Telling the OTAG Ozone Story with Data
Final Report, Vol. I: Executive Summary
OTAG Air Quality Analysis Workgroup
Dave Guinnup and Bob Collom, Co-chair
June 2, 1997
Table of Contents
Origins and Patterns of the Ozone Problem*
What is the OTAG Ozone Problem?*
What is the pattern of ozone precursor emissions?*
What are the spatial patterns of ozone?*
What is the range of ozone concentrations?*
How does ozone vary in time?*
Ozone transport in the OTAG Domain*
Ozone transport: beneficial or harmful?*
What are the implied ranges of ozone transport?*
Air Quality Comparisons to UAM-V Episodic Model Results*
How do model results compare to measured concentrations?*
How does transport during modeling episodes compare to episodes in general?*
What kind of episodes do model results represent?*
How do model episode concentrations compare to episodes in general?*
Air quality management implications*
Is ozone controllable by measures within the OTAG domain?*
Is there evidence that precursor emission changes cause ozone concentration changes?*
Do air quality data suggest high-leverage means for controlling ozone?*
Recommendations to Foster Future AQ Analyses*
Cover image: Superposition of ozone concentration contours and resultant transport winds during the high 90th percentile ozone conditions.
This report was prepared by the OTAG Air Quality Analysis (AQA) Workgroup to aid the deliberations of the OTAG Policy Group with policy-relevant information. The ozone problem addressed by the Workgroup stems from the existence of nonattainment areas in the OTAG domain and that some nonattainment areas are experiencing considerable influx of ozone across their boundaries.
The analysis of more than 600 monitoring stations' data, shows that the highest average daily maximum ozone concentrations (60-80 ppb) within the domain roughly coincide with the highest emission densities of anthropogenic NOx and VOC: near major metropolitan areas and along the industrial Ohio River Valley. The O3 levels at the edges of the OTAG domain correspond to the tropospheric background range of 30-50 ppb. Ten-year trend analysis shows a decline of 120 and 80 ppb exceedances in the Northeast. However, the reductions of these exceedances over the entire domain is less pronounced, particularly if one disregards the anomalous high ozone year of 1988.
Low wind speeds, <3m/sec, cause ozone accumulation near local source areas. High winds, >6m/sec reduce the concentrations, but contribute to the long-range transport of ozone. The average range of ozone transport implied from an array of diverse methods is between 150 and 500 miles. The transport of ozone manifests itself differently at the local, sub-regional and regional scales. In general, local (30-150 miles) transport contributes most to the non attainment of the 120 ppb standard. Beyond 100 - 200 miles the ozone concentrations tend to decrease with increasing transport distances However, the perceived range, depends whether one considers the average concentrations (300-500 miles) or peak concentrations (tens of miles at 120 ppb). The relative importance of ozone transport for the attainment of a new 8-hour standard is likely to be higher due to the closer proximity of nonattainment areas.
The modeling results have been evaluated based on model performance and the representativeness of the selected modeling periods compared to climatological conditions. The model simulations have captured the large-scale features of each episode. However, there was a tendency to under-predict (10-20 ppb) the regional daily maximum ozone concentrations in the North and over-predict in the South. The transport wind fields as well as the average ozone concentration pattern during the four episodes (36 days), are representative for OTAG domain-wide episodes that occur 3-8 times a year. Stagnation over multi-state areas, followed by swift transport is an important characteristic of such O3 episodes that are prevalent over the central portion of the OTAG domain, where the NOx emissions are also high. The modeling periods are not representative of the highest ozone concentrations occurring locally. In particular, high O3 events over the Gulf states are under-represented in the modeling periods.
The anthropogenic ozone in the OTAG domain originates from within the domain and therefore it is controllable with measures inside the domain. The strong weekly cycle of peak ozone concentrations along with the observed parallel 10-year trends of ambient ozone and precursor emissions in some sub-regions suggests that ozone reductions are feasible. Urban areas contribute to their own ozone problems and VOC controls appear to be effective for ‘peak shaving’ of the 120 ppb standard. NOX emission reductions are likely to be effective for regional ozone reductions. Emissions in the central part of the OTAG domain along the industrial-urban Ohio River Valley appears to be associated with many regional-scale ozone episodes. Reductions in that area would benefit many receptor areas downwind.
The Workgroup has recommended preserving the OTAG AQA infrastructure and the stakeholder-based approach. Future monitoring and assessment programs should set as further goals the quantification of ozone source attribution, flow across political boundaries and routine evaluation of photochemical models.
The Air Quality Analysis (AQA) Workgroup of OTAG has been formed to provide assessments of air quality and meteorological data relevant to the mission of OTAG. The Workgroup placed special emphasis on ozone transport since some nonattainment areas are experiencing considerable influx of ozone across their boundaries and they cannot demonstrate attainment by local measures only. Specifically, the workgroup's purpose statement reads:
The Air Quality Analysis Workgroup shall identify, characterize, compare, and assess observational data and studies including but not limited to air quality trends analysis, overflight data, and meteorological studies for the purpose of evaluating the effects of the transport of ozone and its precursors on ozone nonattainment in the eastern United States.
The Workgroup process has entailed the development of individual work products, presentation of results at Workgroup meetings, followed by open review by the group. The Workgroup has collectively developed policy-relevant summaries and subjected these to repeated group review. This process has been greatly facilitated by the early development of an interactive Internet website which has been used to communicate data sets, analytical tools, results, interpretations, and critical feedback.
The Workgroup members were affiliated with the EPA, state agencies, industry (power, transportation) consultants and academia. The specific activities undertaken by this group have included reviewing existing air quality studies and analyses; developing analyses and visualizations of air quality and meteorological data to help in the understanding of ozone formation and transport; comparing modeling results against available air quality data; and integrating air quality analyses and modeled results into conceptual interpretations of ozone transport for use in policy development. The multiple types of analyses included: spatial and temporal pattern and trend analysis, trajectory and residence time analysis, correlation and cluster analysis, detailed evaluations of intensive field studies, (SOS and NARSTO-NE) and comparison of model results with the data.
The efforts of the Modeling and Air Quality Analysis Workgroups are intended to provide complementary input to support the OTAG policy development. The modeling effort evaluates the effects of future control strategies using photochemical grid modeling for specified episode periods. On the other hand, the air quality analysis results assess the ozone problem using long-term measurements from monitoring networks. A comparison of the modeling results for the four modeling episodes with corresponding observations as well as with climatological values provides a good indication of the strengths and weaknesses of photochemical grid modeling, including simulation performance and representativeness.
The air quality assessments help to "set the stage" for the Policy Group by providing broader perspectives on the current ozone problem and its characteristics. While modeling is uniquely suited for evaluating the consequences of future emission scenarios, the complexities of the ozone problem are such that comparisons with data are a necessity. The combination of modeling and AQ analysis results can help to improve confidence for, and identify uncertainties in, the outcome of future control scenarios.
The paragraphs below present the major policy-relevant results, conclusions and recommendations from the AQA Workgroup. The summary falls into the following major categories:
In each major category a set of policy-relevant technical questions are posed, followed by and brief statement of pertinent results. The supporting materials for this Volume I - Executive Summary, can be found in Volume II - Summary and Integration of Results and in Volume III - Summaries of Individual Analyses, all accessible through the AQAWG website, http://capita.wustl.edu/OTAG.
Origins and Patterns of the Ozone Problem
in the OTAG Domain
What is the OTAG Ozone Problem?
Recent health and ecological studies suggest that adverse biological effects can result from ozone exposures at any level above natural background. All sections of the OTAG region periodically experience ozone higher than the natural background, and share a common interest in addressing the ozone problem. However, the severity, frequency and duration of high ozone events exhibit complex patterns and source receptor relationships, such that the most efficient strategies to reduce local or regional ozone exposures are not obvious.
Control strategies depend directly on how the OTAG ozone problem is defined. Current federal health standards focus on peak 120 ppb, 1-hour concentrations; proposed health standards focus on 80 ppb, 8-hour averages; ecological effects result from chronic exposures accumulated over the entire growing season.
The spatial pattern of the current (1993-1995) exceedances (120 ppb, 1 hr) shows that the Washington-New York corridor, Chicago, Atlanta, Dallas-Ft. Worth, Houston, St. Louis, are the major metropolitan areas that exceed the current standard (Figure 1). However, there are other metropolitan areas throughout the OTAG region which remain in nonattainment with the current standard.
Virtually all areas where exceedances of the current standard occur are confined to the near vicinity (<150 miles) of metropolitan areas. In contrast, areas exceeding the proposed 8-hour 80 ppb ozone standard are more numerous, extend further from metropolitan areas and include a large portion of the central OTAG domain. The distances between nonattainment areas projected under this proposed standard are significantly shorter than those under the current standard and therefore there is more likelihood that one area influences the exceedances in its neighboring nonattainment areas.
What is the pattern of ozone precursor emissions?
Ozone precursors are volatile organic compounds (VOC) and nitrogen oxides (NOx) from area and point sources. Anthropogenic area sources of both VOC and NOx are most dense in large urban metropolitan areas (e.g., the Washington-New York corridor, Chicago, Atlanta, Dallas-Ft. Worth, Houston, St. Louis). The largest, elevated point sources of (predominantly) NOx emissions are prevalent in industrial regions including the Ohio River Valley (Figure 2). Anthropogenic NOx area sources tend to have a strong diurnal and weekly cycle, while point sources typically vary less in time (Animation 1). VOC emissions from both anthropogenic and biogenic sources are heavily influenced by sunlight and temperature, and so tend to exhibit stronger diurnal and seasonal variations than NOx emissions.
What are the spatial patterns of ozone?
The Workgroup has evaluated the regional ozone concentration patterns using a comprehensive set of over 600 monitoring stations from routine EPA and research networks. The entire data set includes 102 urban sites, 259 suburban sites, and 238 rural or remote sites (13 sites in the network are not classified).
During the summer ozone season, the OTAG domain is periodically ventilated by air coming from outside the domain where ozone concentrations average ranges between 30 and 50 ppb, corresponding to typical tropospheric background levels measured in the northern hemisphere (Figure 3). It is reasonable to assume that in the absence of anthropogenic emissions, the average summertime ozone concentration would be about 30-50 ppb throughout the OTAG domain. Thus, with the notable exception of the Canadian border along the Windsor-Quebec corridor, there are no significant external source impacts on the domain, at least on a regional scale. Large sections of the domain experience average daily maximum ozone levels of 60 to 80 ppb, double the tropospheric background. The highest average concentrations are observed near major metropolitan areas and in a large central sub-region of the of OTAG domain along the Ohio River (Figure 3).
What is the range of ozone concentrations?
Ozone exhibits strong day-to-day variation that can be quantified by examining "cleaner" and "dirtier" days across the domain (Figure 4). The urban impact is virtually undetectable during cleaner, low-ozone days (i.e., the lowest 10th percentile of all measured ozone concentrations). However, even these cleanest days exhibit a broad area of higher average ozone (>40 ppb) from Illinois to Pennsylvania relative to the rest of the OTAG domain. During the dirtier, high-ozone days (the 90th percentiles) the urban influence is very pronounced but confined to about one or two hundred miles from major metropolitan areas. It is therefore observed that urban areas are making substantial local contributions to the highest 1-hour daily maximum ozone concentrations as well as exhibiting the highest range in ozone concentration as measured by difference between 90th and 10th percentiles.
How does ozone vary in time?
Ozone exhibits temporal variability over hourly, diurnal, synoptic (3-5 days), weekly, seasonal, and long-term (5-20 years) time scales. The ozone changes on weekly and long-term scales are caused primarily by anthropogenic emission changes, while changes at the hourly, diurnal, synoptic and seasonal scales are also influenced by meteorology.
Ten year trends show that the number of 120 ppb, 1-hr exceedances has declined markedly over the past decade in areas like the northeastern Ozone Transport Region (Figure 5a) and Southern California. The percentage reductions of the 80 ppb, 8-hr exceedances were less.
These areas have also experienced substantial emissions reductions of both VOC and NOx. For the OTAG region as a whole, improvements have been much less dramatic (Figure 5b). If 1988 is considered an anomalous ozone year, the OTAG region has experienced relatively small changes in the number of 120 ppb and 80 ppb exceedances and OTAG-wide average ozone concentration over the past 10 summers (Figure 5b). Evidently, historical reductions of VOC and NOx emissions have been successful at reducing peak ozone levels on a local or sub-regional scale. However, broader control approaches may be necessary to reduce the more regionally distributed 80 ppb, 8-hr average ozone levels.
Figure 1. Counties not meeting the 0.12 and 0.08 ppm standard according to EPA.
Figure 2. a) Area source emissions densities for NOx. The high emission densities are located in the vicinity of urban-metropolitan areas.
Figure 2. b) Point source emission densities for NOx. The major emission regions are located along the Ohio River Valley.
Figure 3. a) Average 1-hour daily maximum ozone concentration in the OTAG region. All four corners have about 30-50 ppb ozone, corresponding to the tropospheric background entering the region.
Figure 3. b) The crossection through the center of the OTAG domain indicates a general increase of ozone from west to east.
Figure 4. a) Spatial pattern of the 10th percentile of daily maximum ozone. Elevated ozone concentrations are evident throughout the central OTAG region.
Figure 4. b) Spatial pattern of the 90th percentile of daily maximum ozone. The highest ozone concentrations occurs near major metropolitan areas.
Figure 5. a) Exceedance trends over the Northeast.
Figure 5. b) Trend of the yearly 120 and 80 ppb exceedances for the 1986-95 period; over the OTAG domain.
Animation 1. a) Total hourly NOx source emissions during 7/14/95. The NOx emissions vary diurnally with an afternoon peak. A zip file is also available.
Animation 1. b) Total hourly VOC source emissions during 7/14/95. The VOC emissions have a stronger diurnal cycle than the NOx emissions. The high VOC emission densities are in the south and east of the OTAG domain. A zip file is also available.
Ozone transport in the OTAG Domain
Ozone transport: beneficial or harmful?
Atmospheric conditions can exert a powerful influence on the distribution of pollutant concentrations in space and time. Low wind speeds lead to the buildup of high local pollutant concentrations (Figure 6a) (Animation 2). Strong ventilation with high wind speeds prevents the local build-up near the sources (Figure 6b), but contributes to long-range transport and regional ozone particularly during directionally persistent wind conditions. For southern urban areas, where episodes are caused by local stagnation, ozone levels decline rapidly with increasing wind speed (Figure 7a). In northern cities, more heavily influenced by transport, ozone levels decrease much less rapidly, with increasing wind speeds.
Vertically, ozone transport takes place in synoptic (>800m), channeled (200-800m) and near surface (<200m) flow regimes (Figure 7b). The potential for transport from elevated (>100m) emission sources is substantially higher than for low-level sources, due to higher wind speeds aloft, particularly at night during channeled, 'nocturnal jet' conditions.
Examination of dispersion conditions during locally high-ozone days (90th percentile), shows that dispersion in the Southeast is typically poor due to stagnating air masses (Figure 8a). However, the western and northern sections of the domain experience stronger and more persistent southerly and westerly winds, respectively. This supports the notion that ozone exceedances in the central and southeastern areas are predominantly "homegrown," while exceedances in other sections of the OTAG domain are also influenced by regional transport. In contrast, on low-ozone days, the transport is predominantly from the outside (e.g., Canada and the Gulf of Mexico) into the OTAG domain (Figure 8b).
The widespread, regional-scale ozone transport episodes result from several days of stagnation over the central portion of the OTAG domain, followed by strong unidirectional flow, generally to the northeast (Animation 3). Three (88, 91, 95) out of four episodes chosen for OTAG modeling were such stagnation-followed-by-transport regional episodes.
In summary, the "good news" about transport is that it can disperse, or clean up, the ozone formed in an area during a stagnation event. The central and southeastern portions of the OTAG domain, which experience relatively more stagnation, can benefit from this aspect of transport. The "bad news" about transport is that it can carry significant ozone concentrations from one portion of the domain to another, causing influx of regional ozone across the boundaries of the nonattainment areas, particularly over the midwestern and northeastern portions of the domain.
AQA WG researchers have assessed the transport issue based on examination of meteorological data, trajectory analyses and field observations of ozone and ozone precursors. These analyses have not estimated ozone transport at future times.
What are the implied ranges of ozone transport?
The distance of O3 transport between the precursor emissions and O3 removal is in the range of 150 to 500 miles (Figure 9a). The transport of ozone manifests itself differently at the local, sub-regional and regional scales. In general, local (30-150 miles) transport contributes most to the non attainment of the 120 ppb standard. Beyond 100 - 200 miles the ozone concentrations tend to decrease with increasing transport distances.
Statistical correlation analyses of the regional ozone pattern suggest ozone transport distances of up to 300-500 miles, but it is not clear to what extent this actually represents transport of ozone and/or precursors, or is a meteorological correlation. Time-lagged correlation analyses also suggest linkages between ozone concentrations separated in time by one to two days. Interpretation of such a time lagged correlation as an ozone lifetime of one to two days corresponds to a transport distance of about 400 miles.
While an estimate of ozone impact distance based on any single method is rather uncertain, the coinciding range of the various methods increases confidence in the accuracy of these estimated ozone transport distances.
The perceived distance of ozone impact can vary considerably depending on the measures used to describe the ozone impact. In general, the spatial scale of perceived ozone transport decreases rapidly with an increasing concentration threshold (or increasing percentiles of the ozone distribution). For example, the average daily maximum ozone has a scale of impact of hundreds of miles, while ozone exceedances of 1-hour, 120 ppb thresholds may extend only to tens of miles. This finding underscores the notion that an 8-hour, 80 ppb ozone standard will implicate larger areas and longer transport scales, than the 120 ppb 1-hour standard.
Figure 6. a) Average ozone concentration during low (< 3 m/s) wind speed conditions. Low wind speeds cause ozone to accumulate near the source areas.
Figure 6. b) Average ozone concentration during high (>6 m/s) wind speed conditions. Higher wind speeds cause more regional ozone transport.
Figure 7 .a) Relative change of ozone concentration with wind speed of different cities. In the South, higher local wind speed diminishes ozone, in the North it does not.
Figure 7 .b) Schematics of transport regimes in the Northeast. Ozone transport occurs in synoptic, channeled, and near surface flow regimes.
Figure 8. a) Transport winds during high (90%-ile) local ozone days. High ozone in the central OTAG domain occurs during slow transport winds. In the north and west, high ozone is associated with strong winds.
Figure 8. b) Transport winds during low (10%-ile) local ozone days. Low ozone occurs on days with transport from outside the region. The regions of influence (yellow shaded areas) are also higher on low ozone days.
Figure 9. a) Ozone pattern and air mass histories during the 1995 episode.
Figure 9. b) Daily maximum ozone averaged over the OTAG domain. Regional episodes above 70 ppb have a duration of up to nine days.
Animation 2. Average ozone concentrations as a function of wind speeds varying from <3 m/s to >6 m/s. The low wind speeds cause the buildup of high local ozone concentrations. The higher wind speeds prevent this buildup, but contribute to regional scale transport. A zip file is also available.
Animation 3. Atmospheric flow visualization superimposed on 2pm ozone concentration patterns during the June 1991 regional scale ozone transport episode. An airmass meanders over the Midwest from 6/7 – 6/10/95 causing the accumulation of high ozone concentrations (>90 ppb). This is followed by swift transport of the ozone laden airmass eastward, where the ozone concentrations peak a day later on the Northeastern seaboard. A zip file is also available.
Air Quality Comparisons to UAM-V Episodic Model Results
How do model results compare to measured concentrations?
From the point of view of AQA, the utility of modeling results as a basis of policy making can be evaluated based on two criteria: model performance during the selected (36 day) episodes, and secondly the representatives of these episode conditions for "typical" or climatological conditions.
Visual comparison of maps of measured and modeled ozone values (Animation 4) shows that the simulations capture the large-scale dynamic and spatial features of each episode. Although not seen in every episode, there appear to be tendencies for the model to underpredict daily maximum ozone levels in the northern portion of the domain by 10-20 ppb (Figure 10a, 10b, 11a), where high ozone is frequently associated with strong, synoptic-scale flows. The model tends to overpredict the ozone for the southern portion of the domain where high ozone is typically associated with local stagnation. Further, aircraft measurements in the Northeast for a few simulation days indicate that ozone levels above the surface layer (>200m) may be underpredicted by the model. One possible interpretation of these observations (although not the only one) is that transport impacts may be understated by the model. This, and the generally longer scales of transport indicated by the climatological air quality analyses should be kept in mind when interpreting model results regarding transport distances.
Comparison of model predictions to ozone precursor data, while limited by the availability of measurements, shows marginal to poor agreement, especially for isoprene from biogenic VOC emissions. In addition, comparison of modeled and observed CO (a tracer of automotive emissions) shows poor agreement for the 1995 episode. Comparison of ozone to total reactive nitrogen ratios in non-urban locations tends to agree with model predictions.
Based on the ability of the model to capture the large-scale features of the ozone concentration patterns and non-urban ozone to total reactive nitrogen ratios, it is suggested that the UAM-V model is a useful tool for evaluating the general features of future large-scale control options. However, the limitations of model applications, such as those suggested by these analyses, reinforce the notion that modeling results should be interpreted not as definitive or absolute, but rather as one part of a full assessment. The full assessment, relative to the mission of OTAG, would thus include modeling, air quality analysis, and information relating to control strategy feasibility and cost.
How does transport during modeling episodes compare to episodes in general?
The transport conditions during the selected OTAG modeling episodes are similar to the transport during OTAG domain-wide regional ozone episodes in general (Figure 12a). Such episodes are characterized by slow meandering transport over Kentucky, Tennessee, and West Virginia, with a strong clockwise transport around this region of stagnation.
However, the transport during OTAG modeling periods differs significantly from the conditions of highest local ozone concentrations (Figure 12b). The main difference is that the net transport speeds are mostly higher during the OTAG modeling episodes, and the direction of net transport over the Illinois-Pennsylvania corridor is from the west, rather than the typical southwesterly flow.
What kind of episodes do model results represent?
Within the OTAG domain, high ozone concentration, above 80 or 120 ppb, can occur during regional, sub-regional, or local episodes. The OTAG domain-wide regional episodes are caused by slow-moving or recirculating airmasses over the center of the OTAG domain (Figure 9a) (Animation 5). Coincidentally, the central Ohio River Region is a high NOx emissions area (Figure 2). A regional episode is defined here as the condition when the daily maximum ozone concentration averaged over all the OTAG domain monitoring sites, exceeds 70 ppb (Figure 9b). All four periods selected for OTAG modeling are regional. During the 1988 episode, the OTAG domain average ozone exceeded 70 ppb for 9 consecutive days with an OTAG domain average peak of 103 ppb. The 1991 and 1995, episodes lasted for about 6 days with peaks between 80-90 ppb. The 1993 episode is best characterized as a subregional episode over the Southeast, since the OTAG domain-average ozone barely exceeded 70 ppb on one day.
The pattern of average measured ozone concentration (Figure 10a) shows that during the four modeling episodes ozone is high over the industrial Midwest, as well as over Pennsylvania and New Jersey and low along the Gulf Coast.
The OTAG domain-scale regional episodes occur about 3 (1993) to 8 (1988) times, covering about 10% of the April-September ozone season. Multi-day ozone accumulation causes about half of the OTAG-wide recorded 120 ppb exceedances to occur during these regional episodes. However, these episodes account for only 30% of the 80 ppb exceedances. Hence, their role for the proposed new standard is diminished, compared to the old standard.
Figure 10. a) Average daily maximum ozone concentrations during the four OTAG domain episodes.
Figure 10. b) Average daily maximum UAM-V model ozone concentrations during the four OTAG episodes.
Figure 11. a) Difference between the average UAM-V and measured ozone concentrations.
Figure 11. b) Difference between the OTAG domain episodes and 90th percentile ozone concentration. The OTAG domain episodes under represent the ozone in the deep South.
Animation 4. A hour by hour comparison of the UAM-V modeled ozone concentrations to measured concentrations for the 1991 OTAG episode. The model captures the large scale dynamic features of the midday ozone concentrations. However, midday concentrations in the North are underpredicted (10 – 30 ppb) and overpredicted (10-30 ppb) in the South. A zip file is also available.
Animation 5. Anatomy of the July 1995 Regional Ozone Episode. Elevated ozone most frequently occurs near large metropolitan areas like the Washington-New York corridor, Atlanta and Chicago. However, about five times each summer, regional episodes develop when accumulated ozone blankets large, multi-state areas of the Eastern U.S with unhealthy levels of ozone. The accumulated regional ozone can be transported from one state to another contributing to exceedances of the ozone standard. One such episode occurred in July of 1995 and this animation is an anatomy of that regional episode. A zip file is also available.
Scene 1: Ozone accumulation
Over the course of several days the daytime ozone concentrations increased throughout the Ohio River Valley reaching over 80-100 parts per billion. The high ozone concentrations persisted in the region for seven days.
Scene 2: Airmass stagnation over high NOX emission areas
The ozone accumulation during the episode is caused by airmass stagnation over multi-state areas of high emissions. The rapid movement of the arrows during the first day and a half indicates swift transport of a "relatively clean" airmass from Canada towards the industrial Midwestern states. The red shading on the map indicates that this region contains high NOx emissions densities. Upon arrival to the Midwest, the clean Canadian airmass meanders over several states for about five days accumulating ozone precursors as well as ozone.
Scene 3: Ozone accumulation and transport
In this scene the airmass histories are superimposed on observed ozone concentration patterns. The rise in the ozone concentrations over the industrial Midwest coincides with the airmass meandering over the high NOX emission region. Near the end of the episode, Midwestern concentrations drop, as the polluted airmass is swiftly transported eastward, carrying high ozone concentrations toward the Northeastern seaboard where regional ozone levels peak a day later. Hence, large scale ozone transport across state boundaries occurs when airmasses stagnate over multi-state areas of high emissions creating ozone "blobs" which are subsequently transported to downwind states.
How do model episode concentrations compare to episodes in general?
The modeled impacts derived from any specific historical episodic periods can not be taken as representative of the full range of meteorological flow conditions which may be anticipated in the future. However, the average ozone during the four modeling periods (Figure 10a) appear to be typical for regional ozone episodes that occur 3-8 times during every ozone season. The average modeled episode concentrations are within about 5 ppb of the averages for climatological (1991-1995) regional episode conditions.
The concentrations during the OTAG modeling episodes do not appear to be representative of high concentrations that occur locally. A measure of modeling period overall representativeness is a comparison with the highest ozone (90th percentile) concentrations (Figure 4b). In the Pennsylvania-New Jersey region, the modeling periods roughly correspond to the 90th percentile values. However, over the Gulf Coast including Texas, the modeling episodes represent only about the 60th percentile of ozone. Thus, the high ozone concentrations in the Gulf Coast belt and Texas are under-represented in the OTAG domain modeling selection days (Figure 11b). This should also be kept in mind when using the episode simulations for policy development.
Figure 12. a) Comparison of transport winds during the 1991, '93, and '95 OTAG domain episodes and the regionally high ozone conditions.
Figure 12. b) Comparison of transport winds during the three OTAG domain episodes and the locally high ozone conditions.
Air quality management implications
of the data analysis results
Is ozone controllable by measures within the OTAG domain?
Ozone is indeed controllable by measures within the OTAG domain. The high ozone concentrations in excess of the tropospheric background originate from within the OTAG region (Figures 13 and 14) (Animation 6 and 7); tropospheric background levels of ozone are found at nearly all of the borders of the OTAG domain (Figure 3a), with the exception of the Windsor-Quebec corridor. This means that the high ozone concentrations are not due to external influences but contributed by sources internal to the OTAG domain. Furthermore, the high ozone concentration regions roughly coincide with the pattern of anthropogenic precursor emissions as modified by atmospheric dispersion. Consequently, it can be inferred that most of the excess ozone concentrations observed within the domain result from anthropogenic emissions within the domain.
Is there evidence that precursor emission changes cause ozone concentration changes?
There is empirical evidence that anthropogenic emission changes do cause changes in ambient ozone concentrations. The data show that there is a pronounced weekly cycle of ozone exceedances, with the 1-hour 120 ppb exceedances on Sundays reduced by a factor of three compared to Friday exceedances. This reduction is most pronounced in urban areas, while in the central portion of the OTAG domain, the weekly ozone cycle is virtually nonexistent (Figure 15). Hence, any control scenario that simulates the weekday-weekend emission changes would be effective in reducing the 1-hour 120 ppb exceedances. It should be noted that the 8-hour 80 ppb exceedances show less weekly fluctuation, indicating that such a control scenario would be less effective in reducing exceedances of the new standard. While this natural "emission control" experiment provides interesting results, we do not have good information on the actual emission changes that occur between weekdays and weekends.
Examination of long-term trends of ozone as well as NOx and VOC emissions showed that the largest ozone declines occurred over approximately the same geographic area where simultaneous VOC and NOx emission reduction occurred. This provides further evidence, but not proof, that emission reductions cause declines of ozone concentrations.
Figure 13 a) Frequency of back trajectories for 22 receptor sites for low ozone days. These back trajectories point to outside the OTAG region as the source of low ozone.
Figure 13 b) Frequency of back trajectories for high ozone days (upper 50th percentiles). These back trajectories point to the OTAG region as the source of high ozone.
Figure 14. Ozone roses for selected 100 mile size sub-regions. The roses show the average concentration in excess of 50 ppb for each wind direction.
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Animation 6. Frequency of back trajectories for selected individual and groups of receptors throughout the OTAG domain for low (left) and high (right) ozone days. The trajectories tend to point to outside of the OTAG domain as the source of low ozone and point to inside the OTAG domain as the source of high ozone for the individual and group of receptors. A zip file is also available. Animation 7. Average ozone concentrations as a function of surface wind direction. The high ozone concentrations occur when the wind blows from the center of the OTAG domain. The low ozone concentrations occur when the wind blows from outside of the OTAG domain. A zip file is also available. Do air quality data suggest high-leverage means for controlling ozone?
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If you are using Netscape then click here to play the movie:
Animation 6. Frequency of back trajectories for selected individual and groups of receptors throughout the OTAG domain for low (left) and high (right) ozone days. The trajectories tend to point to outside of the OTAG domain as the source of low ozone and point to inside the OTAG domain as the source of high ozone for the individual and group of receptors. A zip file is also available.
Animation 7. Average ozone concentrations as a function of surface wind direction. The high ozone concentrations occur when the wind blows from the center of the OTAG domain. The low ozone concentrations occur when the wind blows from outside of the OTAG domain. A zip file is also available.
Do air quality data suggest high-leverage means for controlling ozone?
Based on spatial pattern, temporal pattern, and transport considerations, some general control approaches appear to have higher leverage. Clearly, urban nonattainment areas contribute significantly to their own ozone problems as well as to downwind areas within their area of influence, 150-500 miles away. Evidently, urban VOC controls are effective for peak shaving near metropolitan areas.
Spatial pattern analysis indicates that the central area of the OTAG domain along the Ohio River is chronically exposed to moderately high ozone levels (Figure 4). Forward and backward trajectory analyses (Figure 12) and surface wind-ozone rose analyses (Figure 14) implicate the central portion of the OTAG domain for being involved in transport-related ozone events more than any other portion of the domain, although the percentage of impact cannot be quantified from this type of analysis. Nonetheless, controls implemented in this area may be effective at reducing ozone transport more often than controls elsewhere. Further, given the density of NOx-rich point sources in this portion of the domain and the observation that regional ozone formation appears to be determined by the magnitude of NOx emissions, it follows that NOx controls may be more effective for regional ozone reductions. This suggestion is consistent with regional modeling results to date.
Figure 15. a) Map of exceedances on Fridays.
Figure 15. b) Map of exceedances on Sundays. Throughout the OTAG domain the Sunday 120 ppb exceedances are one third of the Friday values.
Recommendations to Foster Future AQ Analyses
A sizable data analysis infrastructure has been created by the OTAG process consisting of a community of analysts, data resources as well as a set of new analysis methods and tools. The associated communications capabilities through regular meetings, conference calls, mailing lists and web sites, has proven to be an effective way of integrating scientific research and policy-making. Efforts should be made to maintain this data analysis and communications infrastructure in the future.
EPA is in the process of reviewing its current attainment status and trend networks, and it is involved in the planing of future monitoring programs. Now that the ozone transport problem has been recognized, the Workgroup urges the Agency to consider enhancing the existing monitoring and assessment programs to allow (1) quantification of ozone formation and source attribution, (2) quantification of transport across political boundaries and (3) routine evaluation of photochemical model performance.
Additional data are needed over less-populated areas, along key political boundaries as well as for ozone aloft including ridgetops and tall buildings. Spatially representative sampling and more accurate measurement methods (notably for NOy) should be pursued. Co-located measurements of ozone, precursors and fine particle composition are needed to better understand pollutant interactions and source contributions.
The Workgroup recommends further development of data analysis methods to quantify ozone transport and to better evaluate the performance of photochemical models. In particular, the analysis of the PAMS network data should be enhanced and used in the evaluation of control strategy effectiveness.
Finally, based on the inherent scientific uncertainties in air quality analysis and modeling along with the multiple legitimate interests of the participants, the Workgroup recommends a continued development of and a commitment to the stakeholder-based air quality management process as being undertaken through OTAG.
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