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 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 front of the entire workgroup, 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, policy-relevant summaries and interpretations have been developed and subjected to group reviews in developing this final report.

In the paragraphs below, the major policy-relevant results, conclusions and recommendations from the AQA workgroup are presented. 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 are answered with brief statements of pertinent analytical results. Technical expositions of the individual analyses and their integration are contained in the supporting documentation for this Executive Summary (Volumes II and III).

In general, these results provide the background assessment of the current ozone problem necessary 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 demonstrate independent corroboration of many of the lessons learned from 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 control strategy impacts.

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 anthropogenic ozone on top of the tropospheric background. Point sources are generally tall stacks located mostly in rural areas with NOx-rich emissions. Area source emissions arise mostly from low-level 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 but industrial regions such as the Ohio River Valley. Temporally, area sources tend to have both a diurnal and seasonal cycle, while point sources are typically more invariant with time.

What is the pattern of ozone on the edges of and within the OTAG domain?

The OTAG region is ventilated typically by prevailing winds coming from outside the domain. The air masses entering the OTAG region originate in Canada in the North and Northeast, from the Gulf of Mexico and Atlantic Ocean in the South, and from the western United States to the Midwest. Ozone concentrations in these air masses are about 30-40 ppb, which corresponds to typical tropospheric background levels measured at remote sites of the world. 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 ozone concentrations). However, even during the cleanest days there is a broad area of elevated ozone (>40 ppb) over the north-central OTAG states from Illinois to Pennsylvania. During the dirtier, or high-ozone days (the 90th percentile) the urban influence is very pronounced but confined to about a few hundred miles from major metropolitan areas. It is therefore implied that urban areas are causing the highest 1-hour daily maximum ozone concentrations as well as the highest variation in ozone concentration (90th - 10th percentile difference).

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 from 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; 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 (>100m) is substantially higher than the transport of low-level sources, due to higher wind speeds aloft, and this difference tends to be amplified 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 well-ventilated by southeasterly and westerly winds, respectively. In contrast, low-ozone days exhibit strong flow 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 periods of high ozone across OTAG results 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 these patterns are not always seen for every ozone episode, they point out the necessary condition for regional ozone events; namely stagnation followed by transport. Such conditions are seen in every episode 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 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.

What is the lifetime of ozone and precursors over the OTAG region?

The atmospheric lifetime of ozone influences the distance over which it is transported. The "lifetime" discussed here includes the initial formation time of ozone (6-12 hours), as well as the decay time due to removal processes (24-30 hours), yielding a total lifetime of about 1-2 days. There is evidence that the ozone formation time and removal time in urban plumes is shorter, with a resulting lifetime of about 20 hours while in elevated, NOx-rich point source plumes, the ozone lifetime can be twice as long. Temporal analysis of ozone data indicates that on average, ozone events tend to last longer in the southern portion of the OTAG domain than in the North.

What are the ranges of ozone transport implied from various studies?

The areas impacted by ozone from large urban areas in the OTAG domain were deduced from multiple types of analysis, resulting in downwind distances ranging from less than 150 miles to over 500 miles. The direct influence of specific urban areas can be easily traced for specific episodes some 150-200 miles into their surrounding rural background. Beyond that, the urban influence tends to merge indistinguishably into the regional ozone pattern. Statistical correlation analyses of the regional ozone pattern suggest downwind distances on the order of 300-500 miles. 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 urban impact areas of about 400 miles. While an estimate of impact distance based on any single method is rather uncertain, the coinciding range of the various different methods increase confidence in the accuracy of these calculated ranges.

How do ozone concentrations during the OTAG modeling episodes compare to the

climatological values?

(In preparation)

How do transport conditions compare to the climatological pattern?

(In preparation)

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, and the low ozone concentrations tend to originate outside the OTAG region. 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 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 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 nonattainment with respect to the new standard.

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. Geographically, the region located in the center of the OTAG domain tends to impact on downwind areas regardless of which direction the wind blows. In addition, trajectory residence-time 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, so 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 all 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 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 over predict daily maximum ozone levels in the northern portion of the domain and under predict the same for the southern portion of the domain. 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. Comparison of model predictions to ozone precursor data, while limited by the availability of measurements, shows generally poor agreement, especially for isoprene (from biogenic VOC emissions). Nonetheless, 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 to evaluate control strategy impacts on regional concentrations (where NOx concentrations typically limit the formation of ozone).

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. In addition, 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.

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.