AECOM 250 Apollo Drive, 978-905-2100 tel 

250 Apollo Chelmsford, MA 01824 978-905-2101 fax 

Memorandum 

To Scott Bohning Page 1 

cc Shirley Rivera, Lisa Beckham, Sara Head 

Subject Justification of the use of the 3-year average 98th percentile ambient 

background concentration for PHPP 1-hour NO2 NAAQS Modeling. 

From 

Richard Hamel 

Date July 21, 2011 

 

I. Background 

On June 27th, 2011, representatives of AECOM, Inland Energy, and the City of Palmdale met with EPA 

regarding outstanding issues still to be resolved related to the Prevention of Significant Deterioration 

(PSD) permit application for the Palmdale Hybrid Power Project (PHPP). One question that arose was 

whether the choice of the 3-year average of the 98th percentile background value for 1-hour NO2 was 

suitably conservative to satisfy the requirements of the PSD program with regard to air dispersion 

modeling. The purpose of this memo is to clarify the reasoning behind AECOM’s choice of this approach

and to demonstrate that the 1st-Tier approach, as outlined in the March 1st, 2011 EPA Memo “Additional 

Clarification Regarding the Application of Appendix W Modeling Guidance for the 1-hour NO2 National Air 

Quality Standard”, would be overly conservative as relates to the PHPP because of the monitor’s location 

and the localized NO2 emissions that occur nearby. Much of the data presented in this memo has been 

included in previous submissions to EPA, but is reproduced here so that all relevant data are located in 

one place. 

II. NO2 Emissions Sources Close to the Lancaster Division Street Monitor 

The Lancaster Division Street Monitor is located approximately 4.1 kilometers from the PHPP site. The

monitor is located in downtown Lancaster and has a number of NO2 line sources in close proximity. The 

primary sources in the area are the Metrolink rail line, the Sierra Highway, the Antelope Valley Freeway 

(SR-14), and commute related traffic on Division Street. The locations of these line sources with respect 

to PHPP and the monitor are shown in Figure 1. Table 1 shows the distance between these sources and 

the project site as compared to the monitor. 

The maximum modeled 1-hour NO2 impacts for normal operations, as well as startup / shutdown, occur 

on the eastern fence line of the PHPP. The distance from the line sources to the location of the maximum 

impacts would thus be slightly higher than the “Distance from PHPP” in Table 1 below. As can be seen in 
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the table, the monitor is much closer to all of the local line sources of NO2 emissions than the Project and 

location of the maximum impacts. More importantly, these sources are generally north-south roadways 

and rail line that would be upwind of both the monitor and PHPP (at different points along the line) during 

the meteorological conditions that lead to the maximum impact, so the distance is particularly relevant. 

Table 1: Distance from Project Site and Lancaster Monitor to Local Sources of NO2

Emission Source Distance From 

Monitor

Distance From 

PHPP

Daily Volume 

(vehicles/day) 

Sierra Highway 110 meters ~2 km 20,000+ 

Southern Pacific Rail 80 meters ~2 km 24 trains / weekday 

6-12 trains on weekends 

Antelope Valley Freeway1Just over 2 km Just over 4 km ~80,000 at Ave. L exit 

Division Street250 meters About 3 km ~18,000 near the monitor 

Avenue K 0.6 km ~4.0 km 30,000 / 25,000 to the 

west / east of monitor 

Avenue L ~1.0 – 1.5 km ~3.5 km 30,000+ west of monitor 

Avenue M ~2.5 – 3.0 km 0.5 km 18,500 in vicinity of PHPP 

1. http://www.laalmanac.com/transport/tr26b.htm#14

2. This portion of Division Street ends at E Avenue L and starts again in Palmdale west of Sierra Highway 

The east-west streets present near PHPP have significantly less traffic than those near the monitor. East 

Avenue M, an east-west road that defines the northern boundary of the PHPP site, experiences traffic 

volumes of around 18,500 vehicles / day in the vicinity of the Project according to the City of Lancaster 

Traffic Engineering Division1. Due to the extensive area covered by USAF Plant 42, there are no eastwest streets within 4 km south (upwind) of the PHPP power block that would have substantial traffic 

volumes (the closest is Avenue P/Rancho Vista Blvd). The traffic volumes of the east-west streets near 

the monitor however are significantly higher: Avenue K experiences traffic volumes of approximately 

25,000 vehicles / day to the east of the monitor, and over 30,000 vehicles / day to the west. Avenue L 

experiences daily volumes well in excess of 30,000 vehicles / day to the southwest of the monitor, directly 

upwind of the monitor in the predominant wind direction. There are also other streets (Avenue K6 and 

K8) that run east-west near and downwind of the monitor. 

Additionally, the monitor is significantly affected by commute traffic idling at the intersection in front of the 

building where the monitor is sighted. In a telephone conversation with Mr. Chris Anderson of the 

Antelope Valley Air Quality Management District (AVAQMD), on September 22nd, 2010, Mr. Anderson 

stated that NO2 concentrations recorded by the monitor visibly spike a few moments after each train 

passes by. Additionally, Mr. Anderson reported that traffic idling at the intersection of Division Street and 

Avenue K6 East, directly in front of the AVQAMD office where the monitor is located, causes daily spikes 

in recorded NO2 levels during the morning and afternoon commutes. This scenario is borne out in 

reviewing the distribution of the background data by hour and season, as shown in Table 2 below. As 

seen in the table, the hours with the highest maximum recorded background combination occur from 5 

am to 8 am, and 5 pm to 8 pm, corresponding closely to the daily commute, and the time at which the 

Metrolink commuter trains begin running from Lancaster to Los Angeles each morning (commencing at 

 

1

 http://www.cityoflancasterca.org/Modules/ShowDocument.aspx?documentid=4370 
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3:58 am, then 4 more trains between 5 am and 7 am)2. Because this congestion does not occur at or 

near the PHPP site, the background values recorded during these hours, which constitute 33% of the 

total background data collected, significantly overstate the ambient background concentration at the 

Project location. 

Because of the much closer proximity of the vehicle and rail traffic sources to the monitor during the wind 

conditions that produced the highest modeled impacts, it can be assumed that the background 

concentrations during these periods are conservative compared to those occurring near the Project site

during the worst case modeling scenario. 

Table 2: Maximum Ambient Concentration by Hour and Season 2008-2010 (ppb) 

Hour 

Starting Winter Spring Summer Fall Overall 

0:00 38 45 20 40 45 

1:00 33 32 25 39 39 

2:00 30 28 24 35 35 

3:00 33 43 42 41 43 

4:00 32 47 49 47 49 

5:00 42 53 65 56 65 

6:00 45 54 61 57 61 

7:00 49 53 45 47 53 

8:00 45 36 34 46 46 

9:00 43 27 19 41 43 

10:00 38 20 11 28 38 

11:00 29 16 10 21 29 

12:00 27 14 18 16 27 

13:00 25 14 11 16 25 

14:00 23 15 13 16 23 

15:00 32 15 12 23 32 

16:00 42 17 12 40 42 

17:00 47 23 14 62 62 

18:00 51 36 16 56 56 

19:00 47 42 24 50 50 

20:00 45 41 33 49 49 

21:00 47 47 30 47 47 

22:00 41 42 25 43 43 

23:001

29 ND ND ND 29 

1. Most of the data from the hour beginning at 23:00 is missing from the dataset as the monitor is 

recalibrated nightly at that time. 

2. Winter = Dec, Jan, Feb. Spring = Mar, Apr, May. Summer = Jun, Jul, Aug. Fall = Sep, Oct, Nov 

 

2

 http://www.metrolinktrains.com/schedules/pdf.php?id=1041
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III. NO2 Emissions Sources Near the PHPP Site 

The primary non-Project point sources contributing to the background at the PHPP site are the LockheedMartin and Northrop-Grumman facilities located on U.S. Air Force (USAF) Plant 42 property adjacent to 

the PHPP. Emissions from sources at these two facilities are included in the cumulative NO2 modeling. 

Since these sources are located about 4 km from the Lancaster monitoring station, they will also 

contribute to the ambient background, hence when using a background value that is not matched to the 

meteorological conditions will lead to some “double counting”. There are no other major point sources of 

NO2 near (within 6 miles [10 km]) the PHPP (or the monitor) beyond those that are explicitly included in 

the modeling. 

The only potentially significant area source of NO2 emissions closer to the Project site than the monitor is 

air traffic at the Palmdale Regional Airport/USAF Plant 42. The airport is operated by Los Angeles World 

Airports (LAWA), but air traffic at the airport is minimal since no commercial airlines currently operate 

there. In a 1989 agreement with the Air Force, commercial traffic is currently allowed up to 50 operations 

per day, with an operation being either a landing or a takeoff3. There are provisions in the agreement, 

which is in place until 2017, that would increase the allowed number to 400 should another commercial 

airline commence operations there. Eight airlines have attempted to establish a viable business at 

Palmdale Regional Airport since 1971, all of which have failed. In 2009, including military operations, 

there were a total of 26,905 landing or take-off operations, an average of 73.7 operations per day4

. In 

2007 and 2008, the average operations per day were 92.4 and 80.2. Because the runways are located to 

the south and southeast of the PHPP, these emissions should not impact the receptors showing the 

maximum modeled impacts at the same time as PHPP sources, which occur when the winds are blowing 

from the west. Any long term contributions to the ambient background concentration should be picked up 

by the Lancaster Division Street monitor, less than 4 kilometers away. No other significant sources of 

NO2 exist upwind of the highest modeled impacts that would contribute significantly to the modeled 

concentrations. 

IV. Effect of Commute Related Sources on the Daily Maximum Monitored Concentrations. 

When considering the appropriate 1-hour NO2 background data to use for the PHPP PSD application 

modeling, the background data were sorted in a number of different ways: monthly, seasonal, hourly, etc. 

(see Table 2 for the seasonal and hourly results). All of this analysis showed that regardless of how the 

data were sorted, the impacts from commute-related background sources overwhelmed any other 

variation in the background concentrations. Further analysis was then done to determine how the daily

maxima correlated to commute related emissions. Table 3 shows the breakdown, by hour, of the number 

of occurrences of the daily maximum background concentrations from 2008-2010. As shown in the table, 

more than half of the days experience their maximum background concentration during the morning 

commute (5-8 am) time when traffic is at its height and commuter rail trains, both passing the monitor and 

idling at the station nearby, are most present. When the data related to the evening commute hours (5-8 

pm) are added, that value increases to 70%. 

 

 

3

 http://www.jpbroker.com/pairport.html

4

 http://www.city-data.com/airports/Palmdale-Regional-Usaf-Plant-42-Airport-Palmdale-California.html#PMD
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Table 3: Occurrences of Daily Maximum Background Concentrations by Hour 

Hour Beginning 2008 2009 2010 Total 

0:00 19 22 19 60 

1:00 3 5 4 12 

2:00 0 3 4 7 

3:00 8 6 3 17 

4:00 24 17 21 62 

5:00 84 94 93 271 

6:00 73 83 66 222 

7:00 28 23 16 67 

8:00 5 7 6 18 

9:00 4 0 3 7 

10:00 1 2 0 3 

11:00 1 1 0 2 

12:00 1 2 1 4 

13:00 2 1 1 4 

14:00 2 2 1 5 

15:00 0 1 1 2 

16:00 1 4 0 5 

17:00 19 21 20 60 

18:00 22 17 36 75 

19:00 15 9 14 38 

20:00 14 12 7 33 

21:00 12 10 10 32 

22:00 27 17 23 67 

23:00 0 0 0 0 

% from 5-8 am: 50.7% 55.7% 51.9% 53.9% 

% from 5-8 pm: 15.3% 13.1% 20.1% 16.1% 

Total Commute: 66.0% 68.8% 71.9% 70.0% 

V. Discussion and Recommendation of Appropriate Background. 

The March 1st EPA memo describes the 1st-tier determination of ambient background, which would be 

accepted without further justification, as the 98th-percentile of the annual distribution of daily maximum 

1-hour values averaged across the most recent three years of monitored data. Other approaches may be 

allowed on a case by case basis with appropriate justification. The 1st-tier ambient background 

concentration is calculated as follows: 

1. For each day included in the three years of data, determine the highest recorded 1-hour ambient 

NO2 concentration, discarded the remaining 23 hours of data.

2. For each of the three years, determine the 98th percentile of the daily maxima calculated in step 

1.
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3. Average the 98th percentile value determined for each of the three years together to determine 

the design ambient background concentration.

The 1st-tier ambient background using the above calculations for 2008 through 2010 would be 52 ppb 

(98.3 μg/m3). As shown in Tables 2 and 3, the ambient background concentrations at the Lancaster 

Division Street Monitor are largely impacted by nearby emission sources that do not exist at the PHPP or 

at the location of the maximum modeled 1-hour NO2 impacts. The large majority of the daily maximum 

impacts in particular are driven by these commute related emissions. Because these commute emissions 

skew the data significantly (as shown above, by 33-50%), the use of the daily maxima as the basis for 

developing a 98th percentile background value as described above, is unrealistically conservative 

because it would be derived almost entirely from emissions that are neither present at the PHPP site, nor 

upwind of the modeled maxima such that they would contribute to those concentrations and in fact would

result in an design background concentration that is higher than the maximum ambient 1-hour NO2

concentration recorded at the Lancaster monitor during any non-commute hour over the entire 3 year 

period. 

Because the use of the daily maxima would be unrealistically conservative, and in order to preserve the 

probabilistic form of the standard, AECOM recommends the use of the 3-year average of the 98th

percentile of all hours modeled. Table 4 shows these values in both ppb and μg/m3 from 2008-2010. By 

including all of the hours in the calculations, the high impact hours occurring during the daily commute 

would still be included in the calculations, and thus the resulting 98th percentile would still be conservative 

for the project site. The proposed ambient background to be added in the 1-hour NO2 NAAQS modeling, 

as shown in the table, is 39.3 ppb (74.0 μg/m3). Note that the background data for 2010 were not

available when the updated 1-hour NO2 modeling was performed. In that modeling, the three year 

average of the 98th percentile background from 2007-2009 of 41.0 ppb (77.1 μg/m3 ) was used. The 2010 

data shows that the ambient background concentration of NO2 in the Palmdale and Lancaster areas 

continues to diminish, as has been the trend since at least 2002. 

Table 4: Lancaster Division Street Monitor 1-hour NO2 Annual 98th Percentile 

Concentrations from 2008 – 2010

Units 2008 2009 2010 3-year Average 

 ppb 41.0 39.0 38.0 39.3 

μg/m3

77.1 73.4 71.5 74.0 

Our prior memo dated March 30, 2011 contained updated cumulative NO2 modeling results as shown in 

Table 5 below. The model was run for both the normal operations and the startup / shutdown scenario 

with the following assumptions regarding the PHPP ancillary equipment sources:

a. The PHPP emergency generator and fire water pump will not be tested while the combustion 

turbines are in the startup or shutdown process. 

b. The PHPP emergency generator and fire water pump engine will not be tested simultaneously. 

The EPA modeling guidance issued in March 2011 suggests that these intermittent sources do not need 

to be modeled at all, but in order to not redo the modeling again, we continued to include these sources in 

the modeling. In addition to the PHPP sources, the U.S. Air Force Plant 42 sources were also included. 

We note that this modeling shows that even if the overly conservative 1st-tier background is used with the 

normal operations scenario and no emergency generator testing, the total cumulative result is less than 

the 1-hour NO2 NAAQS (88.2 μg/m3

 + 98.3 μg/m3 = 186.5 μg/m3, which is less than 188 μg/m3). 
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Table 5: Revised Cumulative 1-hour NO2 NAAQS Modeling Results 

Modeled Concentrations (μg/m3) 

Modeling Scenario 

PHPP with 

updated USAF 

Plant 42 Sources 

and Receptors 

Backgrounda Sum 

Normal Operations - No PHPP 

Emergency generator 88.2 77.1 (74.0) 165.3 (162.2) 

Normal Operations - No PHPP Fire 

Water Pump 108.2 77.1 (74.0) 185.3 (182.2) 

Startup/Shutdown - No PHPP 

Emergency generator 109.7 77.1 (74.0) 186.9 (183.7) 

a. Background based on 3-year average 98th percentile background from 2007 – 2009 (or 2008 – 2010) 

The other scenarios could arguably be excluded under the recent EPA guidance, as they involve 

intermittent sources or activities (emergency engine testing or startup/shutdown events). These 

scenarios are also very conservative, as it was previously felt that the modeling was sufficiently 

conservative with the background approaches taken such that further refinement was not needed. For 

instance, the startup/shutdown scenario is actually a shutdown scenario where the plant stack 

parameters are assumed to remain constant throughout the event, which will never be the case, as in 

actuality the stack temperature and flow rates are decreasing as the combustion turbine shuts down (and 

would also be changing during startup). We note that assuming that the maximum background 

concentrations (hours with values over 100 μg/m3 only occur at the monitoring station about 2 percent of 

all hours) is in the first place representative, but then also occurs at the same time and under the same 

meteorological conditions as when the units are being started or shutdown or engines are being tested, is 

very remote. 

In conclusion, we offer two perspectives under which the impacts from the PHPP are determined to be 

acceptable: 1) EPA agrees that in this specific situation (a case by case determination) use of a 3 year 

average 98th percentile value as background is sufficiently conservative as it reduces the influence of 

local mobile sources near the monitor but not near the Project; and/or 2) That intermittent sources or

activities not be included in the modeling as allowed by EPA guidance. 

Please feel free to call Rich Hamel at 978-905-2231 (note the new phone number, our office has moved) 

or Sara Head at 805-388-3775 if you have any questions or comments regarding this information. The 

spreadsheets 2008-2010 Background Data Analysis.xlsx, 2008-2010 Daily Max Background Data 

Analysis.xlsx, and PHPP Impacts by Hour.xlsx are included as Attachment A to this memo for your 

review. 

 
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Figure 1: Relative Location of the PHPP Power Block, Lancaster Monitor, and 

Significant Background Sources