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 Background 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, Global 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 standard 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) O3 daily afternoon 

concentrations. Based on modeling results, the EPA (2010) notes that PRB O3 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 O3 episodes. Correctly estimating PRB is very 

important because characterizing PRB O3 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 O3

concentrations can be lowered as a result of emission reduction requirements. 

2. Estimating policy-relevant background ozone concentrations 

2.1 The influence of PRB concentrations at Trinidad Head (CA) 

1 
EPA’s Policy-Relevant Background (PRB) estimates 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 diurnal concentrations experienced during PRB 

conditions range from approximately 0.030 ppm to 0.050 ppm (Lefohn, 2007). Measured hourly 

average PRB O3 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 Trinidad Head, which is a small town of about 400 people located 

on the California’s north coast. A location in southern California (Channel Islands National Park) 

demonstrates many of the characteristics during the spring as Trinidad Head in terms of air flow 

patterns and O3 amounts suggesting that background levels of O3 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. 

2 
Table 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 concentration 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 occurrences 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 frequent 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 observed 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) 

3 
Figure 2 summarizes the vertical distribution of O3 that was observed between August 

1997 and July 2007 at Trinidad Head. The figure illustrates the variability of the O3

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 steepest 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 O3 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.

4 
2.2 Long-range transport effects on PRB in the western US from Eurasian biomass 

burning 

Eurasian emissions associated with biomass burning and their easterly transport 

contribute to O3 concentrations at west coast O3 monitoring sites under PRB conditions, as well 

as inland O3 monitoring sites in Montana, Wyoming, and North Dakota (Oltmans et al., 2010). 

Published information indicates that biomass burning events in Eurasia are not infrequent. The 

effects of Eurasian biomass burning in producing O3 enhancements in surface O3 in the western 

US has been reported in the literature (Jaffe et al., 2004; Bertschi et al., 2004; Bertschi 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 O3 readings were recorded in April 2008 at surface O3

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 O3 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 O3 readings exceeding 50 ppb were experienced during this period. As the biomass burning 

enhanced O3 plume moved further into the interior of the US between 18-20 April through a 

northern tier of states (Montana, Wyoming, North Dakota), surface O3 measurements at several 

monitoring sites appeared to have intercepted the plume (Oltmans et al., 2010). Trajectories from 

each site suggest that the enhancements observed during this period could have come from the 

burning region. The 8-h average O3 enhancements were above the normal background 

concentrations observed at these monitoring sites (i.e., 45-55 ppb for Montana and North Dakota 

and 50-60 ppb for Wyoming). The 8-h daily maximum at Yellowstone on 19 April (69 ppb) 

suggests an enhancement during the period of suspected plume influence of 5-10 ppb above the 

5 
other relatively high values observed at this site. This is also about the amount of the 

perturbation seen at the other interior monitoring sites (Oltmans et al., 2010). At Trinidad Head 

in April 2008, the occurrences of hourly averaged O3 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 found that biomass burning impacted the west 

coast of North America (Pfister et al., 2010) and may have been the cause of the elevated surface 

O3 amounts at Trinidad Head in April 2003 as well (Oltmans et al., 2010). 

Figure 3 illustrates the 3-year average of the fourth highest annual daily maximum 8-h 

average concentrations for four background O3 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. 

6 
Figure 3. Four O3 monitoring sites influenced by PRB 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 ozone concentrations 

Natural processes, such as stratospheric-tropospheric exchange (STE) and biogenic 

emissions, contribute to the replenishment of PRB O3 concentrations across the entire United 

States (U.S. EPA, 2006). Naturally occurring STE processes contribute to background O3

concentrations at the surface at both high and low-elevation monitoring sites (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; Ordóńez et al., 2007; Langford et al., 2009). Hourly 

7 
average concentration ≥ 0.05 ppm in the springtime can be attributed at times to these natural 

processes (Lefohn et al., 2001; Cooper 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 O3 concentrations have 

been reported (Ordóńez et al., 2007). Reed (1955) and Danielsen (1968) used instrumented highaltitude aircraft measurements of radioactivity and O3 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 the behavior of a surrogate for stratospheric 

O3 by characterizing the behavior of radioactive debris injected into the stratosphere during 

nuclear weapons testing in the 1960s. Using 90Sr, Ludwig et al. (1977) concluded that a 

significant stratospheric contribution to ground level O3 concentrations in the middle latitudes of 

the Northern Hemisphere, with a maximum occurring in the springtime, was evident at both 

high- and low-elevation surface-monitoring sites.

At some monitoring sites, the stratospheric contribution to surface O3 can be significant 

and can lead to exceedances of the 0.075 ppm 2008 NAAQS O3 standard. During 1999, STE 

contributed to one of the 4 highest daily 8-h maxima at one or more surface sites in the 

metropolitan Denver area on at least 4 occasions during the March–October O3 season (Langford 

et al., 2009). In addition to the Denver area, STE makes an important contribution in the 

springtime to the replenishment of O3 concentrations at Yellowstone National Park in Wyoming. 

The range of the 3-year average of the 4th highest 8-h daily maximum 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 O3 standard for the 3-year 2006-2008 

8 
period if the level were set at 0.065 ppm. Natural processes, such as stratospheric-tropospheric 

exchange (STE) events and long-range transport from Eurasian biomass burning events 

contribute to the enhanced concentrations observed at Yellowstone National Park (Oltmans et al., 

2010). 

2.4 Background contribution at the surface in the western US is larger than 

 current models indicate 

Jaffe (2010) have examined O3 data from 11 rural CASTNET sites 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, indicating that there are large scale influences 

on O3 that operate across the entire western US. These sites also show a good correlation 

between site elevation and annual mean O3, indicating a significant contribution 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 threshold value of 65 ppb at 

least 4 times per year. The transition to lower threshold values increased substantially the 

number of exceedance days, especially during spring, reflecting the fact that background O3

peaks during spring. The author next examined the correlation between surface O3 and free 

tropospheric O3 in the same region, as measured by routine balloon launches from Boulder, CO. 

Using O3 measured by the balloon sensor in the range of 3-6 km above sea level, Jaffe (2010) 

reported statistically significant correlations between surface and free tropospheric O3 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 

9 
the flux of O3 transport from the free troposphere to the surface. In summer, free tropospheric 

and surface concentrations of O3 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 O3 was strongest at lower exceedance 

thresholds, which suggested according to the author that a greater importance of background O3

at the lower thresholds. Jaffe (2010) concluded that the large interannual variations and the 

correlation between surface and free tropospheric O3 observed in his study during both spring 

and summer indicate that background O3 varies strongly from year-to-year and that the 

background contribution at the surface in the western US is larger than current models indicate. 

2.5 The importance of stratospheric-tropospheric exchange processes in replenishing 

ozone in the free troposphere and its influence 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 O3 levels (Manuscript in Preparation). The statistically 

significant coincidences between the number of days with a daily maximum hourly average O3

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 Wyoming exhibited the most statistically significant 

coincidences (i.e., more than 19 days a month) during the spring and summer for hourly average 

O3 concentrations ≥ 50 ppb for the 3 years studied. At this site, the maximum hourly springtime 

average O3 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 

10 
stratosphere-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 statistically significant 

coincidences at low-elevation monitoring sites across the US. At many of these monitoring sites, 

there was a preference for O3 enhancements to be coincident with stratosphere-to-troposphere 

transport to the surface during the springtime, although summertime occurrences were 

sometimes observed. When enhancements occurred, the daily maximum hourly average 

11 
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. 

3. Conclusion 

Correctly estimating PRB is very important because inadequate characterization can lead 

to levels that are too low, resulting in 1) overestimated human health risk predictions and 2) 

optimistic policy expectations of the levels to which hourly average O3 concentrations can be 

lowered as a result of emission reduction requirements. Observations indicate that O3

concentrations measured under conditions representative of PRB are significantly higher than the 

EPA's modeled range of concentrations. 

The frequent number of hourly average O3 concentrations ≥ 50 ppb at sites influenced by 

PRB conditions indicates that the human health risk results may have been overestimated. 

Measured hourly average PRB O3 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 characteristics during the spring as Trinidad Head in terms of air flow 

patterns and O3 amounts suggesting that background levels of O3 entering southern California 

from the Pacific Ocean are similar to those in northern California. Eurasian emissions associated 

with biomass burning and their easterly transport enhance O3 concentrations at west coast O3

monitoring sites under PRB conditions, as well as inland O3 monitoring sites in Montana, 

Wyoming, and North Dakota. 

Natural processes, such as stratospheric-tropospheric exchange (STE) and biogenic 

emissions, contribute to the replenishment of PRB O3 concentrations across the entire United 

12 
States. Naturally occurring STE processes contribute to background O3 concentrations at the 

surface at both high and low-elevation monitoring sites. Large interannual variations and the 

correlation between surface and free tropospheric O3 observed during both spring and summer 

indicate that background O3 varies strongly from year-to-year and that the background 

contribution at the surface in the western US is larger than current models indicate. Statistically 

significant coincidences between the number of days with a daily maximum hourly average O3

concentration ≥ 50 ppb and stratosphere-to-troposphere transport to surface indicates that highelevation and low-elevation monitoring sites exhibit the most statistically significant 

coincidences during the springtime and in some cases during the summertime. At the highelevation monitoring site at Yellowstone National Park in Wyoming, where the greatest 

frequency of stratosphere-to-troposphere transport to the surface (STT-S) events occurred, the 

maximum hourly springtime average O3 concentrations were usually in the 60-70 ppb range; the 

maximum daily 8-h average concentrations mostly ranged from 50 to 65 ppb. Statistically 

significant coincidences at low-elevation monitoring sites across the US showed a preference for 

O3 enhancements to be coincident with stratosphere-to-troposphere transport to the surface 

mostly during the springtime. When enhancements 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 O3

concentrations across the US by stratospheric-tropospheric exchange is an important process. 

Stratosphere-to-troposphere transport to the surface events are frequent during the springtime 

and sometimes during the summertime and influence hourly average concentrations ≥ 50 ppb. 

13 
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