UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Office of Air Quality Planning and Standards (OAQPS) Research Triangle Park, North Carolina 27711

MEMORANDUM [DATE] SUBJECT: Analysis of Overlapping 8

- hour Daily Maximum Ozone Concentrations FROM: Benjamin Wells, Air Quality Assessment Division, OAQPS TO: Ozone NAAQS Review Docket ( # EPA

- HQ

- OAR

- 2008

- 0699 )
I. Overview Appendix P to 40 CFR Part 50 contains the data handling conventions for determining whether the 2008 National Ambient Air Quality Standar ds (NAAQS) for ozone (O 3 ) are met at an ambient air quality monitoring site. This appendix contain s instructions for calculating daily maximum 8

- hour average (MDA8) values from hourly O 3 concentration measurements collected at ambient monitoring sites. In particular, these instructions state that “ the daily maximum 8

- hour concentrations from two consecutive days may have some hourly concentrations in common. Generally, overlapping daily maximum 8

- hour averages are not likely, except in those non

- urban monitoring locations with less pronounced diurnal variation in hourly concentrations. ” T his may lead to “double counting” exceedances of the O 3 NAAQS whenever monit ors are affected by transport which occurs overnight. For the current O 3 NAAQS review , there is concern that O 3 monitors in rural areas may be disproportionately impacted by this phenomenon, which co uld lead to additional exceedance s of a revised O 3 NAAQS in areas which have few anthropogeni c sources of ozone precursors. Therefore, it is useful to quantify the frequency and magnitude of overlapping MDA8 values, and their potential impact on attainment of a revised O 3 NAAQS.This memo discusses the results of a technical analysis which examine s overlapping

MDA8 values at ambient O 3 monito ring sites in the U.S. based on air quality data from 2004 to 2013. First, the analysis identifies the frequency of overlapping MDA8 events at each site, as well as the number of these events with concentrations that exceed the current O 3 standard level of 75 ppb and alternative l evels of 70 ppb, 65 ppb, and 60 ppb. Next, the a nalysis examines the potential for over lapping MDA8 events to impact the annual 4 th highest MDA8 concentration, the metric used to determine attainment of the O 3 NAAQS. Finally, the analysis examines two methods for removi ng overlapping MDA8 events from consideration when calculating MDA8 value s, and quantifies t he impact of each method on annual 4 th highest MDA8 concentration s . II. Data and Analysis Hourly O 3 concentration data from 2004 to 2013 were retrieved from EPA’s Air Quali ty System (AQS) database for 1,654 ambient O 3 monitor ing site s in the U.S. For each monitored day, the MDA8 value and the starting hour of the period where the MDA8 value occurred were determined according to Appendix P to 40 CFR Part 50

. Overlapping MDA8 events were defined as any pair of consecutive days where the MDA8 values were based on 8

- hour periods with one or more hours in common (i.e., the starting hours were less than 8 hou rs apart). Data completeness criteria were then applied to the MDA8 values to remove the impact of sites with little or no data on the analysis. First, for each site, any years where valid MDA8 values were available for fewer than 50% of the days in the O 3 monitoring season were removed from the analysis. Second, from the remaining data, sites with fewer than 5 years of data were removed from the analysis. The final data set used in the analysis consisted of 1,261 O 3 monitoring sites. For the second portion of the analysis, two methods were used to remove overlapping MDA8 events from the data. The first method, referred to as the “minimal” method, iterativelyidentifies each pair of overlapping MDA8 values, keeps the higher MDA8 value in each pair ,

and replaces the lower MDA8 value in each pair with the ne xt highest MDA8 value on that day which does not over lap with any other MDA8 values. The second method, referred to as the “blanket” method, removes the 8

- hour periods with starting hours from 12:00 AM to 7:00 AM each day, then re

- calculates the MDA8 value s based on the remaining data. Alternative sets of MDA8 values were calculated using each method , and annual 4 th highest MDA8 values were determined from each alternative set of MDA8 values. The two sets of alternative annual values were then compared with the annual 4 th highest MDA8 values calculated according to Appendix
P to 40 CFR Part 50

. III. Results and Discussion
A. Frequency and Magnitude of Overlapping MDA8 Events Nationally, overlapping MDA8 events occurred on 3.5% of all monitored days in 2004 to
2013. The percentages of these overlapping MDA8 values greater than 60, 65, 70, and 75 ppb are shown in Table 1. While the vast majority of overlapping MDA8 values were less than or equal

to 60 ppb, the proportion of overlapping MDA8 values greater than each le vel increases rapidly below 75 ppb, roughly doubling for e ach 5 ppb increment

. Table 1. Percentage of overlapping MDA8 values greater than various MDA8 levels
MDA8 Level (ppb) 75 70 65 60 % of Overlapping MDA8 Values > Level 0.8% 1.6% 3.3% 6.3%
Figure 1 shows the distribution of the start hours for all monitored MDA8 values. Under typical conditions, ozone concentrations follow a diurnal pattern, with minimum concentrations occurring in the early morning hours and maximum concentrations occurring during t he late afternoon hours. Since MDA8 value s are defined by their starting hour, the peak 8

- hour averagevalues typically occur near the middle of the day, as reflected in Figure 1. However, Figure 1

also shows small spikes in the fraction of MDA8 values occ urring in the first and last hours of the d ay. These spikes reflect transport situations where ozone formed during the daytime reaches a monitoring site downwind from the source area during the evening or early morning hours. Next, Figures 2 and 3 show the distributions of the MDA8 start hours based on days with MDA8 values greater than 60 ppb and 75 ppb, respectively. These figures show that high MDA8 values are even less likely to occur during evening or early morning hours. Less than 3% of MDA8 values gr eater than 60 ppb occurred between 6:00 PM and 6:00 AM, and less than 2% of MDA8 values greater than 75 ppb occurred during these hours, compared with about 12% of all monitored MDA8 values. Finally, Figure 4 shows the distribution of the MDA8 start hours based on the days with the 4 highest MDA8 values for each site and year. This distribution appears to be similar to the distribution of MDA8 values for 60 and 75 ppb, with a slight shift toward more MDA8 values occurring later in the afternoon.Fig ure 1. Distribution of MDA8 start hours based on all monitored days

Figure 2 . Distribution of MDA8 start hours base d on days with MDA8 values greater than 60

ppbFigure 3 . Distribution of MDA8 start hours base d on days with MDA8 values greater than 75 ppb

Figure 4. Distribution of MDA8 start hours based on the annual 4 highest MDA8 values at each

monitoring siteF igure 5 shows a map of the annual average number of overlapping MDA8 events at each

monitoring site based on 2004

- 2013 air quality data. Nationally, 99.5 % of all monitors recorded at least one overlapping MDA8 event during this period , 81% of all monitors had an average of at least one event per year, and 12% of all monitors had an average of ten or more overlapping
M DA8 events per year, up to a maximum o f 60 events per year . Furthermore, Figure 5 shows that the occurrence of overlapping MDA8 events do es not appear to be limited to rural areas, as Appendix P claims. Next, Figures 6 through 9 show maps of the annual ave rage number of overlapping MDA8 values greater than 60 ppb, 65 ppb, 70 ppb, and 75 ppb, respectively. T he se figures show that the vast majority of high overlapping MDA8 values occur in rural areas. Physically, this pattern is consistent with the underlying principles of atmospheric chemistry related to ozone formation and titration. Near urban areas, higher NOx levels favor increased O 3 titration rates, which cause O 3 concentrations to decrease quic kly after sunset. By contrast, in rural areas with lower NOx levels, titration rates are much slower, and ozone molecul es may be transported long distances overnight. Based on Figures 6 through 9 , t he monitor s which most frequently measured high overlappin g MDA8 values tended to fall into one of two categories. The first category is mountainous areas or regions with complex terrain. Specifically, many moni tors in the Appalachians, the Ozark Plateau, the inter

- mountain We st, and the mountain ranges of e aster n California appear to be frequently impacted by high overlapping MDA8 values. These regions are often characterized by flatter diurnal patterns in O 3 concentrations, especially higher nighttime concentrations, thus O 3 transport events are more likely to c ause higher overlapping MDA8 values in these regions. The second category is coastal areas downwind from major urbanareas. Specifically, monitors along the Northeast Atlantic coast, the Gulf of Mexico, and the

Great Lakes region appear to be frequently im pacted by high overlapping MDA8 values. Low NOx emissions and high winds make favorable conditions for O 3 transport over open water, often resulting in high overlapping MDA8 values downwind from large coastal cities such as Chicago and New York. Finally, Table 2 shows percentages of monitors with various frequencies of overlapping MDA8 values greater than each MDA8 level of interest. Table 2 shows that overlapping MDA8 values exceeding the current O 3 standard of 75 ppb are relatively infrequent, but the fr equency of overlapping MDA8 values exceeding levels of 70, 65, and 60 ppb increases rapidly. Thus, there is increasing con cern that exceedances of the NAAQS may be counted twice if the level of the standard were lowered.

Figure 5 . A verage annual number of overlapping MDA 8 eventsFigure 6 . Average annual number of overlapping MDA8 values greater than 60 ppb

Figure 7 . Average annual number of overlapping MDA8 values greater than 65 ppbFigure 8 . Average annual number of overlapping MDA8 values greater than 70 ppb

Figure 9 . Average annual number of overlapping MDA8 values greater than 75 ppbTable 2. Percentages of sites with various frequencies and levels of overlapping MDA8 values

Frequency of overlapping MDA8 values (% of sites) Occurrence of

any overlapping MDA8 values One or more

overlapping MDA8 values per year Five or more

overlapping MDA8 values per year Ten or more

overlapping MDA8 values per year All Days 99.5 80.9 29.0 11.7 MDA8 > 60 ppb 37.7 7.7 3.3 1.9 MDA8 > 65 ppb 23.3 5.1 1.8 0.8 MDA8 > 70 ppb 13.8 3.1 0.9 0.2 MDA8 > 75 ppb 8.3 2.1 0.3 0.0
B. Alternative Methods for Calculating MDA8 Values The second portion of the analysis focused on applying the minimal and blanket methods described previously in section II to the ambient O 3 monitoring data in order to assess the impact of each method on the distribution of MDA8 values . In general, while the minimal method was designed to remove the fewest 8

- hour average values necessary to ensure that there were no overla pping MDA8 values, this method was also more challenging to implement, and more computationally intensive than the blanket method. The minimal method would also present challenges for real

- time O 3 data reporting systems , because the resulting MDA8 values f or any given day could be affected by concentrations measured on the following day. Figures 10 and 11 show the distribution of MDA8 start hours for all monitored days, after applying the minimal and blanket methods, respectively. T he minimal method has only a small impact w hen compared to the original distribution of MDA8 start hours in Figure 1

. The minimal method appears to reduce the frequency of MDA8 values occurring in the first and last hours of the day, while slightly increasing the frequency of M DA8 values occurring throughout the rest of the day by a small amount. The blanket method has a larger impact upon the distribution, since by design MDA8 values are not allowed to occur between midnight and 6:00 AM. According to Figure 11 , there is a subst antial increase in the number of MDA8 values occurring at 7:00 AM, and modest increases in the frequency of MDA8 values occurring throughout the rest of the day.Figure 10 . Distribution of MDA8 start hours based on all monitored days, with overlapping MDA8 values removed using the minimal method

Figure 11 . Distribution of MDA8 start hours based on all monitored days, with overlapping

MDA8 values removed using the blanket methodNext, the analysis focused on the impact of the minimal and blanket methods on the annual

4 th highest MDA8 values. Figure 12 shows the distribution of the MDA8 start hour based on the 4 highest MDA8 values for each site and year after applying the minimal method, while Figure
13 show s the same distribut ion after applying the blanket method. Given the tendency for the vast majority of peak O 3 concentrations to occur during the daytime hours, it is not surprising that neither method has a large impact upon the distribution of MDA8 values.

Figure 12. Dist ribution of MDA8 start hours based on the annual 4 highest MDA8 values at each

monitoring site, with overlapping MDA8 values removed using the minimal methodFigure 13. Distribution of MDA8 start hours based on the annual 4 highest MDA8 values at each monitoring site, with overlapping MDA8 values removed using the blanket method The analysis concludes by examining the locations where the 4 th highest MDA8 values would be impacted by the removal of overlapping MDA8 values using the minimal or blanket methods. First, Figures 14 and 15 show maps of the number of years during the 2004

– 2013 period where applying the minimal and blanket methods re sulted in a lower 4 th highest MDA8 value. Next, Figures 16 and 17 show maps of the mean decrease in the 4 th h ighest MDA8 value over the 2004

– 2013 period after applying the minimal and blanket methods. Finally, Figures 18 and 19 show maps of the maximum a nnual decrease in the 4 th highest MDA8 value s du r ing this period when applying these two methods. Nationally, 5.5% of all monitoring sites had at least one year where the 4 th hi ghest MDA8 value decreased when applying the minimal method, compared with 7.5% when applying theblanket method. On average, 1.1% of sites per year had a lower 4 th highest MDA8 value using

the minimal method, compared with 1.5% of sites per year using the blanket method. The average decrease in the 4 th highest MDA8 value for tho se site/years affected was 2.0 ppb for both methods

. Overall, the impact of both methods on the 4 th highest MDA8 values was relatively small, and the magnitude of the decreases in the 4 th highest MDA8 values were quite similar between the two methods. Accordi ng to Figures 14 to 19, the spatial patterns in the monitoring sites affected by the minimal and blanket methods were quite similar. Not surprisingly, t he sites where the annual 4 th highest MDA8 values decreased were predominately located in rural areas, p articularly the same mountainous and coastal areas downwind from large urban areas as identified in the previous section. Many of these sites appeared to be affected by overlapping MDA8 events that impacted the annual 4 th highest MDA8 value on a regular ba sis. On the other hand, the two methods did not appear to have any impact upon the annual 4 th highest MDA8 values at monitoring sites near urban areas. The maximum impact upon the 4 th highest MDA8 value was less than 3 ppb in the vast majority of cases, but t here were some isolated instances whe re both methods reduced the annual 4 th highest MDA8 value by 4 ppb or more.Figure 14. Number of years in 2004

– 2013 where applying t he minimal method resulted in a lower annual 4 th highest MDA8 value

Figure 15. Number of years in 2004

– 2013 where applying the blanket method resulted in a

lower annual 4 th highest MDA8 valueFigure 16. Average decrease in the annual 4 th highest MDA8 value when applying the minimal method, 2004

– 2013

Figure 17. Average decrease in the annual 4 th highest MDA8 value when applying the blanket

method, 2004

– 2013Figure 18. Maximum decrease in the annual 4 th highest MDA8 value when applying the minimal method, 2004

– 2013

Figure 18. Maximum decrease in the annual 4 th highest MDA8 value when applying the blanket

method, 2004

– 2013IV. Summary

The first part of the analysis showed that while the vast majority of MDA8 values were based on periods starting near the middle of the day, overlapping MDA8 events accounted for 3.5% of all monitored days, and these events occurred at least once at nearly every monitoring site in the U.S. during the 2004

– 2013 period. However, more than 99% of these events were associated with concentrations meeting the current O 3 standard of 75 ppb, and approximately 94% of these events were associated with concentrations less than or equal to 60 ppb. When looking at overlapping MDA8 events associated with concentrations above 60 to 75 ppb, it is immediately apparent that the vast majority of these events occur in rural areas with few local anthropogenic emissions sources for ozone precursors. Thus, high O 3 concentrations occurring during overlapping MDA8 events are most likely due to transport of O 3 to rural areas with low NOx tit ration rates. The second part of the analysis evaluated two potential methods for removing the impact of overlapping MDA8 events from the calculation of the annual 4 th highest MDA8 value metric used for determining compliance with the NAAQS

. While there we re some minor differences, in general the resulting impacts upon the annual values were quite similar in terms of the frequency, magnitude, and locations affected. On average, l ess than 2% of the annual 4 th h ighest MDA8 values were affected each year using either method, and of those values impacted, both methods showed an average decrease of 2.0 ppb in the annual 4 th highest MDA8 value. The monitoring sites where the annual 4 th highest MDA8 values were impacted were located predominately in rural areas for both methods. In conclusion, the ambient data analysis showed that the current data handling convention in Appendix P to CFR Part 50 whereby exceedances of the NAAQS resulting from overlappingMDA8 events may be counted twice are increasingly likely to affect attainment of the NAAQS if

the level of the standard is lowered from the current level of 75 ppb. In addition, this practice is likely to have the greatest impact on rural areas , which could result in additional areas with few local anthropogenic so urces of ozone precursors being designated nonattainment for a revised NAAQS. Such an outcome may be avoided b y using one of the two methods described in this analysis to remove overlappi ng MDA8 values from the annual 4 th highest MDA8 calculations.