October 4, 2011

Mr. Ken Israels

U.S. EPA Region 9

75 Hawthorne Street

San Francisco, CA 94105

Re: Pechanga Ozone Monitor Data

Dear Mr. Israels:

On behalf of the Pechanga Band of Luiseno Indians (Pechanga or the Tribe), we are 

pleased to submit the following analysis related to the ambient ozone monitoring station 

located at the Pechanga Reservation.

Background 

On May 24, 2011, a U.S. Environmental Protection Agency (EPA) contractor 

performed a scheduled audit of the Pechanga ozone monitor, as required by EPA’s 

National Performance Audit Program (NPAP). The purpose of the NPAP audit is to 

provide quality assurance of monitoring data through independent assessments of the 

monitors. Data quality is important because monitoring data are used to determine 

attainment status for the National Ambient Air Quality Standards (NAAQS).

Pechanga’s monitor passed the NPAP audit on May 24, but subsequently the EPA 

contractor conducted a side-by-side comparison with his device and found a problem 

with the Pechanga monitor that was causing it to show artificially low readings when 

challenged with a side-by-side comparison with the EPA contractor’s analyzer.

Subsequently, on June 22, 2011, EPA provided the Pechanga Ozone Data Assessment 

memo, which gave a description of the testing during the May 24 audit. The EPA 

Assessment memo also compared the data from the Pechanga monitor to data from 

three other California monitors, located at Eureka, Vandenberg, and Oceanside. Based 

on these data, EPA concluded that the problem with Pechanga’s monitor was likely 

related to the manifold system, and likely started in 2009.

After conducting a site visit, reviewing the results of diagnostic experiments performed 

by the Tribe and its contractors, and receiving the EPA Assessment, Sierra performed a

separate analysis of the Pechanga monitoring data, along with data from four other 

nearby monitors: Oceanside (Camp Pendleton), Lake Elsinore, Temecula1(Lake 

Skinner), and the monitor operated by the Pala Tribe.

2

 Available data from all of these 

monitors for the period of June 2008 to August 2011 were processed in accordance with 

 

1 Data from the Temecula monitor were available only beginning in August 2010, when the monitor first 

became operational.

2 Data from the Pala monitor were available only from October 2008 to June 2009 on the EPA web site.

sierra

research

1801 J Street

Sacramento, CA 95811

Tel: (916) 444-6666

Fax: (916) 444-8373

Ann Arbor, MI

Tel: (734) 761-6666

Fax: (734) 761-6755
Mr. Ken Israels -2- October 4, 2011

the EPA Ozone Data Handling Guidance (December 1998).

3

 The data from the 

Pechanga monitor were then compared to the data from the four nearby monitors to 

analyze trends and relative ozone concentrations for the 1-hour daily average, 1-hour 

daily maximum, and 8-hour daily maximum values.

Analysis

For the scope of our analysis, monitors were selected for comparison based on specific 

criteria: the Oceanside (Camp Pendleton) monitor was chosen as the nearest coastal 

monitor, Lake Elsinore was chosen as the nearest long-term inland monitor, and 

Temecula (Lake Skinner) and Pala were chosen for the close proximity to the 

Reservation. Figure 1 shows the location of the monitors used in this analysis. 

Figure 1

Monitor Locations

 

3Available at http://www.epa.gov/ttn/oarpg/t1/memoranda/guidefin.pdf.
Mr. Ken Israels -3- October 4, 2011

While EPA compared the Pechanga data to data from the Oceanside, Vandenberg, and 

Eureka monitors, we believe that the comparison to the Vandenberg monitor (a coastal 

site in Central California) was not necessary because the Oceanside monitor is also 

located at a coastal site. In addition, a comparison with the Eureka monitor would yield 

no relevant information since Eureka is the northernmost California monitor and 

Pechanga Reservation is in Southern California, which has markedly different 

topography and meteorology.

Timing of Pechanga Monitor Issue – The 1-hour daily maximum ozone concentrations

for the Pechanga and Oceanside monitors are shown in Figure 2. As the figure shows, 

the ozone concentrations recorded at both monitors followed a similar pattern 

throughout the time period. The concentration recorded by the Oceanside monitor was 

consistently higher than the concentration recorded by the Pechanga monitor, but the 

peaks and valleys tend to coincide. In June 2009, both monitors had a noticeable drop 

in ozone concentration due to an unusual weather pattern in Southern California during 

that time. In May 2009, ambient temperatures in the area were beginning to warm, as is 

normal for that time of year. However, the first week of June in 2009 brought an 

unusual span of cool weather, resulting in a dramatic decrease in ambient ozone 

concentration during that time.4 

Figure 2

 

4 When comparing the two-week averages and maximums for June 1-14 for 2009 to the same period for 

2008 and 2010, temperatures for San Diego and Riverside County monitors were between 13° and 23° F 

cooler in 2009, as measured at the monitoring sites.
Mr. Ken Israels -4- October 4, 2011

Figure 3 shows the differences between the monthly-averaged 1-hour daily maximum 

ozone concentration readings from the four other monitors and the Pechanga monitor. 

The Pechanga monitor readings were subtracted from each of the four monitor 

measurements and the results were averaged monthly to show a comparison of the 

difference between the Pechanga monitor and the other monitors.

5

Figure 3

As shown in Figure 3, the differences between Pechanga and Lake Elsinore monitors 

have a definite seasonality, with the difference being higher in summer than in winter, 

including in 2008, when EPA and Pechanga both believe that the Pechanga monitor was 

working properly (as verified by previous NPAP audits). The difference between the 

Pechanga and Oceanside monitors was fairly consistent, with less of a seasonal 

influence, with the exception of one specific dip in July 2008. The difference with the 

Temecula monitor actually decreases over the second half of 2010, which indicates the 

monitors had readings coming closer together from August 2010 to February 2011. 

This information points to the fact that there is no clear point in time when a change 

took place that might indicate a problem with the Pechanga monitor. The consistent 

seasonal variation in the Lake Elsinore plot shows that the difference in readings 

between Pechanga and Lake Elsinore has always been larger in the summer and smaller 

in the winter. 

As Figures 2 and 3 demonstrate, there is no clear point showing when the problems 

began with the Pechanga monitor—the only minor change that can be pointed to in the 

monitor differences is that the difference between the Pechanga and Oceanside monitors 

increases slightly around January 2010, from approximately 0.02 ppm to 0.03 ppm on 

average. This slight difference is the only evidence that can be found as to when a 

problem might have occurred.

 

5 Note the gap in the figure for March-May of 2011 due to Pechanga monitor downtime.
Mr. Ken Israels -5- October 4,2011 

Conclusions 

As discussed above, EPA has suggested that problems with the Pechanga monitor began 

in 2009; however, based on the comparisons of the Pechanga monitor readings to 

readings from the four nearby monitors, we believe that the data do not clearly show a 

specific time when any problem with the Pechanga monitor may have begun. In 

addition, based on the comparisons with the nearby monitors, we believe that the 

problem with the monitor did not start prior to January 2010. 

If you have any questions, please call me at (916) 273-5126. 

Sincerely, 

01ML~H.~ 

Gary Rubenstein 

Senior Partner 

cc: Colleen McKaughan, EPA Region 9 

Jeff Wehling, EPA Region 9 

Deborah Jordan, EPA Region 9 

Meredith Kurpius, EPA Region 9