Supplemental NO2 Air Quality 

Impact Analysis 

Avenal Energy Project 

prepared for:

Avenal Power Center, LLC

May 13, 2010

prepared by: 

Sierra Research, Inc. 

1801 J Street 

Sacramento, California 95811 

(916) 444-6666
Supplemental NO2 Air Quality Impact Analysis 

Avenal Energy Project 

 prepared for: 

Avenal Power Center, LLC 

 May 13, 2010 

 Principal author: 

 Eric Walther 

 

Sierra Research, Inc. 

1801 J Street 

Sacramento, CA 95811 

(916) 444-6666 
Supplemental NO2 Air Quality Impact Analysis 

Avenal Energy Project 

Table of Contents

Page

1. Background................................................................................................................. 1

1.1 Project Description.............................................................................................. 1 

1.2 History of Demonstrating Compliance with Ambient Air Quality Standards.... 1 

1.3 The New NO2 National Ambient Air Quality Standard ..................................... 2 

2. Methodology............................................................................................................... 3 

2.1 Overview of Methodology.................................................................................. 3 

2.2 AERMOD Modeling........................................................................................... 4 

2.3 Receptor Grids .................................................................................................... 4 

2.4 Data Requirements—Meteorological Data......................................................... 5 

2.5 Data Requirements—Ambient Monitoring Data................................................ 6 

2.6 Combining Existing Ambient Air Quality Data with Modeled Impacts ............ 7 

3. Results....................................................................................................................... 10 
-1- 

1. BACKGROUND 

1.1 Project Description

The Avenal Energy Project will be a combined-cycle 600-megawatt (MW) power 

generation facility consisting of two GE Energy Frame 7FA natural gas-fired turbinegenerators, a single condensing steam turbine (STG), air-cooled condenser, and 

associated balance-of-plant equipment. The facility will be located on a 148-acre site at 

the eastern edge of the city limits of Avenal, Kings County, California, adjacent to the 

San Luis Canal. Agricultural land uses currently occupy and surround the site. 

1.2 History of Demonstrating Compliance with Ambient Air Quality 

Standards

Key dates in the history of demonstrating compliance with ambient air quality standards 

in the applications to the USEPA for a PSD permit and to the San Joaquin Valley Air 

Pollution Control District (SJVAPCD or District) for a Final Determination of 

Compliance (FDOC) are the following: 

 February 15, 2008 PSD and ATC permit applications submitted to 

USEPA and SJVAPCD, respectively 

 October 30, 2008 District FDOC issued 

 June 16, 2009 EPA draft PSD permit issued 

An ambient air quality impact analysis was prepared to support the February 2008 PSD 

permit and FDOC applications. The air dispersion modeling indicated that the maximum 

annual nitrogen dioxide (NO2) impact from the project of 0.5 μg/m3 would not exceed the 

Significant Impact Level of 1.0 μg.m3 for the annual NO2 National Ambient Air Quality 

Standard (NAAQS). 

A full impact analysis demonstrating compliance with the annual NO2 NAAQS was 

included in the February 2008 application. EPA confirmed that the Project’s maximum 

potential annual NO2 impact of 0.6 μg/m3

 was below the applicable Significant Impact 

Level of 1 μg/m3

 and, hence, that no further analysis was required for potential NO2
-2- 

impacts from the Project’s NOx emissions.1 At the time that the application was 

submitted, there was no hourly NO2 NAAQS. 

1.3 The New NO2 National Ambient Air Quality Standard

On February 9, 2010, EPA revised the primary NO2 NAAQS, establishing a new onehour NO2 standard to supplement the existing annual standard (75 FR 6473). The new 

standard became effective on April 12, 2010. It is a statistically based standard at a level 

of 100 ppb, based on the 3-year average of the 98th percentile of the yearly distribution2

of 1-hour daily maximum concentrations, and will supplement the existing annual 

standard.3

 

This report demonstrates that the Avenal Energy Project will not interfere with attainment 

of the new NO2 standard. The following guidance documents were used to develop the 

methodology for this analysis: 

 USEPA. Notice Regarding Modeling for New Hourly NO2 NAAQS, updated 

February 25, 2010; 

 Federal Register, Volume 75, Number 26, Part III, Environmental Protection 

Agency, 40 CFR Parts 50 and 58, Primary National Ambient Air Quality 

Standards for Nitrogen Dioxide; Final Rule, Appendix S, pages 6532-6534, 

February 9, 2010; 

 USEPA. Guideline on Air Quality Models, published as Appendix W of 40 CFR 

Part 51, Federal Register, Volume 70, Number 216, November 9, 2005; and 

 USEPA. On-Site Meteorological Program Guidance for Regulatory Model 

Applications, report EPA 450/4-87-013, August 1995. 

 

### 

 

1

 USEPA. Draft PSD Permit Statement of Basis and Ambient Air Quality Impact Report, Tables 8-1 and 8-

2, pp.25 and 26, respectively, June 2009. 

2

 For a reasonably complete annual set of daily measurements (i.e., at least 351 valid days), the 98th

percentile is the 8th highest measurement. 3

 75 FR 6474 
-3- 

2. METHODOLOGY 

2.1 Overview of Methodology

This full impact analysis considers project emissions and regional background 

concentrations. The NO2 impacts in the study area from project sources were estimated 

using AERMOD and the Plume Volume Molar Ratio Method (PVMRM) for each hour in 

the five-year analysis period (2000–2004). 

The representative background concentration for each hour was added to the modeled 

impact from the Project plus nearby sources to get a predicted concentration for each 

hour in the study period, for each receptor. 

EPA’s guidance on the use of AERMOD modeling results to calculate NO2 impacts for 

comparison to the new standard was followed, as outlined below.4

 For each day in the study period, the highest one-hour concentration was 

determined for each receptor in the modeling domain. This is the daily maximum 

concentration. 

 At each receptor, for each calendar year in the five-year analysis period, the 98th

percentile daily maximum concentration was determined. If the annual data set is 

essentially complete (i.e., at least 351 valid days), the 98th percentile corresponds 

to the 8th highest daily maximum. If the annual data set is complete to at least the 

required 75% for all four quarters, and the number of valid daily measurements is 

between 301 and 350, the 98th percentile concentration for each receptor was the 

7th highest daily one-hour maximum. A similar scale is provided in Appendix S 

Table 1 of the Final Rule for fewer valid daily measurements in an otherwise 

“complete” year. 

 At each receptor, the 98th percentile daily maximum concentrations were averaged 

across each of the 3 consecutive three-year periods in the five-year analysis 

period.5

 The highest three-year average 98th percentile concentration across all receptors 

represents the modeled 1-hour NO2 concentration for comparison with the 

NAAQS. 

 

4 Notice Regarding Modeling for New Hourly NO2 NAAQS; EPA (Updated February 25, 2010). 5

 This approach is more conservative (results in a higher maximum value) than the approach described in 

the EPA guidance, which calls for an average across all modeled years. This approach was taken to make 

the result more closely match the form of the standard. 
-4- 

 The design value was converted from μg/m3

 (the units used by AERMOD) to ppb 

(the units in the standard) using the ratio 100 μg/m3 = 53 ppb, following EPA 

guidance; the result was rounded to the nearest ppb and compared to the standard. 

2.2 AERMOD Modeling

The following USEPA air dispersion models were used to quantify pollutant impacts on 

the surrounding environment based on the emission sources’ operating parameters and 

their locations: 

 

 American Meteorological Society/Environmental Protection Agency Regulatory 

Model Improvement Committee (AERMIC) model, also known as AERMOD 

(Version 09292); and 

 Building Profile Input Program – Plume Rise Model Enhancements 

(BPIP-PRIME, Version 04274). 

The original air dispersion modeling for the Project was conducted with AERMOD 

Version 07026, but this supplemental air dispersion modeling analysis has been 

conducted using AERMOD Version 09292. The air quality modeling analysis followed 

the January 2008 USEPA AERMOD Implementation Guide and EPA’s 2005 “Guideline 

on Air Quality Models.” USEPA default options for AERMOD were used (model option 

DFAULT). Standard AERMOD control parameters were used, including stack tip 

downwash, non-screening mode, non-flat terrain, and sequential meteorological data 

check. The modeled hourly NO2 concentrations were limited by the available ozone 

according to the use of AERMOD with the PVMRM option. To the extent that the 

meteorological data set contained missing values for any hour such that a valid NO2

concentration could not be modeled, the results for that hour were coded as missing. 

2.3 Receptor Grids

Receptor and source base elevations were determined from USGS Digital Elevation 

Model (DEM) data using the 71⁄2-minute format (10- to 30-meter spacing between grid 

nodes). All coordinates were referenced to UTM North American Datum 1927 

(NAD27), Zone 10. The AERMOD receptor elevations were interpolated among the 

DEM nodes according to standard AERMAP procedure. For determining concentrations 

in elevated terrain, the AERMAP terrain preprocessor receptor-output (ROU) file option 

was chosen. 

Nested Cartesian coordinate receptor grids were developed to provide adequate spatial 

coverage surrounding the project area for assessing ground-level pollution 

concentrations, to identify the spatial extent of significant impacts, and to identify 

maximum impact locations. The resolution and extent of each nested grids were as 

follows: 
-5- 

 25-meter resolution along the facility fence line in a single tier of receptors 

composed of four segments extending out to 100 meters from the fence line; 

 100-meter resolution from 100 meters to 1,000 meters from the fence line; and 

 250-meter resolution from 1 km out to approximately 10 km from the site. 

When maximum first-high or maximum second-high impacts occurred in the 100- or 

250-meter spaced areas, an additional refined receptor grid with 25-meter resolution was 

placed around each maximum coarse grid impact in the form of a circular area extending 

out to a distance of 500 meters (i.e., two coarse grid “spacings”) from the location of each 

coarse grid maximum. Concentrations within the facility fence line were not calculated. 

The following 7.5-minute USGS Digital Elevation Model (DEM) quadrangles in 

California were employed for modeling the Project: 

 Guijarral Hills; 

 Huron; 

 Westhaven; 

 Avenal; 

 La Cima; 

 Kettleman City; 

 Garza Peak; 

 Kettleman Plain; and 

 Los Viejos. 

The approach used for selecting receptor grids for this analysis is identical to that used in 

the air quality impact analysis contained in the February 2008 PSD and FDOC permit 

applications. 

2.4 Data Requirements—Meteorological Data

AERMOD uses hourly meteorological data to characterize plume dispersion. The 

representativeness of the data is dependent on the proximity of the meteorological 

monitoring site to the area under consideration, the complexity of the terrain, the 

exposure of the meteorological monitoring site, and the period during which the data are 

collected. The meteorological data used in this analysis were collected at the Hanford 

monitoring station, located 28 miles northeast of the site. This data set was selected to be 

representative of meteorological conditions at the project site and to meet the 

requirements of the EPA On Site Meteorological Program Guidance for Regulatory 

Model Applications (EPA 450/4 87 013, August 1995). The analysis used 

meteorological data collected during 2000-2004, and prepared by the District in 

AERMOD-compatible format. 
-6- 

The District had preprocessed the meteorological data from Hanford into the format 

required by AERMOD. The surface characteristics appropriate for the land uses 

surrounding the meteorological station at Hanford—namely surface roughness length, 

albedo, and Bowen Ratio—were computed by the District. The meteorological data set 

used for this analysis is unchanged from that used to develop the air quality impact 

analysis in the February 2008 PSD and FDOC permit applications. 

2.5 Data Requirements—Ambient Monitoring Data

Ambient NO2 data collected during the five-year modeling period of 2000-2004 at the 

Hanford monitoring station were used to characterize the representative ambient 

background concentrations. The ambient pollution levels monitored at the Hanford 

monitoring station represent area-wide ambient conditions rather than the localized 

impacts of any particular facility due to the lack of any large local industrial sources in 

the Hanford area. 

Computation of hourly NO2 impacts from the Project also requires use of ambient 

monitored hourly ozone concentrations. The PVMRM adaptation of the Ozone Limiting 

Method6 was used to determine the extent to which the NO emitted from the exhaust 

stacks is converted to NO2 when it reaches the ground. AERMOD PVMRM calculates 

the NO2 concentration using hourly ozone data. Hourly ozone data collected at the same 

Hanford monitoring station during the years 2000-2004 were used in conjunction with 

PVMRM to calculate hourly NO2 concentrations from hourly NOx concentrations, as 

described in the modeling protocol submitted to EPA on August 14, 2007. 

The hourly ambient monitoring data are identical to those used in the air quality impact 

analysis in the February 2008 PSD and FDOC permit applications 

2.5.1 Data Substitution—Ozone

Data substitution for missing hourly O3 and NO2 ambient monitoring data requires 

different data substitution procedures because the two data sets serve different purposes. 

The hourly O3 data are used within the AERMOD air dispersion model when operated 

using the PVMRM option that simulates the atmospheric chemistry of O3 reacting with 

initially emitted nitric oxide (NO) to form NO2. If the amount of O3 in the plume is 

insufficient to completely react the NO to NO2, then the reaction is limited, forming less 

NO2 than occurs with the simplifying default assumption of complete conversion. The 

model disperses the initial NOx emissions, which are mostly NO, during each of the 

8,760 hours in a 365-day year. Missing hourly ambient O3 concentrations from the 

 

6

 Cole, Henry S. and John E. Summerhays. The Application of reactive plume models to the estimation of 

short-term NO2 concentration, presentation at the Annual Meeting of the Air Pollution Control Association, 

Houston, Texas, June 1978, and A review of techniques available for estimating short-term NO2

concentrations, Journal of the Air Pollution Control Association, Volume 29, Number 8, pages 812-817, 

August 1979. 
-7- 

Hanford monitoring station were given substituted concentrations with hour-appropriate 

values (e.g., from the previous day, or the next day, for the same hour) as stated in the 

August 14, 2007 modeling protocol. 

2.5.2 Data Substitution—NO2

Unlike the O3 data that are used by the dispersion model to determine modeled impacts, 

the NO2 ambient data are used to establish the baseline ambient conditions for the Project 

impact area. As such, based on EPA guidance related to determinations of compliance 

with the new one-hour average NO2 NAAQS,7 no data substitution was performed for the 

NO2 data; instead, it was necessary to determine whether there are sufficient hourly data 

available for a complete day, quarter, and year. Under EPA guidance,8

 a day is classified 

as complete if it has at least 75% of the hourly concentrations recorded (i.e., at least 18 

hourly readings per day). A quarter is classified as complete if it has at least 75% of the 

sampling days with complete data (i.e., at least 67 to 69 depending on quarter). A year is 

classified as complete if it has four complete quarters. An incomplete year cannot be 

used to demonstrate compliance (although under certain circumstances it could be used to 

demonstrate noncompliance). 

To determine whether a day, quarter, or year is complete, missing hourly NO2 data were 

identified. Missing hourly NO2 ambient concentrations were replaced with the value -99, 

which informs the computation algorithm that no valid NO2 concentration was measured 

for that hour. If more than six hourly concentrations were missing in the same day, the 

entire day was identified as invalid, again following the same EPA regulatory guidance.9

2.6 Combining Existing Ambient Air Quality Data with Modeled Impacts

Modeled concentrations were added to background NO2 concentrations to determine total 

concentrations for comparison with the new NAAQS using the procedure summarized 

below. 

The modeled and monitored 1-hour NO2 concentrations were combined as outlined 

below. 

 As described above, the impacts of the project at each receptor in the study area 

and for each hour in the analysis period were modeled using AERMOD. 

 The predicted total hourly NO2 concentration at each receptor was calculated by 

adding the modeled hourly project impact at the receptor to the corresponding 

hour NO2 concentration measured at the Hanford ambient monitoring station. 

 

7

 Federal Register, Volume 75, Number 26, Part III, Environmental Protection Agency, 40 CFR Parts 50 

and 58, Primary National Ambient Air Quality Standards for Nitrogen Dioxide; Final Rule, Appendix S, 

pages 6532-6533, February 9, 2010. 

8

 Ibid, p. 6532. 

9

 Ibid. 
-8- 

 For hours with missing ambient monitored background NO2 concentrations, the 

resulting total hourly concentrations (modeled impacts plus background 

concentrations) were labeled as missing using the value -99 at every receptor. 

 The daily maximum total hourly NO2 concentration for each day was determined 

for each receptor. If there were fewer than 18 complete hours for a day, the value 

for that day was labeled as missing using the value of -99. 

 If there were four complete quarters for a given complete year, the 98th percentile 

daily maximum total hourly NO2 concentration was determined for that year for 

each receptor as follows from EPA guidance: 

- 8th highest value if the annual number of valid daily maximum total hourly 

NO2 concentrations is at least 351; 

- 7th highest value if the annual number of valid daily maximum total hourly 

NO2 concentrations is between 301 and 350; and 

- 6th highest value if the annual number of valid daily maximum total hourly 

NO2 concentrations is between 251 and 300. 

 At each receptor, the 98th percentile daily maximum concentrations were averaged 

across each three-year period in the analysis period. 

 From the field of receptors, the receptor with the highest three-year average was 

selected to represent the maximum total hourly NO2 concentration for the project 

impact area. This maximum total hourly NO2 concentration was converted from 

μg/m3

 to ppb (using the annual NO2 NAAQS conversion ratio of 100 μg/m3 = 

53 ppb); the result was rounded to the nearest ppb and compared to the new 

one-hour NO2 NAAQS. 

Sierra Research developed a postprocessor in FORTRAN to perform these calculations. 

The postprocessor uses the two following input files: 

 AERMOD 1-hour average NO2 DAYTABLE10 output file (processed with 

PVMRM); and 

 Hourly ambient background NO2 concentrations (in units of μg/m3

). 

To allow the postprocessor to handle the large DAYTABLE output files for tens of 

thousands of receptors in the grids and the five years of hourly meteorological data, the 

postprocessor reads and processes concentrations from both files in 24-hour blocks. 

 

10 A February 25, 2010 USEPA Notice Regarding Modeling for New Hourly NO2 NAAQS, suggests the 

same computational post-processing with the AERMOD-generated POSTFILE 

(http://www.epa.gov/scram001/no2_hourly_NAAQS_aermod_02-25-10.pdf). The AERMOD generated 

DAYTABLE and POSTFILE contain identical data, but in different formats. 
-9- 

The AERMOD output file is read, receptor by receptor, for each hour. Then the 

background concentration is read for that hour and added to the modeled concentrations. 

By the end of a day’s values, if 75% completeness is not achieved, then the hourly 

maxima at all the receptors for that day are replaced with an incompleteness indicator 

(-99). 

Once the daily maxima are determined for each receptor for the entire year, two tables are 

generated. The first table calculated is the daily maxima for all receptors, and becomes 

the first output table from the postprocessor. 

The daily maxima for each receptor are sorted by the postprocessor through use of the 

HEAPSORT utility from Numerical Recipes.11 The second table is calculated, and 

output for the eight highest daily maximum concentrations for each receptor (tagged by 

Julian day) is generated. 

At each receptor, the 98th percentile daily maximum concentrations were averaged across 

each of the 3 three-year periods in the 2000-2004 analysis period. Examination of the 

second table yields the highest 8th-highest concentration from among all the receptors, for 

comparison to the federal standard. 

### 

 

11 The HEAPSORT sorting routine is taken from Press, W.H., B.P. Flannery, S.A. Teukolsky, and W.T. 

Vetterling,, Numerical Recipes, The Art of Scientific Computing (FORTRAN Version), Section 8.2, 

page 229, Cambridge University Press, New York, 1989. 
-10- 

3. RESULTS 

The three-year average of 98th percentile concentrations at any receptor was 119.8 μg/m3, 

or 63 ppbv, and occurred during startup conditions. This is lower than the one-hour NO2

NAAQS of 100 ppb. The project will therefore not cause or contribute to an exceedance 

of the standard. 

Except for the last two quarters of 2001, each quarter in the study analysis period met the 

validity requirements for a compliance demonstration under EPA guidelines. For 2000, 

2002, and 2004, more than 350 of the daily measurements were valid; thus the 98th

percentile concentration for each receptor was the 8th highest daily one-hour maximum. 

The year 2001 was invalid for compliance calculation because of the invalid last two 

quarters. For 2003, the 342 valid daily measurements were between 301 and 350; 

therefore, the 98th percentile concentration for each receptor was the 7th highest daily onehour maximum. Failure of the year 2001 left only one consecutive three-year period over 

which to calculate the three-year average of 98th percentile concentrations—2002-2004; 

this was the period used to determine the three-year average 98th percentile value of 

63 ppbv.