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 s ys tem passed this performance audit, which indicates that the ozone analyzer is being properl y operated, maintained, and calib rated. 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 s ys tem, by comparing it directl y with an EPA - certified mon itor. During this side - by - side testing, the Pechanga s ys tem 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 ty picall y low, with ambient air to occur upstream of the instrument resulting in lower ozone readings at the i nstrument. 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 b y the follo wing 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 tes t .  Lower readings b y 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 Pech anga 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 with in the sampling line for Pechanga’s ozone monitor, EPA conducted subsequent data analy ses 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 coasta l 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 durin g 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 ye ar, even as other sites including Eureka were more elevated.Figure 1.

EPA then more closel y compared Pechanga data with Oceanside (Camp Pendleton) , which is the nearest ozone monitor along the coast operated b y San Diego Count y APCD (Figure s 2 and 3 ).Figure 2.

Figure 3.

Similar to Figure 1, Figure 2 show s 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 reasonab ly well in 2008 and have deviated since. Figure 4 also shows that the difference is not a set offset but varies b y day and has drifted. Figure 4.Finally , EPA examined the diurnal patterns between 2008 and 20 10 , which further show the

difference that developed between nearb y San Diego Count y APCD sites and the Pechanga ozone monitor. Figure 5 shows diurnal patterns of two similar day s, one in 2008 and one in 2010 for Pechanga and the nearest ozone sites operated by San Diego Count y AP CD. 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.