Pechanga Ozone Data Assessment

6/22/2011

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. 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.

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 1). Figure 1 shows that 

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

was higher than Eureka during this period. At the end of 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 1.

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 2 and 3).
Figure 2.

Figure 3.

Similar to Figure 1, Figure 2 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 3 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 4 further shows that Pechanga and Oceanside agreed reasonably well in 2008 and have 

deviated since. Figure 4 also shows that the difference is not a set offset but varies by day and 

has drifted.

Figure 4.
Finally, EPA examined the diurnal patterns between 2008 and 2010, which further show the 

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

ozone monitor. Figure 5 shows diurnal patterns of two similar days, one in 2008 and one in 2010 

for Pechanga and the nearest ozone sites operated by San Diego County APCD. Figure 6 shows 

annual diurnal patterns for 2008 and 2010 for Pechanga, the nearest coastal site to Pechanga 

(Oceanside), and Eureka (a pristine coastal location). Both Figures show that the diurnal pattern 

at Pechanga changes substantially between 2008 and 2010 and by 2010 does not reflect ambient 

ozone patterns for coastal areas.
Figure 5.

Figure 6.