1 UNITED STATES ENVIRONMEN TAL PROTECTION AGENCY Office of Air Quality Planni ng and Standards (OAQPS) Research Triangle Park, North Carolina 27711

MEMORANDUM [DATE] SUBJECT: Comparison of Ozone Metrics Consid ered in the Current NAAQS Review FROM: Benjamin Wells, Air Quality Assessment Division, OAQPS TO: Ozone NAAQS Review Docket (# EPA-HQ-OAR-2008-0699) I. Overview The final policy assessment for the current review of the national ambient air quality standards (NAAQS) for ozone (O 3 ) (U.S. EPA, 2014) identifies tw o metrics that are appropriate for consideration of the secondary standard. One metric is identical to the form and averaging time for the current primary and secondary O 3 standards: the annual 4 th highest daily maximum 8-hour O 3 concentration, averaged over three consecu tive years, in part s per billion (ppb). For convenience, this metric is he reafter referred to as the “4 th max” metric. The other metric identified for the secondary standard is the annual W126 index, averaged over three consecutive years, in parts per million-hours (ppm-hrs), he reafter referred to as the “W126” metric. This memo provides the results of a technical analysis which examines the relationship between the 4 th max metric and the W126 metric based on ambient air quality data from 2001 to 2013. The analysis focuses on levels of 75, 70, 65, and 60 ppb for the 4 th max metric, with particular emphasis on 70 and 65 ppb, and levels of 17, 15, 13, 11, and 7 ppm-hrs for the W126 metric, with particular emphasis on 17, 15, and 13 pp m-hrs. These particular levels were chosen for evaluation based on EPA staff recomme ndations in the final policy assessment.2 II. Data Handling Hourly O 3 concentration data were retrieved from EPA’s Air Quality System (AQS) database for 1,849 O 3 monitors which operated between 2001 and 2013. These data were used to calculate 4 th max and W126 values for each 3-yearperiod from 2001-2003 to 2011-2013. Before calculating these values, some initial processi ng was done on the hourly data. First, data collected using monitoring methods other than federal reference or equivalent methods, and data collected from monitoring sites not meeting EPA’ s quality assurance or siting criteria in 40 CFR part 58 were removed. Second, data collected by multiple monitoring instruments operating at the same site were combined by selecting the monitor with the most hourly observations each year, then filling in missing hourly concentration values using datacollected from the remaining monitors at the site. Finally, data were combin ed across 62 pairs of m onitoring sites approved by the appropriate EPA regional offices, in order to maintain a continuous data record when these sites were replaced or relocated a short distance away. The final dataset consisted of hourly O 3 concentration data for 1,657 monitoring sites. The 4 th max values were calculated according to the data handling in the proposed rulemaking notice for the O 3 NAAQS (Appendix U to 40 CFR part 50). First, moving 8-hour averages were calculated from the hourly O 3 concentration data for each site. For each 8-hour period, an 8-hour average valu e was calculated if there we re at least 6 hourly O 3 concentrations available, and stored in the first hour of the period. Daily maximum 8-hour average values were found using the 8-hour periods beginning from 7:00 AM to 11:00 PM each day. These daily maximum values were used if at least 13 of the 17 possible 8-hour averages were available, or if the daily maximum value was greater than the 4 thmax level being evaluated. Finally, the annual 4 th highest daily maximum value was found for each year, then averaged across each consecutive3 3-year period to obtain the final set of 4 th max values. The annual 4 th highest daily maximum 8- hour values and the 3-year average values were truncated to the next lowest unit ppb for applications requiring direct comparison to a 4 th max level, otherwise, all decimal digits were retained in the calculations. The 4 th max values were considered valid if daily maximum values were available for at least 90% of the days in the O 3 monitoring season (defined in Appendix D to 40 CFR part 58) on average acro ss the three years, with a mini mum of 75% of the days in the O 3 monitoring season in any single year. In addition, 4 th max values were considered valid if they were greater than the 4 th max levels to which they were being compared. To calculate the W126 values, the hourly O 3 concentration values (in parts per million) for daytime hours (defined as the 12-hour period from 8:00 AM to 8:00 PM each day) at each site were weighted using the following equation: Weighted O 3 = O 3 / (1 + 4403*exp(-126 * O 3 )). These weighted values were summed over each calendar month, then adjusted for missing data (e.g.; if 80% of the daytime hourly concentrations were available, the sum would be multiplied by 1/0.8 = 1.25) to obtain the monthly W126 index values. Monthly W126 index values were not calculated for months where fewer than 75% of the possible daytime hourly concentrations were available. Next, moving 3-month sums were calcu lated from the monthly index values, and the highest of these 3-month sums was determinedas the annual W126 index. Three-month periods spanning multiple years (e.g., November to January, December to February) were not considered in these calculations. The annual W126 index values were averaged across each consecutive 3- year period to obtain the final W126 values, withunits in parts per million-hours (ppm-hrs). The annual W126 index values and the 3-year average values were truncated to the next lowest unit ppm-hour for applications requiring direct comparison to a W126 level, otherwise, all decimal4 digits were retained in the cal culations. The W126 values were considered valid if hourly O 3 concentration values were available for at least 90% of the daytime hours during the O 3 monitoring season on average across the three ye ars, with a minimum of 75% of the daytime hours during the O 3 monitoring season in any single year. In addition, W126 values were considered valid if they were greater than the W126 levels to which they were being compared. In the final datase t, 1,419 of the 1,657 O 3 monitoring sites had sufficient data to calculate valid 4 th max and W126 values for at least one 3-year period between 2001-2003 and 2011-2013. The number of sites with valid 4 th max and W126 values ranged from 1,023 in 2001-2003 to 1,104 in 2011-2013, and 729 sites had valid 4 th max and W126 values for all eleven 3-year periods. III. Analysis The analysis based on the 4 th max and W126 values consisted of three parts. The first part focused on the metric values for the most r ecent period, 2011-2013, to determine how many sites had values greater than the various 4 th max and W126 levels of intere st, and to gain a sense of how the 4 th max and W126 metrics relate to one another based on obs erved air quality data. The second part of the analysis focused on trends in the 4 th max and W126 values from 2001-2003 to 2011-2013, to examine how those values have change d over time. The final part of the analysis focused on the relative changes in the 4 th max and W126 values from 2001-2003 to 2011-2013, to assess how those values have responded to cha nges in precursor emissions, and to gain a sense of how future control programs de signed to meet a revised primary O 3 standard could impact both 4 th max and W126 values.5 A. Recent Air Quality Data The first portion of the analysis focused on the 4 th max and W126 values for the most recent 3-year period, 2011-2013. Figures 1 and 2 below show maps of the 4 th max and W126 values based on 2011-2013 data. These values were compared to the various 4 th max and W126 levels of interest in order to determine the numbe r of sites with values greater than each level. Table 1 shows the number of sites with 4 th max values greater than each 4 th max level, and Table 2 shows the number of sites with W126 values greater than each W126 level. The number of sites with 4 th max values greater than 70 ppb was approximately double the number of sites with 4 th max values greater than 75 ppb, and the number of sites with 4 th max values greater than 65 ppb was approximate ly triple the number of sites with 4 th max values greater than 75 ppb. In general, gi ven similar points with in the range of leve ls being considered, there were fewer sites exceeding the W126 levels than the 4 thmax levels.6 Figure 1. Map of 4 th max values based on 2011-2013 data

Figure 2. Map of W126 values based on 2011-2013 data7 Table 1. Number of sites with 4 th max values greater than various 4 th max levels of interest based on 2011-2013 data

4 th Max Level (ppb) 75 70 65 60 # of Sites Exceeding Level 1 273 566 884 1080

Table 2. Number of sites with W126 values greater than various W126 levels of interest based on 2011-2013 data W126 Level (ppm-hrs) 17 15 13 11 7 # of Sites Exceeding Level 2 106 165 262 381 736

The 4 th max and W126 values were also compared to each combination of 4 th max and W126 levels of interest based on 2011-2013 data. Fi rst, Table 3 shows the number of sites with 4 th max values greater than each 4 th max level, and W126 values le ss than or equal to each W126 level (e.g., 305 sites had 4 th max values greater than 70 ppb a nd W126 values less than or equal to 13 ppm-hrs). Next, Table 4 s hows the number of sites with 4 th max values less than or equal to each 4 th max level, and W126 values greater th an each W126 level (e.g., 16 sites had 4 th max values less than or equal to 70 ppb and W126 valu es greater than 13 ppm-hrs). Finally, Table 5 shows the number of sites with 4 th max values greater than each 4 th max level, and W126 values greater than each W126 level (e.g., 244 sites had 4 th max values greater than 70 ppb and W126 values greater than 13 ppm-hrs).

1 There were 1,180 sites with sufficient 2011-2013 data to calculate a valid 4 thmax value. 2 There were 1,152 sites with sufficient 2011-2013 data to calculate a valid W126 value.8 Table 3. Number of sites with 4 th max values greater than various 4 th max levels of interest and W126 values less than or equal to various W126 levels of interest based on 2011-2013 data # of Sites Exceeding Only 4 th Max Level W126 Level (ppm-hrs) 17 15 13 11 7

4 th Max Level (ppb) 75 176 139 86 41 2 70 439 386 305 206 28 65 726 669 576 463 150 60 903 846 753 640 312

Table 4. Number of sites with 4 th max values less than or equal to various 4 th max levels of interest and W126 values greater than various W126 levels of interest based on 2011-2013 data # of Sites Exceeding

Only W126 Level W126 Level (ppm-hrs) 17 15 13 11 7

4 th Max Level (ppb) 75 15 35 75 143 433 70 0 4 16 30 178 65 0 0 0 0 15 60 0 0 0 0 0

Table 5. Number of sites with 4 th max values greater than various 4 th max levels of interest and W126 values greater than various W126 le vels of interest based on 2011-2013 data # of Sites Exceeding Both Levels W126 Level (ppm-hrs) 17 15 13 11 7

4 th Max Level (ppb) 75 89 127 182 230 269 70 106 161 244 348 534 65 106 165 262 381 715 60 106 165 262 381 7359 According to Table 4, there were no sites with 4 thmax values less than or equal to 70 ppb and W126 values greater than 17 ppm-hr s, and there were no sites with 4 th max values less than or equal to 65 ppb and W126 values greater than 11 ppm-hrs. Figure 3a shows the locations of the 35 sites with 4 th max values less than or equal to 75 ppb and W126 values greater than 15 ppm-hrs, as well as the 16 sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs. All of these sites are located in the S outhwestern U.S. (i.e., the region bounded by the red border in Figure 3a), which indicates that there may be regional differences in the relationship between the 4 th max and W126 metrics. Figure 3b shows an enlarged version of the same map focused on the Southwestern U.S. In order to examine these differences, many of the further analyses were stratified into the nine NOAA climate regions (Karl and Koss, 1984), which are shown in Figure 4. Note that since the NOAA climate regions only cover the contiguous U.S., Alaska was added to the Northwest region, Hawaii was added to the West region, and Puerto Rico was added to the Southeast region.10 Figure 3a. Map of sites with 4 th max values less than or equal to 75 ppb and W126 values greater than 15 ppm-hr s, and sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs. The red border i ndicates the Southwestern U.S. region enlarged in the figure below.

Figure 3b. Map of sites with 4 th max values less than or equal to 75 ppb and W126 values greater than 15 ppm-hr s, and sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs, enlarged for emphasis on the Southwestern U.S. region.11 Figure 4. Map of the nine NOAA climate regions Figure 5a shows a scatte r plot comparing the 4 th max (x-axis) and W126 (y-axis) values based on 2011-2013 data, with points colored by region, while Figure 5b shows an enlarged version of the same scatter plot withemphasis on the region containing the 4 th max and W126 levels of interest. These figures indicate that there is a strong, positive, non-linear relationship between the 4 th max and W126 metrics. The amount of variability in the relationship between the 4 th max and W126 metrics appears to increase as the metric values themselves increase. The relationship between the 4 th max and W126 metrics also appear s to vary across regions. In particular, the Southwest and West regions (i.e., the southwestern U.S.) appear to have higher W126 values relative to their respective 4 th max values than the rest of the U.S.12 Figure 5a. Scatter plot comparing the 4 th max (x-axis) and W126 (y-axis) values based on 2011- 2013 data

Figure 5b. Scatter plot comparing the 4 thmax (x-axis) and W126 (y-axis) values based on 2011- 2013 data, enlarged to emphasize po ints within the range of 4 th max and W126 levels of interest. Dotted lines mark cutoffs for meeting 4 th max levels of 65 and 70 ppb, and meeting W126 levels of 13, 15, and 17 ppm-hrs.13 B. Air Quality Trends The second portion of the analys is focuses on trends in the 4 th max and W126 values from 2001-2003 to 2011-2013. Figure 6 shows the trend in the 4 thmax values, and Figure 7 shows the trend in the W126 values based on th e 729 sites which collecte d sufficient data to calculate valid 4 th max and W126 values for all eleven 3-y ear periods. In general, the trends in both the 4 th max and W126 values were decreasing. The amount of variability in the 4 th max and W126 values also tended to decrease over time (i.e., the 90 th percentile values decreased more quickly than the 10 th percentile values), especially for the W126 values.14 Figure 6. Trend in 4 th max values, 2001-2003 to 2011-2013. The bold center lines show median values, the dotted lines show 25 th and 75 th percentile values, and the outer lines containing the shaded area show 10 th and 90 th percentile values.

Figure 7. Trend in W126 values, 2001-2003 to 2011- 2013. The bold center lines show median values, the dotted lines show 25 th and 75 th percentile values, and the outer lines containing the shaded area show 10 th and 90 th percentile values.15 Next, Figure 8 compares sites based on their 4 th max values relative to 70 ppb, and their W126 values relative to 13 ppm-hr s. This combination of levels was chosen because it has the highest potential withinthe levels of greatest interest (e.g., 65 to 70 ppb, 13 to 17 ppm-hrs) for sites with 4 thmax values less than or equal to the 4 th max level to also have W126 values greater than the W126 level. Figure 8 uses four panels to group these trends: the top left panel shows the trend in the number of sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs; the t op right panel shows the trend in the number of sites with 4 th max values greater than 70 ppb and W126 values gr eater than 13 ppm-hrs; the bottom left panel shows the trend in the number of sites with 4 th max values less than or equal to 70 ppb and W126 values less than or equal to 13 ppm-hrs; and the bottom right panel shows the trend in the number of sites with 4 th max values greater than 70 ppb and W 126 values less than or equal to 13 ppm-hrs. The bars in each panel are colored to show the number of sites located within each NOAA climate region for each 3-year period. Figure 8 shows that fewer than 20 sites would have 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs in any 3-year period, and that these sites are predominately located in the Southwes t region. The number of sites with 4 th max values less than or equal to 70 ppb and W126 values less than orequal to 13 ppm-hrs has increased over time, while the number of sites with 4 th max values greater than 70 ppb and W126 values greater than 13 ppm-hrs has decreased by a similar amount. The number of sites with 4 th max values greater than 70 ppb and W126 values less th an or equal to 13 ppm-hrs has remained relatively constant over the past decade.16 Figure 8. Trends in the number of sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs (top left pa nel); the number of sites with 4 th max values greater than 70 ppb and W126 values greater than 13 ppm-hrs (top rightpanel); the number of sites with 4 th max values less than or equal to 70 ppb and W126 values less than or equa l to 13 ppm-hrs (bottom left panel) ; and the number of sites with 4 th max values greater than 70 ppb and W126 values less than or equal to 13 ppm -hrs (bottom right panel). Bars are colored to show the number of site s located within each NOAA climate region for the 3-year period ending with the year shown on the x-axis.17 C. Relative Changes in 4 th Max and W126 Values The final portion of the analysis compared the respective 4 thmax and W126 values from 2001-2003 to those from 2011-2013 for the 777 sites with valid 4 th max and W126 values in both periods. Figure 9 shows a map of the changes in the 4 th max values, while Figure 10 shows a map of the changes in the W126 va lues. Most sites in the Eastern U.S. and California saw large, widespread decreases in both 4 th max and W126 values over the pa st decade as a result of regional and national NOx control programs, su ch as the NOx SIP Call and Federal mobile source emissions standards. In the inter-mountain West, where NOx control programs have been more localized, the decreases observed in the 4 th max and W126 values were typically much smaller in magnitude, and a few sites even showed increases.18 Figure 9. Map of changes in 4 th max values from 2001-2003 to 2011-2013

Figure 10. Map of changes in W126 values from 2001-2003 to 2011-201319 Figure 11 shows a scatter plot comparing the changes in 4 thmax (x-axis, ppb) and W126 (y-axis, ppm-hrs) values from 2001-2003 to 2011- 2013. The relationship between the changes in the 4 th max and W126 metrics was fair ly linear and positive (Pears on correlation coefficient R = 0.80). The regression line shown in Figure 11 indicat es that, on average, there was a change of approximately 0.7 ppm-hr in the W126 va lues per unit ppb ch ange in the 4 th max values. Similar to the direct comparison between the 4 th max and W126 metrics based on 2011-2013 data, the amount of variability in the relative changes between these tw o metrics generally increased with the magnitude of the changes, and regional differences were apparent.

Figure 11. Scatter plot comparing the changes in 4 th max (x-axis) and W126 (y-axis) values from 2001-2003 to 2011-2013.20 Figure 12 shows scatter pl ots of the changes in 4 thmax (x-axis) and W126 (y-axis) values from 2001-2003 to 2011-2013 in each NOAA climate region and the associated regression lines fit using the sites within each region. Table 6 provides some summary statistics based on the regional analyses. Figure 12 and Table 6 show th at the positive, linear relationship between the changes in the 4 th max and W126 values persists within each region, with Pearson correlation coefficients ranging from 0.57 to 0.92. The regressi on lines shown in Figure 12 with slopes listed in Table 6 indicate that the Southwest and West regions, which had the great est potential for sites having 4 th max values less than or equal to the various 4 th max levels of interest and W126 values greater than the various W126 leve ls of interest, also exhibite d the greatest response in W126 values per unit change in 4 th max values.21 Figure 12. Scatter plots showi ng the changes in 4 th max (x-axis, ppb) and W126 (y-axis, ppmhrs) values for sites within eachof the nine NOAA climate regions. Table 6. Summary statistics based on regional analysis of changes in 4 th max and W126 values from 2001-2003 to 2011-2013

NOAA Climate Region Number of O 3 Sites Mean Change in 4 th Max Value (ppb) Mean Change in W126 Value (ppm-hrs) Regression Slope Pearson Correlation Coefficient Central 158 -13.0 -7.4 0.67 0.89 East North Central 49 -11.4 -4.3 0.39 0.85 Northeast 120 -17.6 -9.1 0.75 0.57 Northwest 16 -8.9 -2.5 0.24 0.63 South 94 -8.5 -2.6 0.41 0.75 Southeast 134 -13.7 -7.0 0.76 0.91 Southwest 57 -3.3 -1.9 1.12 0.87 West 142 -11.3 -7.4 1.09 0.75 West North Central 7 -0.3 -0.5 0.49 0.92 National 777 -12.0 -6.3 0.71 0.8022 IV. Summary The analysis based on 2011-2013 data showedthat nearly one quarter of U.S. O 3 sites had 4 th max values greater than 75 ppb, nearly half of U.S. O 3 sites had 4 th max values greater than 70 ppb, and roughly three quarters of U.S. O 3 sites had 4 th max values greater than 65 ppb. In contrast, less than 10 percent of U.S. O 3 sites had W126 values great er than 17 ppm-hrs, and less than one quarter of U.S. O 3 sites had W126 values gr eater than 13 ppm-hrs. When examining both metrics in combina tion, the 2011-2013 data showed that there were 35 sites with 4 th max values less than or equal to 75 ppb and W126 values greater than 15 ppm-hrs, and 15 of these sites also had W126 va lues greater than 17 ppm-hrs. These sites were located exclusively in the Southwestern U.S (i .e., the West and Southwest climate regions). There were no sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 17 ppm-hrs. There were 16 sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs, and 4 of these sites also had W126 valu es greater than 15 ppmhrs. These sites were also located exclusively in the Southwestern U.S. There were no sites with 4 th max values less than or equal to 65 ppb and W126 values greater than 11 ppm-hrs. The trends analysis showed that 4 th max and W126 values have generally decreased over the past decade. The number of sites with 4 th max values less than or equal to 70 ppb and W126 values less than or equal to 13 ppm-hrs has in creased substantially, wh ile the number of sites with 4 th max values greater than 70 ppb and W126 va lues greater than 13 ppm-hrs has decreased by a similar amount. The number of sites with 4 thmax values greater than 70 ppb and W126 values less than or equal to 13 ppm-hrs has remain ed relatively constant over the past decade, while sites with 4 th max values less than or equal to 70 ppb and W126 values greater than 13 ppm-hrs have consistently been limited to a few locations in the Southwestern U.S.23 Finally, the analysis of th e relative changes in the 4 th max and W126 values from 2001- 2003 to 2011-2013 showed that there was a positive, linear relationship between the changes in the 4 th max and W126 metrics. Nationally, the W 126 values decreased by approximately 0.7 ppm-hr per unit ppb decrease in the 4 thmax values over this peri od. This relationship varied across the NOAA climate regions, and the Southw est and West regions which showed the greatest potential for ex ceeding only the W126 levels of inte rest also showed the greatest improvement in the W126 va lues per unit decrease in 4 th max values. In conclusion, the ambient data analyses showed that the 4 th max and W126 metrics were highly correlated, and so were the relative change s in these two metrics over the past decade. Thus, it is expected that future control programs designed to help meet a primary O 3 standard based on the 3-year average of the 4 th highest daily maximum 8-hour concentration would provide similar improvements in terms of th e 3-year average of the annual W126 metric.

V. References Karl, T. R.; Koss, W. J. (1984). “Regiona l and National Monthly, Seasonal, and Annual Temperature Weighted by Area, 1895-1983.” Historical Climatology Series , 4-3, National Climatic Data Center, Asheville, NC, 38 pp. U.S. Environmental Protection Agency (2014). Policy Assessment for Ozone. Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA-452/R-14-006. Available at: http://www.epa.gov/ttn/naaqs/ standards/ozone/s_o3_index.html