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 500-1000 km from the source of its precursors. However, a full quantification of the ozone source-receptor 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

EPA

1991-1995

CASTNet

EPA

1991-1995

EMEFS

Eulerian Model Evaluation and Field Study

1988

SCION

Southern Oxidant Study

1993, 1995

LADCO

Lake Michigan Air Directors Consortium

1991 (88, 93, 95)

GEORGIA

State of Georgia

1988, 91, 93, 95

NORTH CAROLINA

State of North Carolina

 

 

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. Data from individual networks were quality controlled and formatted uniformly.

2. The hourly ozone data from all the networks were combined into a single database.

3. The daily maximum (1-hour average) ozone was extracted from the hourly data.

4. 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°-90°, (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(3-6m/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 0-90°, 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 - make it a link] 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.



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