Weekly Pattern of Ozone over the OTAG Region

Contents:

• Background
• Relevance to OTAG Mission and Goals
• Data Sources and Processing
• Results
• Figure Captions

Background

Human activities follow a pronounced weekly cycle which results in cyclic emissions of ozone precursors. It is therefore expected that ambient ozone concentrations would also exhibit a cyclic weekly pattern. The weekly cycle is unique among the ozone periodicities in that it is explicitly driven by human activities. Both the seasonal and the diurnal cycles are also influenced by natural driving forces such as solar radiation in addition to the human influence. Since the atmospheric variables are not expected to vary from one day of the week to another, the weekly cycling of the ozone signal provides an outstanding opportunity to study the man-induced ozone contribution. In particular, the weekly pattern analysis reveals the consequences of emission reductions from weekday to weekends. Hence, it represents an emission reduction scenario that can be verified by the available data. This report was prepared in support of the OTAG Air Quality Analysis Workgroup.

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.

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 ozone on weekdays and weekends. The specific question addressed is: Are the ozone levels different during weekdays and weekends?

1. The weekly emission cycle results in a weekly ozone cycle. Over the entire OTAG region, the 90th percentile ozone is reduced from 87 to 81 ppb from Friday to Sunday. The daily exceedences (>120 ppb) are reduced on Sundays to one third of the exceedences on Fridays. This analysis indicates that if the OTAG-wide weekday emissions would be reduced to the current weekend levels, then ozone exceedences would decline by at least factor of three.
   

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
EMEFS	Eulerian Model Evaluation and Field Study	1988
SCION	Southern Oxidant Study	1993, 1995
LADCO	Lake Michigan Air Directors Consortium	1991 (88, 93, 95)
GEORGIA	State of Georgia	1988, 91, 93, 95
NORTH CAROLINA	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.
6. The average ozone concentration for each station was obtained by averaging the daily maximum ozone over the five year period, 1991-1995. For each day of the week, the average daily maximum ozone as well as the 50th, 75th, 90th and 95th percentiles of ozone was obtained by averaging the ozone concentration over the entire OTAG region for a specific day of the week. The exceedences were calculated by counting the number of days the daily maximum ozone exceeded 120 ppb during the 1991-1995 period and each day of the week.

Results

Weekly Cycle of Summertime Daily Maximum Ozone Averaged Over the OTAG Region

The OTAG scale features of the ozone weekly cycle are examined by averaging the ozone concentration over the entire OTAG region for each day of the week. The results are depicted in Figure 1. The average daily maximum ozone as well as the 50th, 75th, 90th and 95th percentiles of ozone are shown for each day of the week. The weekly cycle is repeated twice in order to convey the weekly periodicity of the ozone signal.

Both the average and the 50th percentiles of the OTAG-wide ozone are virtually constant. The weekday-weekend concentration difference is only about 2 ppb, or about 3% of the mean signal of approximately 60 ppb. The 75th percentile is marginally periodic with about 4% periodicity. However, the 90th and 95th percentiles of OTAG-wide ozone show a substantial weekly cycle. The lowest concentrations in the 90th percentile occur on Sundays (81 ppb), following a buildup of concentration during Mondays and Tuesdays. By Wednesdays, Thursdays and Fridays the concentration is roughly constant at a level of 87 ppb. The Saturday values are again lower (85 ppb) and drop back down to 81 ppb on Sundays. The overall amplitude of the OTAG-averaged 90th percentile is 6 ppb, or about 7% of the signal. Recognizing that the 90th percentile at background sites (at four corners of OTAG region) is about 65 ppb, the excess ozone concentration within OTAG is about 20 ppb (85-65 ppb). Hence, a weekly amplitude of 6 ppb represents about 1/3 of the OTAG excess ozone signal. It would be highly desirable to explore the amplitude of the weekly cycle in ozone precursor emissions, particularly in oxides of nitrogen.

Spatial Pattern of Ozone for Each Day of the Week

Given that the entire OTAG region exhibits a substantial weekly ozone fluctuation it is of interest to explore the ozone spatial pattern for each day of the week. It has been established elsewhere that the anthropogenic ozone signal is most pronounced at the higher percentile levels. For this reason, the weekday-weekend difference is examined using maps of 90th percentile daily maximum ozone. A map has been produced for each day of the week as shown in Figures 2a-f. It is beyond our scope here to examine each day of the week in significant detail. Instead, attention is focused on the maps of Fridays and Sundays.

On Fridays, the entire Atlantic seaboard from Virginia to southern Maine shows 90th percentile daily maximum ozone in excess of 90 ppb. Also, Dallas-Ft. Worth, and Houston and their surroundings have >90 ppb. Virtually all metropolitan areas have high ozone in their surrounding areas.

The corresponding map of Sundays shows a remarkably different picture. The size of the high ozone (>90 ppb) region over the eastern seaboard has been reduced to a narrow strip between Washington, DC and New York. The lower New England states, Connecticut and Rhode Island show reduction in concentration from >90 ppb to <80 ppb. Similar dramatic reductions are evident in the areas surrounding Atlanta, Birmingham, Dallas-Ft. Worth, St. Louis, Chicago as well as for other population centers.

The change in the ozone concentration between Fridays and Sundays is further highlighted by mapping the Fridays-Sundays difference (Figure 3). For each monitoring station the Friday values were subtracted from the corresponding Sunday value and the difference is shown in the contour map. Red and yellow shades represent the magnitude by which Fridays exceed the Sundays value. Blue shades indicate regions where Sundays exceed Friday concentrations. It is clear that the Friday excess values are most pronounced surrounding the major metropolitan areas such as the northeastern megalopolis, Atlanta, Dallas-Ft. Worth, etc. This is another indication that the urban centers strongly influence the ozone concentrations in their surrounding regions as evident by their pulsating concentrations.

It is interesting to note, that the ozone levels just south of the Ohio River Valley, west Tennessee, Kentucky, West Virginia, west Pennsylvania. Evidently, the ozone precursor emissions in this region have a weekly cycle which differs markedly from the metropolitan areas. It is conceivable that the ozone in this region is largely contributed by major point sources with modest weekday-weekend emission changes. However, this hypothesis needs to be supported by further evidence.

Spatial Pattern of Ozone Exceedences for each Day of the Week

The ozone exceedences (days/year with >120 ppb) for each day of the week are displayed in Figures 4a-f. The maps show that throughout the week, the exceedences occur in the vicinity of large urban areas. They also show, that the number of exceedences is significantly lower on Sundays compared to the weekdays. The Friday-Sunday exceedence difference map (Figure 5) shows that the largest weekly ozone exceedence pulsing is over the metropolitan areas and less pronounced in the Ohio River Valley.

The magnitude of the weekly ozone cycle is further illustrated in the weekly chart of exceedences (Figure 6). It shows the number of exceedences over the entire OTAG region for each day of the week, repeated twice to illustrate periodicity. The values were further normalized for the weekly average to indicate the relative amplitude of the weekly exceedence cycle.

The OTAG-average number of relative exceedences (> 120 ppb), are lowest on Sunday, at 0.4 (40%) of the weekly average. On Fridays, exceedences occur 1.35 (135%) times the weekly average. This shows that on Fridays, the exceedences are more the threefold of the Sunday values.

The weekly pulsing of the ozone exceedences at 100, 80 and 60 ppb levels is much less significant. In fact, counting the ozone values that exceed 60 ppb shows virtually no weekly cycle. This further suggests the existence of a tropospheric background concentration (at<60 ppb) that is uniform throughout the week.

Another interesting feature of the 120 ppb exceedence cycle is the linear increase of exceedences from Monday through Friday. This behavior is consistent with a mixed box model for the OTAG region: Every weekend, the ozone in the OTAG region is reduced due to emission reductions. As the week progresses, the regional OTAG ' box' is filled up, gradually increasing the concentration "spillovers" (>120 ppb) until Friday.

The policy implication of this observation is that by reducing the current weekday emissions to the current weekend values would reduce the number of ozone exceedences by a factor of three or more. Unfortunately, we are not aware of the nature and magnitude of the weekend/weekday emission changes that yield such a significant reduction.

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Figure Captions