Comments for Consideration by the CASAC Ozone Review Panel on the Reconsideration of the 2008 NAAQS for its teleconference deliberations on February 18, 2011 

### Estimating Policy-Relevant Back ground Ozone Concentrations Allen S. Lefohn, A.S.L. & Associates 302 North Last Chance Gulch, Suite 410, Helena, Montana 59601 asl-associates.com/ Samuel J. Oltmans*, NOAA, Earth System Research Laboratory, Globa l Monitoring Division, 325 Broadway, Boulder, Colorado 80305 *The views expressed herein are exclusively the author’s and do not represent any official policy of NOAA February 7, 2011

1. Introduction The EPA's proposed range for the primary st andard is based on a combination of (1) human laboratory experimental results and (2) risk assessments that incorporate the assumptions of the modeled estimates for policy-relevant background (PRB) O 3 daily afternoon concentrations. Based on modeling resu lts, the EPA (2010) notes that PRB O 3 concentrations are generally predicted to be in the range of 15 to 35 ppb in the afternoon and are generally lower under conditions conducive to man-made O 3 episodes. Correctly estimating PRB is very important because characterizing PRB O 3 levels that are too low can result in 1) overestimated human health risk predictions (e .g., human laboratory experimental results compared to a control of 0 ppb) and 2) optimistic policy expectations of the levels to which hourly average O 3 concentrations can be lowered as a result of emission reduction requirements.

2. Estimating policy-relevant back ground ozone concentrations 2.1 The influence of PRB concentrations at Trinidad Head (CA)

1EPA’s Policy-Relevant Background (PRB) estima tes for risk analysis used a maximum monthly diurnal concentration mostly in the 0.015 to 0.025 ppm range (US EPA, 2007). Unlike the predicted PRB concentrations, actual di urnal concentrations experienced during PRB conditions range from approximately 0.030 pp m to 0.050 ppm (Lefohn, 2007). Measured hourly average PRB O 3 concentrations ≥ 0.050 ppm frequently occurred with maximum hourly average concentrations up to 0.065 ppm (Lefohn, 2007). The predicted PRB concentrations using models are below observed PRB levels that influence the low-elevation monitoring site at Trinidad Head (CA) (Oltmans and Lefohn, 2005; Lefohn, 2007; Oltmans et al., 2008; Oltmans et al., 2010). Figure 1 shows the location of Trin idad Head, which is a small town of about 400 people located on the California’s north coast. A location in sout hern California (Channel Islands National Park) demonstrates many of the characte ristics during the spring as Trinid ad Head in terms of air flow patterns and O 3 amounts suggesting that background levels of O 3 entering southern California from the Pacific Ocean are similar to those in northern California.

Figure 1. The location of Trinidad Head, California. Source: MapQuest.com, Inc.

2Table 1 summarizes the number of hourly average concentrations ≥ 0.05 ppm for the nine-year period 2002- 2010 for the months of March – July. In 2003, the first complete year of data collection, the maximum hourly average concentra tion of 64 ppb was measured in April, which was the month that experienced the most frequent occurrences of hourly average concentrations

≥ 0.05 ppm (188). In 2008, the maximum hourly average concentration of 61 ppb occurred during the month of April with 120 occurr ences of hourly average concentrations ≥ 0.05 ppm. In 2009, the maximum hourly average concentration of 65 ppb occurred during the month of April. April 2009 experienced the most freque nt occurrences during the year of hourly average
concentrations ≥ 0.05 ppm (73) for the year. In 2010, the latest period of record, the maximum hourly average concentration of 58 ppb was observe d in March and April and the most frequent occurrences of hourly average concentrations ≥ 0.05 ppm (51) occurred in April. As shown in Table 1, numerous occurrences of hourly average concentrations ≥ 0.05 ppm are exhibited the springtime . Table 1. Summary of the number of hourly average concentrations ≥ 0.05 ppm and maximum hourly average value (in parentheses ) measured at Trinidad Head, California for the 9-year period, 2002 through 2010. Source: Modified from Oltmans and Lefohn (2005). Year March April May June July 2002 No Data 36 (54 ppb) 18 (53 ppb) 0 (41 ppb) 0 (45 ppb) 2003 23 (52 ppb) 188 (64 ppb) 95 (57 ppb) 13 (54 ppb) 0 (41 ppb) 2004 38 (62 ppb) 30 (54 ppb) 3 (50 ppb) 2 (50 ppb) 0 (38 ppb) 2005 58 (55 ppb) 72 (56 ppb) 6 (52 ppb) 0 (46 ppb) 0 (39 ppb) 2006 37 (55 ppb) 56 (60 ppb) 43 (64 ppb) 0 (47 ppb) 0 (35 ppb) 2007 21 (52 ppb) 46 (57 ppb) 30 (55 ppb) 0 (40 ppb) 0 (33 ppb) 2008 26 (53 ppb) 120 (61 ppb) 34 (56 ppb) 3 (52 ppb) 0 (42 ppb) 2009 18 (54 ppb) 73 (65 ppb) 11 (54 ppb) 0 (43 ppb) 0 (35 ppb) 2010 38 (58 ppb) 51 (58 ppb) 6 (52 ppb) 0 (42 ppb) 0 (32 ppb)

3Figure 2 summarizes the vert ical distribution of O 3 that was observed between August 1997 and July 2007 at Trinidad Head. The figure illustrates the variability of the O 3 concentrations as a function of elevation as well as period of the year. From the surface to 2 km, the mixing ratio nearly doubles with the steep est gradient between the surface and 1 km and during the summer months when surface values are lowest. At 2 km during all months, the average is ≥ 50 ppb. This suggests that there is a significant reservoir of O 3 just above the boundary layer at Trinidad Head that can be mixed down to the surface.

Figure 2. Average monthly ozonesonde profiles (1997-2007) of ozone mixing ratios (ppbv) at Trinidad Head, California in the lower troposphere for the 6 months of April to September. Source: Oltmans et al., 2008.

42.2 Long-range transport effects on PRB in the western US from Eurasian biomass burning Eurasian emissions associated with bi omass burning and their easterly transport contribute to O 3 concentrations at west coast O 3 monitoring sites under PRB conditions, as well as inland O 3 monitoring sites in Montana, Wyomi ng, and North Dakota (Oltmans et al., 2010). Published information indicates that biomass burning events in Eurasia are not infrequent. The effects of Eurasian biom ass burning in producing O 3 enhancements in surface O 3 in the western US has been reported in the litera ture (Jaffe et al., 2004; Bertschi et al., 2004; Be rtschi and Jaffe, 2005; Pfister et al., 2010; Oltmans et al., 2010). The year 2008 was one in which large Eurasian biomass burning occurred. Unusually high O 3 readings were recorded in April 2008 at surface O 3 monitoring sites from northern Alaska to northern California as well as inland monitoring sites in Montana, Wyoming, and North Dakota. At Denali National Park in central Alaska, an hourly average of 79 ppb was recorded during an 8-h period in which the 8-h average was over 75 ppb. At Trinidad Head, hourly O 3 readings were >50 ppb almost continuously for a 35-h period. At several sites in northern California, located to the east of Trinidad Head, numerous occurrences of O 3 readings exceeding 50 ppb were experienced during this period. As the biomass burning enhanced O 3 plume moved further into the interior of the US between 18-20 April through a northern tier of states (Montana, Wyoming, North Dakota), surface O 3 measurements at several monitoring sites appeared to have intercepted the plume (Oltmans et al., 2010). Trajectories from each site suggest that the enhancements observe d during this period could have come from the burning region. The 8-h average O 3 enhancements were above the normal background concentrations observed at these monitoring si tes (i.e., 45-55 ppb for Montana and North Dakota and 50-60 ppb for Wyoming). The 8-h daily ma ximum at Yellowstone on 19 April (69 ppb) suggests an enhancement during the period of su spected plume influence of 5-10 ppb above the

5other relatively high values obser ved at this site. This is also about the amount of the perturbation seen at the other in terior monitoring sites (Oltmans et al., 2010). At Trinidad Head in April 2008, the occurren ces of hourly averaged O 3 concentrations ≥ 50 ppb were similar in magnitude to the number of events in April 2003, which over a 9-yr period experienced the highest occurrences of hourly average concentrations ≥ 50 ppb. Although a thorough study of 2003 was not undertaken, modeling of 2003 data f ound that biomass burning impacted the west coast of North America (Pfister et al., 2010) and may have been the cause of the elevated surface
O 3 amounts at Trinidad Head in Apr il 2003 as well (Oltmans et al., 2010). Figure 3 illustrates th e 3-year average of the fourth highest annual daily maximum 8-h average concentrations for four background O 3 monitoring sites for the 2008-2010 period that appear to have been influenced in April 2008 by a PRB component that was associated with the long-range transport of emissions from Eurasian biomass burning.

6Figure 3. Four O 3 monitoring sites influenced by PR B long-range transport with their fourth highest 8-h daily maximum concentrations averaged over 3 years for 2008 to 2010. The concentrations are in units of ppm.

2.3 The importance of the stratosphere in influencing surface oz one concentrations Natural processes, such as stratospheri c-tropospheric exchange (STE) and biogenic emissions, contribute to the replenishment of PRB O 3 concentrations across the entire United States (U.S. EPA, 2006). Naturally occurri ng STE processes contribute to background O 3 concentrations at the surface at both high and lo w-elevation monitoring s ites (e.g., Ludwig et al., 1977; Haagenson et al., 1981; Schuepbach et al ., 1999; Stohl et al., 2000; Lefohn et al., 2001; Cooper et al., 2005; Hocking et al., 2007; Ordó ń ez et al., 2007; Langford et al., 2009). Hourly

7average concentration ≥ 0.05 ppm in the springtime can be attributed at times to these natural processes (Lefohn et al., 2001; Coope r et al., 2005; Langford et al ., 2009). Key to the importance of STE processes in replenishing PRB is the frequency of STE events. Indications of the importance of the frequency of STE events affecting lower tropospheric O 3 concentrations have been reported (Ordó ń ez et al., 2007). Reed (1955) and Daniel sen (1968) used instrumented highaltitude aircraft measurements of radioactivity and O 3 to document the exchange of air and trace constituents from the stratosphere into the troposphere. Building upon this research methodology, Ludwig et al. (1977) also examined th e behavior of a surrogate for stratospheric
O 3 by characterizing the behavior of radioactive debris injected into the stratosphere during nuclear weapons testing in the 1960s. Using 90 Sr, Ludwig et al. (1977) concluded that a significant stratospheric c ontribution to ground level O 3 concentrations in the middle latitudes of the Northern Hemisphere, with a maximum occu rring in the springtime, was evident at both high- and low-elevation surface-monitoring sites. At some monitoring sites, the stra tospheric contribution to surface O 3 can be significant and can lead to exceedances of the 0.075 ppm 2008 NAAQS O 3 standard. During 1999, STE contributed to one of the 4 hi ghest daily 8-h maxima at one or more surface sites in the metropolitan Denver area on at least 4 occasions during the March–October O 3 season (Langford et al., 2009). In addition to the Denver area, STE makes an important contribution in the springtime to the replenishment of O 3 concentrations at Yellowst one National Park in Wyoming. The range of the 3-year average of the 4 th highest 8-h daily maximu m values at Yellowstone National Park is from 63 to 67 ppb. For the period 1997-2008, 75% of the top four daily maximum 8-h average concentrations occurred during the springtime. The EPA has indicated that Yellowstone National Park would violate the primary O 3 standard for the 3-year 2006-2008

8period if the level were set at 0.065 ppm. Natural processes, such as stratospheric-tropospheric exchange (STE) events and long-range transp ort from Eurasian biomass burning events contribute to the enhanced concen trations observed at Yellowstone National Park (Oltmans et al., 2010).

2.4 Background contribution at the surfa ce in the western US is larger than current models indicate Jaffe (2010) have examined O 3 data from 11 rural CASTNET s ites in the western US for the period 1995-2009. The 11 surface sites show a similar seasonal cycle and generally a good correlation in the deseasonalized monthly means, i ndicating that there are large scale influences
on O 3 that operate across the en tire western US. These sites also show a good correlation between site elevation and annual mean O 3 , indicating a significant c ontribution from the free troposphere. Jaffe (2010) examined the number of exceedance days for each site, defined as a day when the Maximum Daily 8-h Average (MDA8) exceeded a threshold value (i.e., 65 ppb, 70 ppb, or 75 ppb). Over this time period, more than half of these sites exceeded an MDA8 threshold of 70 ppb at least 4 times per year, and all sites exceeded a thre shold value of 65 ppb at least 4 times per year. The transition to lower threshold values increa sed substantially the number of exceedance days, especially during spring, reflecting the fact that background O 3 peaks during spring. The author next exam ined the correlatio n between surface O 3 and free tropospheric O 3 in the same region, as measured by r outine balloon launches from Boulder, CO. Using O 3 measured by the balloon sensor in the range of 3-6 km above sea level, Jaffe (2010) reported statistically signif icant correlations between surface and free tropospheric O 3 in spring and summer months using both monthly means, daily MDA8 values, and the number of surface exceedance days. The author suggested that during spring this correlation reflects variations in

9the flux of O 3 transport from the free troposphere to th e surface. In summer, free tropospheric and surface concentrations of O 3 and the number of exceedance days were all significantly correlated with emissions from biomass burning in the western US . Jaffe (2010) noted that the correlation between surface and free tropospheric O 3 was strongest at lower exceedance thresholds, which suggested according to the au thor that a greater importance of background O 3 at the lower thresholds. Jaffe (2010) concluded that the large interannual variations and the correlation between surface and free tropospheric O 3 observed in his st udy during both spring and summer indicate that background O 3 varies strongly from y ear-to-year and that the background contribution at the surfa ce in the western US is larger than current models indicate.

2.5 The importance of stratos pheric-tropospheric exchange processes in replenishing ozone in the free troposphere and its infl uence on surface ozone concentrations ≥ 50 ppb across the US In our most current research, our team has used the Lagrangian Analysis Tool (LAGRANTO) trajectory model to identify specific days when stratosphere-to-troposphere transport was optimal to elevate surface O 3 levels (Manuscript in Preparation). The statistically significant coincidences between the number of days with a daily maximum hourly average O 3 concentration ≥ 50 ppb and stratosphere-to-troposphere transport to surface (STT-S) were quantified. In evaluating several monitoring sites across the US, we found that the high-elevation site at Yellowstone National Park in Wyomi ng exhibited the most statistically significant coincidences (i.e., more than 19 days a month) during the spring and summer for hourly average
O 3 concentrations ≥ 50 ppb for the 3 years studied. At this site, the maximum hourly springtime average O 3 concentrations were usually in the 60- 70 ppb range. The maximum daily 8-h average concentrations mostly ranged from 50 to 65 ppb. Figure 4 illustrates the frequency of the

10stratosphere-to-troposphere transport events to the surface at Yellowstone NP for the 2006-2008 period.

Figure 4. Frequency of stratosphere-to-troposphere transport to surface (STT-S) at Yellowstone National Park in Wyoming for April – August 2006, 2007, and 2008. Source: Manuscript in Preparation.
In addition to our applying the LAGRANTO trajectory model to the high-elevation Yellowstone National Park site in Wyoming, we investigated stat istically significant coincidences at low-elevation monitoring sites acr oss the US. At many of these monitoring sites, there was a preference for O 3 enhancements to be coincident with stratosphere-to-troposphere transport to the surface during the springtime, although summertime occurrences were sometimes observed. When enhancements o ccurred, the daily maximum hourly average 11concentrations were mostly in the 50-65 ppb range and the daily maximum 8-h average concentrations were usually in the 50-62 ppb range.

3. Conclusion Correctly estimating PRB is very important b ecause inadequate characterization can lead to levels that are too low, resu lting in 1) overestimated human health risk predictions and 2) optimistic policy expectations of th e levels to which hourly average O 3 concentrations can be lowered as a result of emi ssion reduction requirements. Ob servations indicate that O 3 concentrations measured under conditions representa tive of PRB are significantly higher than the EPA's modeled range of concentrations. The frequent number of hourly average O 3 concentrations ≥ 50 ppb at sites influenced by PRB conditions indicates that the human health risk results may have been overestimated. Measured hourly average PRB O 3 concentrations ≥ 0.050 ppm frequently occurred with maximum hourly average concentrations up to 0. 065 ppm at a low-elevation monitoring site at Trinidad Head (CA). A location in southern California (Channel Islands National Park) demonstrates many of the characte ristics during the spring as Trinid ad Head in terms of air flow patterns and O 3 amounts suggesting that background levels of O 3 entering southern California from the Pacific Ocean are similar to those in no rthern California. Eurasian emissions associated with biomass burning and their easterly transport enhance O 3 concentrations at west coast O 3 monitoring sites under PRB conditions, as well as inland O 3 monitoring sites in Montana, Wyoming, and North Dakota. Natural processes, such as stratospheri c-tropospheric exchange (STE) and biogenic emissions, contribute to the replenishment of PRB O 3 concentrations across the entire United

12States. Naturally occurring STE pr ocesses contribute to background O 3 concentrations at the surface at both high and low-elevation monitoring sites. Large interannual variations and the correlation between surface and free tropospheric O 3 observed during both spring and summer indicate that background O 3 varies strongly from year-to- year and that the background contribution at the surface in the western US is la rger than current models indicate. Statistically significant coincidences between the number of days with a daily maximum hourly average O 3 concentration ≥ 50 ppb and stratosphere-to-troposphere tr ansport to surface indicates that highelevation and low-elevation m onitoring sites exhibit the mo st statistically significant coincidences during the springtime and in so me cases during the summertime. At the highelevation monitoring site at Yellowstone Na tional Park in Wyoming, where the greatest frequency of stratosphere-to-troposphere transpor t to the surface (STT-S) events occurred, the maximum hourly springtime average O 3 concentrations were usually in the 60-70 ppb range; the maximum daily 8-h average concentrations mo stly ranged from 50 to 65 ppb. Statistically significant coincidences at low-elevation monito ring sites across the US showed a preference for
O 3 enhancements to be coincident with strato sphere-to-troposphere transport to the surface mostly during the springtime. When enhancemen ts occurred, the daily maximum hourly average concentrations were mostly in the 50-65 ppb range and the daily maximum 8-h average concentrations were usually in the 50-62 ppb range. The replenishment of PRB O 3 concentrations across the US by stratospheric-tr opospheric exchange is an important process. Stratosphere-to-troposphere transport to the su rface events are frequent during the springtime and sometimes during the summertime and in fluence hourly average concentrations ≥ 50 ppb.

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