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 Standards (NAAQS) for ozone (O3) are met at 

an ambient air quality monitoring site. This appendix contains instructions for calculating daily 

maximum 8-hour average (MDA8) values from hourly O3 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.” This

may lead to “double counting” exceedances of the O3 NAAQS whenever monitors are affected 

by transport which occurs overnight. For the current O3 NAAQS review, there is concern that O3

monitors in rural areas may be disproportionately impacted by this phenomenon, which could 

lead to additional exceedances of a revised O3 NAAQS in areas which have few anthropogenic 

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 O3 NAAQS.
This memo discusses the results of a technical analysis which examines overlapping 

MDA8 values at ambient O3 monitoring 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 O3 standard level 

of 75 ppb and alternative levels of 70 ppb, 65 ppb, and 60 ppb. Next, the analysis examines the 

potential for overlapping MDA8 events to impact the annual 4th highest MDA8 concentration, 

the metric used to determine attainment of the O3 NAAQS. Finally, the analysis examines two

methods for removing overlapping MDA8 events from consideration when calculating MDA8 

values, and quantifies the impact of each method on annual 4th highest MDA8 concentrations.

II. Data and Analysis

Hourly O3 concentration data from 2004 to 2013 were retrieved from EPA’s Air Quality 

System (AQS) database for 1,654 ambient O3 monitoring sites 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 hours 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 O3 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 O3 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, iteratively 
identifies 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 next highest MDA8 value on that day

which does not overlap 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 values based on the remaining data. Alternative sets of 

MDA8 values were calculated using each method, and annual 4th 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 level increases rapidly 

below 75 ppb, roughly doubling for each 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 the late 

afternoon hours. Since MDA8 values are defined by their starting hour, the peak 8-hour average 
values 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 occurring in the first and last hours of 

the day. 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 greater 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.
Figure 1. Distribution of MDA8 start hours based on all monitored days

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

ppb
Figure 3. Distribution of MDA8 start hours based 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 site
Figure 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 

MDA8 events per year, up to a maximum of 60 events per year. Furthermore, Figure 5 shows 

that the occurrence of overlapping MDA8 events does not appear to be limited to rural areas, as 

Appendix P claims.

Next, Figures 6 through 9 show maps of the annual average number of overlapping 

MDA8 values greater than 60 ppb, 65 ppb, 70 ppb, and 75 ppb, respectively. These 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 O3 titration rates, 

which cause O3 concentrations to decrease quickly after sunset. By contrast, in rural areas with 

lower NOx levels, titration rates are much slower, and ozone molecules may be transported long 

distances overnight.

Based on Figures 6 through 9, the monitors which most frequently measured high

overlapping MDA8 values tended to fall into one of two categories. The first category is 

mountainous areas or regions with complex terrain. Specifically, many monitors in the 

Appalachians, the Ozark Plateau, the inter-mountain West, and the mountain ranges of eastern 

California appear to be frequently impacted by high overlapping MDA8 values. These regions 

are often characterized by flatter diurnal patterns in O3 concentrations, especially higher 

nighttime concentrations, thus O3 transport events are more likely to cause higher overlapping 

MDA8 values in these regions. The second category is coastal areas downwind from major urban 
areas. Specifically, monitors along the Northeast Atlantic coast, the Gulf of Mexico, and the 

Great Lakes region appear to be frequently impacted by high overlapping MDA8 values. Low 

NOx emissions and high winds make favorable conditions for O3 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 O3 standard of 75 ppb are relatively infrequent, but the frequency of 

overlapping MDA8 values exceeding levels of 70, 65, and 60 ppb increases rapidly. Thus, there 

is increasing concern that exceedances of the NAAQS may be counted twice if the level of the 

standard were lowered.

Figure 5. Average annual number of overlapping MDA8 events
Figure 6. Average annual number of overlapping MDA8 values greater than 60 ppb

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

Figure 9. Average annual number of overlapping MDA8 values greater than 75 ppb
Table 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 O3 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 

overlapping 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 O3 data reporting systems, because the resulting MDA8 values for 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. The minimal method has only a small 

impact when 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 MDA8 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 substantial 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 method
Next, 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 shows the same distribution after applying the blanket method. Given the tendency for the 

vast majority of peak O3 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. Distribution of MDA8 start hours based on the annual 4 highest MDA8 values at each 

monitoring site, with overlapping MDA8 values removed using the minimal method
Figure 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 4th 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 resulted in a lower 4th highest MDA8 

value. Next, Figures 16 and 17 show maps of the mean decrease in the 4th highest MDA8 value 

over the 2004 – 2013 period after applying the minimal and blanket methods. Finally, Figures 18 

and 19 show maps of the maximum annual decrease in the 4th highest MDA8 values during this 

period when applying these two methods.

Nationally, 5.5% of all monitoring sites had at least one year where the 4th highest MDA8 

value decreased when applying the minimal method, compared with 7.5% when applying the 
blanket method. On average, 1.1% of sites per year had a lower 4th highest MDA8 value using 

the minimal method, compared with 1.5% of sites per year using the blanket method. The 

average decrease in the 4th highest MDA8 value for those site/years affected was 2.0 ppb for both 

methods. Overall, the impact of both methods on the 4th highest MDA8 values was relatively 

small, and the magnitude of the decreases in the 4th highest MDA8 values were quite similar 

between the two methods.

According to Figures 14 to 19, the spatial patterns in the monitoring sites affected by the 

minimal and blanket methods were quite similar. Not surprisingly, the sites where the annual 4th

highest MDA8 values decreased were predominately located in rural areas, particularly 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 4th highest MDA8 value on a regular basis. On the other hand, the two methods did 

not appear to have any impact upon the annual 4th highest MDA8 values at monitoring sites near 

urban areas. The maximum impact upon the 4th highest MDA8 value was less than 3 ppb in the 

vast majority of cases, but there were some isolated instances where both methods reduced the 

annual 4th highest MDA8 value by 4 ppb or more.
Figure 14. Number of years in 2004 – 2013 where applying the 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 value
Figure 16. Average decrease in the annual 4th highest MDA8 value when applying the minimal 

method, 2004 – 2013

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

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

method, 2004 – 2013

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

method, 2004 – 2013
IV. 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 O3 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 O3 concentrations occurring 

during overlapping MDA8 events are most likely due to transport of O3 to rural areas with low 

NOx titration 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 4th highest MDA8 value metric 

used for determining compliance with the NAAQS. While there were 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, less than 2% of the annual 4th highest 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 4th highest MDA8 value. The monitoring 

sites where the annual 4th 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 overlapping 
MDA8 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 sources of ozone precursors being designated nonattainment for a revised 

NAAQS. Such an outcome may be avoided by using one of the two methods described in this 

analysis to remove overlapping MDA8 values from the annual 4th highest MDA8 calculations.