OTAG Air Quality Analysis Workgroup Results Summary

Draft to be submitted to the Policy Group on April 15th, 1997


The Air Quality Analysis (AQA) workgroup of OTAG was specifically tasked to "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 primary benefit of the efforts of this workgroup is to "set the stage" for the useful interpretation of the modeling of future-year control strategies. 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. The major findings of this workgroup are listed below. For detailed discussion of these results, consult the AQA workgroup’s 3-volume final report, available on the Internet.


Origins and Patterns of Ozone in 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.


Ozone exhibits strong day-to-day variability that is greater in some locations of the domain than others. The lowest values in urban areas are generally as low as the lowest values in the the broad surrounding regions. On the other hand, the highest values in large urban areas are generally much higher than those in surrounding regions. Thus, the day-to-day variability of ozone in large urban areas is greater than in surrounding rural areas. Outside of the major urban areas, ozone in the north-central OTAG domain (generally from Illinois to Pennsylvania) is notable for having low values that are consistently higher than most other parts of the domain and high values that are nearly as high as those in major urban areas.


The spatial pattern of the exceedances of the current ozone standard shows that the Washington-New York corridor, Chicago, Atlanta, Dallas-Ft. Worth, Houston, and St. Louis, are the major urban areas which exceed the standard two or more times a year. Spatial extrapolation from the monitoring sites indicates 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


Periods of high ozone across the OTAG domain typically result from a stagnation event somewhere in the domain (particularly central and south) followed by strong unidirectional flow to some other part of the domain (particularly in the upper Midwest and Northeast). While the the same transport pattern is not seen for every ozone episode, we do see the same general condition for regional-scale ozone events; namely stagnation followed by transport. Such conditions are seen in every episode chosen for OTAG modeling.


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, could benefit from more of this kind of transport. The "bad news" about transport is that it can carry high 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.


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 analysis, 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 indicate correlations at 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. 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.


Air quality management implications of analytical results


The geographic domain of OTAG is indeed a well-defined air quality control region (with the possible exception of the Windsor-Quebec corridor). A wide range of air quality analysis techniques suggest that most of the excess ozone concentrations observed within the OTAG domain result from anthropogenic emissions originating within the domain.


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 man’s activities. Hence, a weekly ozone cycle must be exclusively due to anthropogenic emission changes. There is indeed a weekly ozone cycle; ozone data show that over large portions of 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 nonattainment with respect to the new standard.


Based on spatial pattern, temporal pattern, and transport considerations, some general control approaches appear to have higher leverage. Geographically, 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 potentially 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.


In general, visual comparison of tile 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 to evaluate future control options. 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 under predicted by the model. One possible implication 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.


Future OTAG Efforts and 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 conducting policy-relevant research and efforts should 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. 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 transport in the future.

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