Pechanga Ozone Data Assessment 

8/4/2011 

On May 24, 2011, the EPA contractor visited Pechanga to conduct the scheduled NPAP audit. 

The Pechanga ozone ambient air quality monitoring system passed this performance audit, which 

indicates that the ozone analyzer is being properly operated, maintained, and calibrated. The 

EPA instrument and the Pechanga instrument deviated by less than 2% during the NPAP audit. 

To rule out sampling line leaks or other sampling issues that would not be detected during the 

NPAP audit, our contractor performed a test of the Pechanga ozone monitoring system, by 

comparing it directly with an EPA-certified monitor. During this side-by-side testing, the 

Pechanga system indicated lower monitored values than the EPA instrument. During 

troubleshooting, EPA's contractor discovered that one possible cause for the lower values could 

be a leak along the sampling line to the ozone monitor at the manifold. Since the manifold is 

located inside the building, such a leak would allow mixing of indoor air, which is typically low, 

with ambient air to occur upstream of the instrument resulting in lower ozone readings at the 

instrument. The EPA contractor reconfigured the Pechanga ozone sampling line to by-pass the 

manifold, which resulted in an increase in ozone levels registered on the Pechanga ozone 

instrument. The EPA contractor examined the sampling line going to Pechanga’s instrument and 

noted a non-standard fitting on the manifold (Figure 1). The fitting is a ball and socket fitting 

with a pinch clamp (on left and right of manifold in the picture); the clamp is not secure. The 

fitting is located at the inlet of the manifold and in between the manifold and the blower motor. 

O-rings are typically used with this kind of fitting as the sealant but none was observed by the 

EPA contractor. There was no other type of sealant (e.g., gasket, ferrule, or vacuum grease) 

observed with the fitting. Even with the appropriate sealant, this type of fitting is known by EPA 

R9 staff to be prone to leaks and insecure. 

Figure 1. Picture of Manifold for the Pechanga Ozone Analyzer. 
That there is a leak at the manifold is suggested by the following information: 

· Agreement between the EPA and Pechanga instruments during the NPAP audit indicates 

both instruments being similarly calibrated and makes instrument performance an 

unlikely reason for any differences in readings during the side-by-side test. 

· Lower readings by the Pechanga instrument during the side-by-side test suggest possible 

dilution somewhere along the sampling line. The most common source of leaks is a 

manifold which draws cleaner indoor air into the sampling line. 

· By-passing the Pechanga manifold caused an increase in levels registered on the 

Pechanga instrument, which indicates some contribution of the manifold to depressed 

ozone levels registered on the instrument. 

· A leak-prone fitting was noted on either side of the manifold. 

Based on the side-by-side test which suggested a possible leak within the sampling line for 

Pechanga’s ozone monitor, EPA conducted subsequent data analyses on ozone trends at 

Pechanga and other relevant sites. EPA reviewed Pechanga 8-hour ozone data since 2008 and 

compared it with Oceanside and Vandenberg (nearby coastal sites), along with Eureka (lowest 

ozone site in California, located on coast in northern California) (Figure 2). Figure 2 shows that 

Pechanga data agreed fairly well with Oceanside and Vandenberg data during part of 2008 and 

was higher than Eureka during this period. By November, 2008 Pechanga data dropped below 

all other sites, somewhat recovered for part of 2009 but then dropped further in 2010 and 

remained low throughout the year, even as other sites including Eureka were more elevated. 

Figure 2. Comparison of Pechanga Ozone Data with Relevant Coastal Sites in California. 
EPA then more closely compared Pechanga data with Oceanside (Camp Pendleton), which is the 

nearest ozone monitor along the coast operated by San Diego County APCD (Figures 3 and 4). 

Figure 3. Ozone Levels at Pechanga and Oceanside. 
Figure 4. Difference in Ozone Levels at Pechanga and Oceanside. 

Similar to Figure 2, Figure 3 shows that the overall magnitude of ozone levels at Oceanside have 

remained consistent since 2008 which Pechanga data has decreased relative to Oceanside since 

2008. Figure 4 shows that in 2008 the ozone levels at Pechanga and Oceanside were on average 

within 5 ppb but then drifted to more than 20 ppb difference in 2009 and 30 ppb difference in 

2010. 
Figure 5 also shows that Pechanga and Oceanside agreed reasonably well in 2008 and have 

deviated since. Figure 5 further shows that the difference is not a set offset but varies by day and 

has drifted. 

 
Figure 5. Ozone Levels at Pechanga and Oceanside for August-October for 2008, 2009, and 

2010.
EPA examined the diurnal patterns between 2008 and 2010 (Figure 6), which further show the 

difference that developed between nearby San Diego County APCD sites and the Pechanga 

ozone monitor. We have included Eureka as a comparison to a pristine coastal location. 

Figure 6. Diurnal Pattern of Ozone at Pechanga, Oceanside, and Escondido for a Typical Day in 

2008 and 2010. 

Finally, EPA compared official ozone data from EPA Headquarters for 2010 for Pechanga with 

the data for the rest of the U.S. (Figure 7). The 4th highest daily 8-hour maximum value, which 

is used to calculate the 3-year averaged design value, was evaluated. Pechanga data is lower 
than all other sites in the country. The other closest site is in Puerto Rico, which is under 

evaluation based on concerns by Region 2. 

Figure 7. National Ozone Data for 2010. 

In Summary, at a Pechanga-EPA meeting in May, both EPA and Pechanga’s contractor 

acknowledged that data at Pechanga were lower than expected. At this meeting, both EPA and 

Pechanga committed to review data and conduct an on-site evaluation. The on-site evaluation, 

conducted with EPA-certified audit equipment, showed that the Pechanga and EPA instruments 

agreed well under audit conditions but the Pechanga instrument recorded lower values than the 

EPA instrument when sampling ambient air. This suggests a leak in Pechanga’s sampling line 

within the trailer. Upon inspection of the manifold, the EPA contractor noted a non-standard, 

leak-prone fitting. Subsequent to the on-site evaluation, EPA conducted numerous analyses of 

Pechanga data. Analyses show that Pechanga data has drifted lower by more than 25 ppb since 

2008 and in 2010 was lower than typical background for the California Coast. The diurnal 

pattern showed a flattening between 2008 and 2010: the 2010 pattern is similar to a diurnal of 

indoor air rather than outdoor air. Finally, comparison of Pechanga data to all other sites in the 

country show that the 2010 Pechanga data is anomalously lower than all other sites in the U.S., 

including numerous sites in pristine coastal locations. The analyses conducted by EPA indicate 

that ozone data collected at Pechanga do not represent actual ambient conditions. It is unclear 

when the deviation from actual ambient conditions began but the data suggest that all data after 

November, 2008 are suspect. The on-site evaluation and data analyses suggest the likely cause 

for the spurious data is a leak at the manifold.