Spatial Pattern of Daily Maximum Ozone over the OTAG Region

Rudolf B. Husar

Center for Air Pollution Impact and Trend Analysis (CAPITA)

Washington University

St. Louis, MO 63130-4899

September 16, 1996



Ozone is a secondary pollutant that impacts receptors near the sources, within few hundred kilometers, as well as remote receptors, through long range transport, thousand or more kilometers away. Regional ozone, transported over long distances may significantly increase the ozone concentration at the boundaries of some nonattainment areas. Ozone attainment in such areas can only be reached by reductions within the nonattainemnt area (urban ozone) as well as through reductions of regional ozone at the area boundaries. OTAG is mostly concerned about the regional ozone, outside of major metropolitan (nonattainment) areas, and with the transport of regional ozone into nonattainment areas.

The Air Quality Analysis Workgroup has identified the evaluation of the general ozone spatial patterns as an activity. This report, prepared for the OTAG Air Quality Analysis Workgroup, is in response to the expressed needs. The quantification of ozone concentration within and outside urban areas can be established based on the available monitoring data.

The Framework for Ozone Pattern Analysis

The air quality analysis below, makes extensive use of the concepts of pattern and pattern analysis. Pattern analysis is a structured approach to the organization and presentation of air quality data. The ozone pattern analysis over the OTAG region is also based on the following physical considerations..

  • 1. The ozone concentration at a given location is composed of contributions from global tropospheric background ozone, the regional ozone from the superimposed emissions within the OTAG region , and from urban ozone that is in excess of the tropospheric and regional background.
  • 2. The spatial pattern can be examined on global, regional (e.g. the OTAG region) or on urban scales, with distinct pattern at each scale. The global-scale of tropospheric ozone constitutes the boundary condition for the regional ozone. Similarly, the regional ozone is the boundary condition of the urban ozone in a hierarchical relationship. In fact, for purposes of this analysis, regional ozone is defined as the ozone concentration at the boundaries of urban/industrial areas.
  • 3. The temporal ozone signal at any location may be decomposed into several temporal scales: secular (1-100 years), yearly, weekly, synoptic (3-5 days), and daily time scale. Each scale exhibits a distinct temporal pattern. The seasonal scale is superimposed on the secular trend, the weekly cycle on the seasonal cycle, the diurnal cycle is superimposed on the weekly cycle, etc.
  • 4. The spatial and temporal scales are linked due to transport at, say at 3 to 5 m/s average transport speed. The corresponding transport distance for a week is on the order of 2-3000 km, for 4 days it is 1-2000 km and for one day is 250-400 km. For purposes of OTAG, the most relevant ozone transport is across the boundaries of nonattainment areas.
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    Relevance to OTAG Mission and Goals

    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 Results

    The policy-relevant section of this report addresses the spatial pattern of regional and urban distributed ozone over the OTAG region. The specific question: What is the magnitude of urban and regional ozone?

    The OTAG region is surrounded by relatively uniform tropospheric ozone concentrations of 30-40 ppb. While the average ozone within OTAG is about 60 ppb, the difference (60-80=20 ppb) average ozone is due to emissions within the OTAG region, hence, it is controllable.

    The main ozone pool in the OTAG region resides over the Ohio River Valley and also over specific major metropolitan areas. A general reduction of ozone throughout the OTAG region can be accomplished by emission reductions in those areas. The regional ozone concentration at the boundaries of nonattainment areas vary regionally. In the south the regional contributions are less pronounced than the ozone levels on the boundaries of the northeastern nonattainment areas. This implies that regional ozone transport is more important in the Northeast.

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    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
  • Eulerian Model Evaluation and Field Study
  • 1988
  • Southern Oxidant Study
  • 1993, 1995
  • Lake Michigan Air Directors Consortium
  • 1991 (88, 93, 95)
  • State of Georgia
  • 1988, 91, 93, 95
  • 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 exeedances of daily maximum ozone was computed.
    5. The results for all stations were contoured and plotted on maps and for easy presentation.

    The average ozone concentration for each station was obtained by averaging the daily maximum ozone over the five year period, 1991-1995. The ozone concentration percentiles for each monitoring station were calculated using June, July, August data during 1991-1995. Data completeness condition of 25% was applied, such that during the 5 summers (5*90=450 days) at least 112 days had valid daily maximum ozone concentration data. The spatial maps were obtained by computing the percentile for each station and contouring the values using standard procedures. Such maps were generated for 10, 50, and 90, percentiles.

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    Spatial Pattern of Average Summertime Daily Maximum Ozone

    This section examines the summertime (June, July, August) 1-hour daily maximum ozone concentration.

    The average summertime daily maximum ozone concentration is shown in Figure 1. The coloration of the contours follows the rainbow pattern, the lower values are blue (50 ppb), and the highest values is red (70 ppb). The entire geographic range of average daily maximum ozone is within a factor of 2, between 40-80 ppb.

    The values at about 40 ppb, roughly correspond to the northern hemispheric global average ozone concentrations in the lower troposphere. It represents the tropospheric background summertime ozone concentration upon which the ozone contributions from eastern US sources are superimposed. On the seasonal time scale, the average midday ozone concentration near urban industrial areas is (80 ppb) about twice of the lower tropospheric background (40 ppb). However, the ozone concentration spatially averaged over the entire eastern US (OTAG) domain is about 60 ppb, which is only about 50% higher than the tropospheric background.

    The spatial pattern of the summer average ozone concentration indicates that all four corners of the OTAG region are bounded by ozone levels that are near the tropospheric background levels. The Florida Peninsula, the Texas Gulf Coast, the Upper Midwest, as well as Northern Maine, all show about 40 ppb summertime average concentrations.

    Within each of the four background "corner" regions, all the monitoring sites show a consistent, spatially uniform ozone pattern that has small or modest day-to-day variation. The spatial-temporal uniformity of ozone is a further indication that these "edges" of the OTAG region represent the tropospheric background. The relative spatial and temporal uniformity of ozone at the OTAG boundaries is rather significant, since it allows the consideration of the OTAG region as an "island" in the sea of the lower tropospheric ozone. Hence, the OTAG process does not have to concern itself with a highly variable ozone boundary condition.

    Notable exception to the uniform boundary condition is at the OTAG boundary that stretches between Texas, Oklahoma, and Nebraska, where the ozone inflow concentrations from the West are about 50 ppb or 10 ppb higher than the background at other boundaries. The eastern OTAG boundary at the Atlantic seaboard is primarily an ozone outflow boundary where the concentration is determined by the ozone pattern within the OTAG region, rather than by external influences.

    The average daily maximum ozone concentration within the OTAG region exhibits a spatially patchy pattern. The highest levels occur in the Washington-New York corridor, where the average daily maximum ozone exceeds 70 ppb. Additional large scale elevated ozone covers Indiana and Ohio, just north of the Ohio River. Smaller geographic ozone hot spots can be observed in the vicinity of the metropolitan areas, such as Atlanta, Dallas-Ft. Worth, Chicago, St. Louis, Memphis, etc., where the average summertime ozone reaches up to 70 ppb.

    Notably absent are average ozone hot spots around Houston, New Orleans, Detroit, and Pittsburgh. Also, virtually the entire New York-New England region has <60 ppb summertime average concentrations. Several low concentration ozone "holes" can be seen over the southern Appalachian Mountains. These are higher elevation sites that may be above the height of the man-made ozone layer.

    The absolute magnitude of). West-east cross sections are plotted at South Dakota-New England, Kansas-Maryland, and southern Texas- northern Florida. The two north-south cross sections were from Texas to North Dakota and from Florida to eastern Ohio (Figure 2a). The northern west-east cross section (Figure 2b) shows that over South Dakota the ozone level (42 ppb) is slightly over the tropospheric background of 35-40 ppb. By Lake Michigan, near Chicago, the average ozone level rises above 62 ppb, and then declines over Michigan to 55 ppb. At about Detroit, it again rises to 60 ppb, followed by a decline throughout New York and New England.

    The middle west-east cross section (Figure 2c) begins at about Kansas City where the concentration rises from 52 to 60 ppb and falls again to 55 ppb. Like the Chicago area, Kansas City exhibits a clear peak ozone compared to the surrounding area. At about St. Louis, the concentration again rises to over 60 ppb, due to the St. Louis urban impact. As the cross section passes through the Ohio River Valley the concentrations further rise to over 65 ppb and remain at that level to western Pennsylvania. The ozone level over south-central Pennsylvania declines somewhat before it rises again over Maryland to 73 ppb. The excess concentration near Baltimore is about 10 ppb over the concentration in south-central Pennsylvania. The ozone "saddle" point (63 ppb) in south-central Pennsylvania may be significant in that it represents the concentration at the boundary of the Washington DC-Maryland urban ozone.

    The west-east cross section over southern Texas (Figure 2d) shows about 45 ppb, which is somewhat above the tropospheric background. Excess concentrations of 5-10 ppb can be observed at Houston, New Orleans and northern Florida.

    The two north-south cross sections (Figure 2e) indicate that North Dakota is close to the tropospheric background at about 40 ppb, and there is a graduate increase of ozone levels to 52 ppb in Oklahoma, followed by a decline to 37 ppb in southern tip of Texas. The north-south cross section from eastern Ohio to Florida, clearly shows a strong ozone peak of 70 ppb over the Ohio River Valley followed by a gradual decline southward to the background of 35 ppb in southern Florida.

    In summary, the spatial cross section of average concentrations further illustrates the existence of a broad ozone pool of about 65-70 ppb within and just north of the Ohio River Valley. Other metropolitan areas also show their impact as smaller ozone bulges. Also, the west-east cross section from Kansas to Maryland across the industrial states seem to indicate generally rising ozone levels toward the east, as if there was a cumulative addition of one ozone source on top of the other as air passes from west to east. This hypothesis requires further supporting evidence.

    Spatial Pattern of Ozone Concentration Percentiles

    Ozone exhibits strong day to day variation throughout the summer. For this reason, the time-averaged concentration is an inadequate measure of spatial pattern. Since the shape of the ozone distribution function of concentrations is also variable from one region to another, it is beneficial to express the ozone distribution using non-parametric statistics, i.e. percentiles.

    The 10th percentile map (Figure 3) represents the daily maximum ozone concentration during the cleanest 10% of the days. At the edges of the OTAG region, 10th percentile ozone daily maximum concentration is below 30 ppb. Elevated 10th percentile values, in excess of 40 ppb extend through the industrial Midwest, from Missouri through Illinois, Indiana, Ohio, Kentucky and Virginias. Throughout this industrial region the 10th percentile ozone concentration varies by about 5 ppb, 42-47 ppb. In other words, there is a persistent and spatially uniform area that has about 15 ppb excess ozone over the tropospheric background even during the cleanest days. Conspicuously absent are higher concentrations in the 10th percentiles for major metropolitan areas. There is virtually no concentration difference between urban areas and their surrounding. This implies that during low-ozone periods both urban and rural areas are well ventilated, preventing the occurrence of local urban ozone buildup. However, this is not the case for the industrial region surrounding the Ohio River Valley, where elevated ozone persists even when the rest of the OTAG region is clear.

    The 90th percentile map given in Figure 4 is remarkably different from the 10th percentile. The ozone concentration during the highest 10% of the days is elevated in the vicinity of major metropolitan centers. In particular, the entire eastern seaboard from Virginia to Rhode Island is covered with high ozone levels (100 ppb) at least 10% of the days. Additional high 90th percentiles in excess of 100 ppb can be observed for Dallas-Ft. Worth, Houston, Atlanta, as well as downwind of Chicago, on eastern shores of Lake Michigan. Southern Indiana and southern Ohio also indicate elevated 90th percentile ozone. The most significant difference between the 10th and 90th percentiles is that the 90th percentile is highest near urban metropolitan centers, while the 10th percentile is highest over the industrial Ohio River Valley.

    The southern Appalachian Mountains have also low 90th percentile ozone. In fact, the entire spine of the Appalachian Mountains has lower 90th percentiles than their surrounding. Evidently, high ozone concentration in lower lying areas do not reach the higher elevation Appalachian sites. A further complication arises in examining the high elevation sites, Whiteface Mountain, NY and Mount Washington, NH. Both sites show higher concentrations than their surrounding low lying areas. A full understanding of the high-low elevation difference can only be reached if the vertical ozone profiles are understood.

    The 50th percentiles daily maximum ozone (Figure 4) shows a pattern that is a combination of the 10th and the 90th percentiles. The elevated ozone concentrations over industrial Midwest, just north of the Ohio River coexist with the high 50th percentile concentrations over the eastern megalopolis. Virtually all major metropolitan areas show higher values than their regional surrounding, but the urban excess is much less than for the 90th percentile.

    Spatial Pattern of Ozone Concentration Percentile Difference

    The magnitude of the day to day ozone variation is reflected in the width of the concentration frequency distribution functions. Monitoring sites that have highly variable ozone levels have a broad distribution function with large difference of upper and lower percentiles. Ozone variability can be viewed as a measure of nearby source strength. Monitoring sites that show great day to day variability tend to be in the vicinity of fresh ozone sources, while sites with small day to day variability tend to be background sites exposed to spatially more homogeneous and temporarily more uniform background concentrations.

    The following discussion will be directed toward the 90th-10th percentile difference (Figure 5). The highest variability (90-10 percentile difference) of daily maximum ozone concentration occurs in the vicinity of metropolitan areas. In particular, in Washington-New York megalopolis, Houston, Dallas-Ft. Worth and Atlanta exhibit the highest range with the 90th percentile exceeding the 10th percentile by 55 ppb. Furthermore, a broad moderately variable ozone region stretches throughout the industrial Midwest from Missouri to Pennsylvania. It is interesting to observe that the corners of the OTAG region covering southern Florida, Texas Gulf Coast, Upper Midwest, and Maine show the least percentile difference (<35 ppb) in ozone concentration. This is consistent with the common knowledge that background monitoring sites that sample aged tropospheric ozone do not have high day to day variation. It is also interesting to observe that over the spine of the Appalachian Mountains the concentrations are much less variable than over their surrounding low lying areas. The phenomenon is related to the vertical ozone distribution, but a full explanation is not available.

    In summary, the spatial pattern of daily maximum summertime ozone over the OTAG region is elevated in the vicinity of populated metropolitan centers. A broad area of elevetad ozone is also present over the industrial states from Illinois to Pennsylvania. The urban impact is virtually undetectable during clean days (10 percentile of the ozone concentration). However, the urban influence during high ozone levels (90th percentiles) is very pronounced, but confined largely to the vicinity (few hundred miles) of metropolitan areas. It is therefore evident, that in establishing and quantifying the urban influence on near surface ozone concentrations the upper percentiles of the ozone concentration are more sensitive indicators than the lower percentiles.
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    Figure 1. Average summertime daily maximum ozone concentration over the OTAG region.

    Figure 2. Cross sectional charts of average summertime daily maximum ozone concentration. a) Location of

    West-East and North-South cross sections b) South Dakota-New England. c) Kansas-Maryland (d) Texas-N.

    Florida e) ( North-South cross section Texas-North Dakota and Florida-E.Ohio

    Figure 3. Spatial pattern of 10th percentile of daily maximum ozone .

    Figure 4. Spatial pattern of 90th percentile of daily maximum ozone

    Figure 5 Spatial pattern of 50th percentile of daily maximum ozone

    Figure 6. Spatial pattern of 90th-10th percentile difference . The figure illustrates that the largest ozone variability is in the vicinity of metropolitan areas.

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