wind direction derived
from the surface winds may be less representative of the wind direction for the bulk of the
mixing layer than at other regions. In fact, Blumenthal et al. 1997
http://capita.wustl.edu/otag/reports/otagrept/otagrept.html suggested that while surface winds
tend to be southwesterly direction, the main transport winds in the 200-800 m elevation tend to
be from the west. Hence, it is not clear whether the high regional ozone levels in Philadelphia
and New York originate from the southwest or from the west.
Figure7a
Figure 7b
Figure 7c
Figure 7d
Figure 7e
Figure 7f
Figure 7g
Figure 7h
Figure 7i
Figure 7j
Figure 7k
Figure 7l
Figure 7m
Figure 7 a - m. Dependence of ozone concentration on wind speed and direction at different
metropolitan areas.
Figure 8. Relative decline of ozone concentrations with wind speed at different metropolitan
areas. Strong decline indicates the influence of local sources, while weak decline suggests
transported ozone.
In summary, the characteristic wind speed dependence at different locations are compared in
Figure 8 which shows the relative change of ozone concentration as a function of wind speed. All
concentrations are normalized to the ozone measured at 1 m/s wind speed. For southern urban
areas, such as Houston and Atlanta, ozone levels decline with increasing wind speed. In northern
cities, like Chicago, New York and Boston, the average ozone levels over the metropolitan area
are virtually independent of wind speed.
Directional Ozone Roses
One of the ways to examine the directionality of ozone transport is through ozone pollution roses
at specific locations. In the current usage an ozone pollution rose is defined as the average ozone
concentration when the wind blows from a given direction. It does not consider the frequency of
occurrence of different wind directions or speeds. The purpose of such a analysis is to evaluate
the magnitude of the ozone concentrations that arrive at receptor site from different directions.
The wind directional sectors that show high values of the ozone rose provide directional "fingers"
that point toward the source areas of high ozone concentrations.
Figure 9 shows ozone concentration roses at nine selected subregions of OTAG. The ozone roses
display only the concentrations that are above 50 ppb. In other words, the origin of the polar
diagrams is at 50 ppb. The ozone rose for each subregion was averaged using monitoring sites in
a 100 km rectangular box centered over the selected urban sub-region.
At Philadelphia, when the wind blows from the north the average concentration is about 60 ppb.
Winds from the southwest are associated with over 80 ppb average ozone concentration. This
points to the southwest as the source of high ozone in Philadelphia. It should be noted, however,
that in the Northeast, the surface wind direction may poorly represent the mean transport winds,
which are from the west.
In Atlanta, southerly winds are associated with average ozone of 60 ppb. On the other hand,
when the wind blows from the north the average concentration is over 80 ppb. The apparent
"source direction" of high ozone in Atlanta is to the north.
In St. Louis, the ozone concentrations are about 55 ppb during winds from the west and
northwest, and increase to about 70 ppb when the winds are from the east, south, and southwest.
Evidently, St. Louis receives elevated ozone from the east and south.
The directionality of concentrations at western Michigan and Detroit is such that concentrations
over 50 ppb occur during southerly winds. On the other hand, in Houston, the average ozone
concentration is over 70 ppb when the winds are northerly, and less than 40 ppb during southerly
winds.
An intriguing pattern illustrated by the ozone roses is that Pittsburgh, PA, Philadelphia, PA,
Charlotte, NC, Atlanta, GA, Nashville, TN, St. Louis, MO, W. Michigan, as well as Detroit. All
these sub-regions show higher O3 concentration when the surface winds are from the direction of
the OTAG domain center. These sites “point” to the center of the OTAG domain as the source of
ozone. The “transport wind vectors at high local ozone also implicate the Ohio River Valley
region as a major source area. The center of the OTAG domain as a major ozone source was first
noted by Poirot and Wishinski, 1996 [LINK].
Figure 9. Ozone concentration roses for selected metropolitan areas.
Discussion
The local surface wind-nased source identification and source apportionment techniques has not
been fully evaluated in this report. It is clear, however, that the wind direction and wind speed
sorting has a potential to yield observation-based evidence regarding ozone transport. This
analysis also brings to the reader’s attention that establishing the spatial scales of zone transport
is much more uncertain and variable than quantifying the temporal scales. In particular, distance
from the source is an ambiguous way to determine local vs regional ozone.
The obvious limitations of this analysis includes the use of surface winds, opposed to mean
transport winds, and location differences between meteorological and ozone sites.
The results indicate that locally generated ozone concentrations are dependent primarily on
emission strength and wind speed. On the other hand, regal ozone depends more on wind
direction. The evidence also indicates that increasing wind speed dilutes locally generated ozone
but also transports ozone to longer distances. A fully defensible apportionment of local and
regional ozone will have to await further analysis. In particular, it is ambiguous as to how the
observed wind-speed independent concentrations are generated.
Substantial future efforts need to be invested in evaluating the consistency of this method with
the other approaches that attempt to illuminate and quantify ozone transport.