Final Report for the
OTAG Air Quality Analysis Workgroup
Volume I: EXECUTIVE SUMMARY
The Air Quality Analysis (AQA) Workgroup of OTAG has been formed to develop analyses based on air quality data which can be used to independently check or complement analytical results developed through OTAG emission-based modeling efforts. 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 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 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. It should be noted that comprehensive model evaluation was performed by the modeling group; the AQA efforts in this area complemented this work.
The Workgroup process has entailed the development and presentation of individual work products in Workgroup meetings, followed by open review of work efforts and results by the group. This process has been greatly facilitated by the early development of an interactive Internet website which has been used to communicate datasets, analytical tools, results, interpretations, and critical feedback. In addition, the Workgroup developed policy-relevant summaries and interpretations and subjected these summaries to group review in developing this final report.
In the paragraphs below, we present the major policy-relevant results, conclusions and recommendations from the AQA workgroup. The conclusions fall into the following major categories:
Origins and patterns of ozone in the OTAG domain
Ozone transport in the OTAG domain
Comparison of episodes with climatological data
Air quality management implications of analytical results
In this summary, each category includes policy-relevant questions which we address with brief statements of pertinent results. The supporting materials for this Executive Summary , Volumes II and III, contain technical expositions of the individual analyses and their integration.
In general, these results provide the background assessment of the current ozone problem necessary to place the modeling results in the appropriate context. That is to say, they "set the stage" for the useful interpretation of the modeling of future-year control strategies. As mentioned above, it is important that these analyses be considered as a complement to the modeling results; because their strength lies in the fact that they are based primarily on actual measurements, and in many cases they can provide a good indication of the inherent strengths and weaknesses of the photochemical grid modeling. They also provide more of a "climatological" view of the ozone problem, which extends beyond the modeled episode days and provides a broader perspective of the ozone problem and its characteristics. Once one has developed an understanding of this broad perspective, the modeling results can be used to subsequently provide a more focused view of individual episodes and potential control strategy impacts.
Origins and Patterns of Ozone in the OTAG Domain
What is the pattern of ozone precursor emissions in the OTAG domain?
Anthropogenic emissions of volatile organic compounds (VOC) and nitrogen oxides (NOx) within the OTAG domain from area and point sources contribute to the formation of excess ozone which adds to tropospheric background levels. Point sources are generally tall stacks located mostly in rural areas with NOx-rich emissions. Area source emissions arise mostly from low-level mobile source emissions in urban metropolitan areas that are rich in VOC as well as NOx. Area sources of NOx occur primarily in large urban metropolitan areas (e.g., Washington-New York corridor, Chicago, Atlanta, Dallas-Ft. Worth, Houston, St. Louis), while elevated point sources of NOx are prevalent in non-urban, industrial regions such as the Ohio River Valley. Temporally, area sources tend to have both a diurnal and seasonal cycle, while many point sources (but certainly not all) are less variant in time.
What is the pattern of ozone on the edges of and within the OTAG domain?
The OTAG region is ventilated generally by air coming from outside the domain whose ozone concentrations average about 30-40 ppb, which corresponds to typical tropospheric background levels measured in the northern hemisphere. Thus, with the notable exception of the Canadian border along the Windsor-Quebec corridor, there are no significant external ozone precursor source impacts on the OTAG domain, at least on a regional scale. It is reasonable to assume that in the absence of anthropogenic emissions, the average summertime ozone concentration would be about 30-40 ppb throughout the OTAG domain.
How much does ozone vary in time and space?
Ozone exhibits strong day-to-day variation that can be quantified by examining "cleaner" and "dirtier" days across the domain. 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 clean days exhibit a broad area of higher average ozone (>40 ppb) over the north-central OTAG states from Illinois to Pennsylvania relative to the rest of the OTAG domain. During the dirtier, or high-ozone days (the 90th percentile of all measured ozone concentrations) the urban influence is very pronounced but confined to about one or two hundred miles from major metropolitan areas. The urban influence can be further enhanced when urban areas of influence overlap, or when nighttime transport occurs through the development of low-level "nocturnal jets". It is therefore implied that urban areas are making substantial local contributions to the highest 1-hour daily maximum ozone concentrations as well as exhibiting the highest variation in ozone concentration (difference between 90th and 10th percentile).
Where are the areas of exceedances for the current and proposed ozone standards?
The exceedances of the current 1-hour 120 ppb ozone standard have been evaluated using data from 600 monitoring stations in the OTAG region. The ozone database compiled for these analyses is extremely comprehensive, including data from routine stations as well as from several research-oriented networks not normally reported in the EPA's national database. The entire network includes 102 urban sites, 259 suburban sites, and 238 rural or remote sites (13 sites in the network are not classified). The spatial pattern of the current (1991-1995) exceedances shows that the Washington-New York corridor, Chicago, Atlanta, Dallas-Ft. Worth, Houston, St. Louis, are the major metropolitan areas that exceed the standard two or more times a year. Spatial extrapolation between the monitoring sites can yield estimated areas where exceedances of the standards occur. The results indicate that virtually all areas where exceedances of the current standard occur are confined to the near vicinity (<100 miles) of the above metropolitan areas. In contrast, areas exceeding the proposed 8-hour 80 ppb ozone standard extend further from the major metropolitan areas and include a large portion of the central part of the OTAG domain.
Ozone transport in the OTAG Domain
Ozone transport; beneficial or harmful?
In general, the atmosphere acts as a dispersing agent for air pollutants. Dispersion can cause both beneficial pollutant reduction near a source and harmful pollutant increases downwind of a source. Dispersion takes place through horizontal transport and vertical mixing in the lower atmosphere. Horizontal transport depends on location as well as on elevation above ground, and to some extent on time of day. The horizontal transport for elevated emission sources (greater than 100m above ground) is substantially higher than the transport of low-level sources, due to higher wind speeds aloft, particularly at night.
Examination of dispersion conditions during high-ozone periods across the OTAG domain shows that dispersion in the Southeast is typically poor due to stagnating air masses, whereas the western and northern sections of the domain are typically ventilated by strong southeasterly and westerly winds, respectively. In contrast, low-ozone days exhibit strong windflow everywhere within the domain, but coming from areas outside the domain (e.g., Canada and the Gulf of Mexico). One interpretation of these results is that most periods of high ozone across the OTAG domain result from a stagnation event somewhere in the domain (particularly central and south) which is followed by strong unidirectional flow to some other part of the domain (particularly in the upper Midwest and Northeast). While this pattern is not always seen for every ozone episode, it points out what appears to be the necessary condition for a region-wide ozone event; namely stagnation followed by transport. Such a condition is seen in three out of four episodes chosen for OTAG modeling.
To summarize, the "good news" about transport is that it can disperse, or clean up, the ozone formed in an area during a stagnation event. The south-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 concentrations of ozone from one portion of the domain to another, and this aspect of transport tends to cause more problems in the midwestern and northeastern portions of the domain.
What are the ranges of ozone transport implied from various studies?
The distance and direction of transport can vary considerably from day-to-day, site-to-site, and source-to-source. The areas impacted to an observable extent by ozone transport from large urban or industrial areas in the OTAG domain, deduced from multiple types of analyses, correspond to downwind distances ranging from less than 150 miles to over 500 miles. The direct influence of specific urban areas can be traced for specific episodes some 150-200 miles before merging indistinguishably into the regional ozone pattern. Elevated point source NOx emissions, subject to higher windspeeds and slower formation and destruction rates, are characterized by larger regions of influence, especially at night. Statistical correlation analyses of the regional ozone pattern suggest downwind distances of up to 300-500 miles, but it is not clear to what extent this actually represents transport of ozone and/or precursors. Time-lagged correlation analyses also suggest linkages between ozone concentrations separated in time by 1 to 2 days within the domain. Based on observed mean wind speeds, such an atmospheric lifetime suggests potential downwind impact areas of about 400 miles, but again it should be noted that these may represent meteorological correlations instead of actual transport, and that the actual impacts (in terms of ozone concentrations) have not been addressed by these analyses. While an estimate of impact distance based on any single method is rather uncertain, the coinciding range of the various methods increases confidence in the accuracy of these calculated distances.
Comparison of episodes with climatological data
How do ozone concentrations during the OTAG modeling episodes compare to the
While the spatial pattern of the highest values of ozone seen during the OTAG modeling episodes resembles the spatial pattern of the 90th percentile of ozone concentrations developed from a climatological perspective, there are clear spatial variances from episode to episode. Since the meteorological conditions associated with any individual ozone episode can never be expected to
replicate exactly, the interpretations derived from investigating impacts and control strategies from one episode can be expected to provide, at best, only part of the story. Relationships derived from long-term data sets provide a more robust basis for probabilistic predictions of future conditions, and therefore provide a more complete picture of all possible outcomes.
How do transport conditions compare to the climatological pattern?
Transport conditions, as assessed through analysis of meteorological conditions, show significant variance from episode to episode, and in comparison to the climatological pattern of high ozone episodes. Nonetheless, there are similar patterns between episodes and the climatological pattern which emphasize the concept that substantial transport in the OTAG domain requires an initial buildup of ozone and/or precursors somewhere in the domain, and that this buildup is associated with stagnation conditions.
Air quality management implications of analytical results
Is the OTAG domain a well-defined air quality control region?
The geographic domain of OTAG is indeed a well-defined air quality control region. The high ozone concentrations in excess of the tropospheric background originate from within the OTAG region; tropospheric background levels of ozone are found at nearly all of the borders of the OTAG domain, with the possible 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 OTAG. 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.
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 weekly cycle of emissions differs from the diurnal and seasonal cycles in that it is exclusively due to manís activities. Hence, any detected weekly ozone cycle must be exclusively due to anthropogenic emission changes. There is indeed a weekly ozone cycle; ozone data show that throughout the OTAG domain, on Sundays, the 1-hour 120 ppb exceedances are reduced by factor of 3 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. 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 80ppb exceedances show less weekly fluctuations, indicating that such a control scenario would be less effective in reducing exceedances of the new standard. While this natural "emission ontrol" experiment provides interesting results, we do not have good information on the actual changes in precursor emission amounts and timing, and this makes it difficult to infer quantitative impacts of emission changes on actual ozone levels; this task is thus reserved for air quality modeling.
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 non-attainment areas contribute significantly to their own ozone problems as well as to downwind areas within their area of influence. It has been noted that the region located in the center of the OTAG domain tends to impact on downwind areas regardless of which direction the wind blows. Spatial pattern analysis indicates that this central area is chronically characterized by moderately high ozone levels nearly all the time. Forward and backward trajectory analyses implicate the central portion of the OTAG domain as being involved in transport-related ozone events more than any other portion of the domain, although the degree of impact cannot be estimated from this type of analysis. Nonetheless, controls implemented in this area may be effective at reducing transport more often than controls anywhere else. Further, given the density of NOx-rich point sources in this portion of the domain and the observation that ozone formation appears to be NOx-limited in non-urban areas, it follows that NOx controls may be more effective in this regard. It should be noted that this suggestion is consistent with regional modeling results to date.
Do air quality data support the use of UAM-V model use to evaluate control options?
In general, visual comparison of maps of measured ozone during modeled episodes with model-predicted ozone values shows that the simulations capture the large-scale features of each episode, suggesting that UAM-V is adequate as a tool for evaluating future regional control options. Although not seen in every episode, there appear to be some tendencies for the model to underpredict daily maximum ozone levels in the northern portion of the domain where high ozone is frequently associated with strong, synoptic-scale flows, and overpredict the same for the southern portion of the domain where high ozone is typically associated with local stagnation. Further, aircraft measurements taken at selected locations for a few simulation days indicate that ozone levels above the surface layer of the atmosphere 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, and this should be kept in mind when interpreting model results regarding transport.
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, again supporting the concept of utilizing model results as a tool to evaluate control strategy impacts on regional concentrations (where NOx concentrations typically limit the formation of ozone).
The limitations of model applications, such as those suggested by these analyses, reinforce then notion that modeling results should be interpreted not as being 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.
Recommendation to Foster Future Analyses
The infrastructure consisting of the community of analysts brought together by the OTAG process and associated communications capabilities (regular meetings, conference calls, web sites, mailing lists) has proven to be a valuable resource for integrating scientific research and policy-making; efforts should clearly be made to maintain this infrastructure in the future.
Now that ozone transport has been identified as a significant problem by atmospheric scientists, efforts are needed to address the lack of routine air quality and meteorological data suitable for analysis of transport over a broad range of episode conditions. In particular, the current monitoring network is primarily focused on the larger metropolitan areas whereas spatial coverage in less-populated areas or along boundaries of political jurisdictions is quite limited. The current monitoring network is also primarily geared towards monitoring ozone attainment status and trends. As well, measurements of ozone aloft or of ozone precursors are quite limited. All these measurements are needed both for transport assessments and to better evaluate the performance of photochemical models used to estimate control strategy impacts. In particular, the PAMS network should be enhanced to provide the information necessary for the evaluation of control strategy effectiveness.
As the EPA reviews its current monitoring networks and plans monitoring programs for the future, this workgroup urges the Agency to consider enhancing the coverage and implementation of such measurement programs to allow for a better understanding of ozone formation and transport in the future. Further, because of the inherent scientific uncertainties in the modeling and associated databases, it is clear that the most prudent course of action toward solving the ozone problem entails the development of and commitment to an air quality management process, such as that being undertaken through the OTAG process, which results in continuous and measurable progress toward attainment.
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