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 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..
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.
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?
The ozone data used in this report were collected from multiple
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.
The data processing was conducted in the following major steps below:
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.
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.
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
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