Attachment A: EPA Region 9 Comments on the May 13, 2010 Supplemental N02 Air Quality 

Impact Analysis for the Avenal Energy Project 

Comment 1: Avenal's May 13, 2010 I-hour N02 submittal did not include necessary information 

indicating which, if any, nearby sources were modeled or the process used to determine which 

nearby sources should be included. A full impact analysis should include modeling of nearby 

sources as well as monitored background values. The recommendations for modeling for nearby 

sources are discussed in EPA's Guideline on Air Quality Models (40 CFR Part 51, Appendix W) 

Subsection 8.2.3, and relevant excerpts are quoted below. All sources expected to cause a 

significant concentration gradient in the vicinity of the source or sources under consideration for 

emission Iimit(s) should be explicitly modeled. Please provide a description of the process used for 

determining which nearby sources should be included in the modeling, as well as the modeling 

analysis for any such sources. 

8.2 Background Concentrations 

8.2.3 Reconlmendations (Multi-Source Areas) 

a. In multi-source areas, two components of background should be determined: 

contributions from nearby sources and contributions from other sources. 

b. Nearby Sources: All sources expected to cause a significant concentration gradient in 

the vicinity of the source or sources under consideration for emission limit(s) should be explicitly 

modeled. The number of such sources is expected to be small except in unusual situations. Owing 

to both the uniqueness of each modeling situation and the large number of variables involved in 

identifying nearby sources, no attempt is made here to comprehensively define this term. Rather, 

identification of nearby sources calls for the exercise of professional judgement by the 

appropriate reviewing authority (paragraph 3.0(b)). This guidance is not intended to alter the 

exercise ofthat judgement or to comprehensively define which sources are nearby sources. 

c. For compliance with the short-term and annual ambient standards, the nearby sources 

as well as the primary source(s) should be evaluated using an appropriate Appendix A model with 

the emission input data shown in Table 8-1 or 8-2. 

Comment 2: Avenal's May 13, 2010 I-hour N02 submittal did not include necessary information 

concerning the source emission input data. Existing NOx emission inventories used to support 

modeling for compliance with the annual N02 standard may not be adequate for use in assessing 

compliance with the new hourly standard, since maximum ground-level concentrations for an 

hourly standard may be more sensitive to operating levels than for the annual standard, and 

maximum allowable hourly emission rates for nearby sources needed for modeling hourly impacts 

may be higher than emission rates that may have been used for the annual standard. The source 

emission inventory discussion should include an analysis of model load as discussed in 40 CFR Part 

51, Appendix W, Subsection 8.1.2: 
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8.1 Source Data 

8.1.2 Recommendations 

a. For point source applications the load or operating condition that causes maximunl 

ground-level concentrations should be established. As a minimum, the source should be modeled 

using the design capacity (100 percent load). If a source operates at greater than design capacity 

for periods that could result in violations ofthe standards or PSD increments, this load) [footnote 

omitted] should be modeled. Where the source operates at substantially less than design capacity, 

and the changes in the stack parameters associated with the operating conditions could lead to 

higher ground level concentrations, loads such as 50 percent and 75 percent of capacity should 

also be modeled. A range of operating conditions should be considered in screening analyses; the 

load causing the highest concentration, in addition to the design load, should be included in 

refined modeling. For a steam power plant, the following (b-h) is typical ofthe kind of data on 

source characteristics and operating conditions that may be needed. Generally, input data 

requirements for air quality models necessitate the use of metric units; where English units are 

common for engineering usage, a conversion to metric is required. 

c. Stack parameters. For all stacks, the stack height and inside diameter (meters), and the 

temperature (K) and volume flow rate (actual cubic meters per second) or exit gas velocity 

(meters per second) for operation at 100 percent, 75 percent and 50 percent load. 

j. Line source modeling of streets and highways requires data on the width of the 

roadway and the median strip, the types and amounts of pollutant emissions, the number of lanes, 

the emissions from each lane and the height of emissions. The location of the ends of the straight 

roadway segments should be specified by appropriate grid coordinates. Detailed information and 

data requirements for modeling mobile sources of pollution are provided in the user's manuals for 

each of the models applicable to mobile sources. 

Comment 3: Avenal's May 13, 2010 I-hour N02 submittal did not include a discussion of whether 

startup or shut-down emissions were included in the analysis. The submittal should include an 

analysis of startup and shutdown emissions and a description of how they were addressed.. 

Comment 4: Avenal's May 13, 2010 I-hour N02 submittal did not appropriately address the instack ratios of N02INOx emissions and background ozone concentrations. Key model inputs for 

both the OLM and PVMRM options are the in-stack ratios of N02INOx emissions and background 

ozone concentrations. While the representativeness of these key inputs is important in the context 

of the annual N02 standard, they will generally take on even greater importance for an hourly 

standard. Recognizing the potential importance ofthe in-stack N02INOx ratio for hourly N02 

compliance demonstrations, we recommend that in-stack ratios used with either the OLM or 

PVMRM options be justified based on the specific application, i.e., there is no "default" ratio for 

either OLM or PVMRM. It is our understanding that the upcoming EPA guidance on the 

applicability of Appendix W Modeling Guidance for honrly N02 NAAQS will also provide further 

clarification on this issue. 

Comment 5a: Avenal's May 13,2010 I-hour N02 submittal did not include appropriate 

justification of the method that was used for combining arrlbient air quality data with the modeled 

source and nearby source impact data, based on combining monitored background concentrations 
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and modeled concentrations on an hour-by-hour basis. The submittal should provide a justification 

as to why the method should be deemed acceptable. 

Comment 5.b: Avenal's May 13,2010 I-hour N02 submittal labeled hours with missing ambient 

monitored background N02 as missing for combined modeled and monitored impact, which 

effectively discards valid modeled data. An alternative method, which would provide a 

conservative monitored background value for every valid modeled hour, should be selected. 

Comment 6: Avenal's May 13,2010 I-hour N02 submittal did not include any discussion of how 

building downwash is addressed. The analysis should include an appropriate discussion of how 

building downwash is addressed, following the recommendations of 40 CFR Part 51, Appendix W 

Subsection 6.2.2, Good Engineering Practice Stack Height. Also note that building downwash 

influences on other nearby sources included in the modeling analysis should be addressed. 

Comment 7: Avenal's May 13,2010 I-hour N02 submittal did not include any justification of the 

representativeness of the meteorological data. The representativeness of meteorological data of the 

conditions at the project site should be consistent with the recommendations of 40 CFR Part 51, 

Appendix W, Subsection 8.3, Meteorological Input Data, as quoted below. The analysis should 

include a discussion of how the proposed site addresses these recommendations. This analysis 

should include a discussion of the representativeness of the surface characteristics. The surface 

characteristics input to AERMET should be based on the land cover characteristics in the vicinity 

of the meteorological tower. Other candidate meteorological stations should be discussed. The 

representativeness of the upper air station used for this analysis should be discussed. The 

AERMOD Implementation Guide, revised March 19, 2009, specifically addresses meteorological 

data representativeness and representativeness of surface characteristics for AERMOD. 

8.3 Meteorological Input Data 

a. The meteorological data used as input to a dispersion model should be selected on the 

basis of spatial and climatological (temporal) representativeness as well as the ability of the 

individual parameters selected to characterize the transport and dispersion conditions in the area 

of concern. The representativeness of the data is dependent on: (1) The proximity of the 

meteorological monitoring site to the area under consideration; (2) the complexity of the terrain; 

(3) the exposure of the meteorological monitoring site; and (4) the period of time during which 

data are collected. The spatial representativeness ofthe data can be adversely affected by large 

distances between the source and receptors of interest and the complex topographic 

characteristics of the area. Temporal representativeness is a function of the year-to-year variations 

in weather conditions. Where appropriate, data representativeness should be viewed in terms of 

the appropriateness of the data for constructing realistic boundary layer profiles and three 

dimensional meteorological fields, as described in paragraphs (c) and (d) below. 

c. Regulatory application of AERMOD requires careful consideration of minimum data 

for input to AERMET. Data representativeness, in the case of AERMOD, means utilizing data of 

an appropriate type for constructing realistic boundary layer profiles. Of paramount importance is 

the requirement that all meteorological data used as input to AERMOD must be both laterally and 

vertically representative of the transport and dispersion within the analysis domain. Where 

surface conditions vary significantly over the analysis domain, the emphasis in to adequate 

characterization of transport and dispersion between the source(s) of concern and areas where 

maximum design concentrations are anticipated to occur. The representativeness of data that were 

collected off-site should be judged, in part, by comparing the surface characteristics in the 
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vicinity ofthe meteorological monitoring site with the surface characteristics that generally 

describe the analysis domain. The surface characteristics input to AERMET should be based on 

the topographic conditions in the vicinity ofthe meteorological tower. Furthermore, since the 

spatial scope of each variable could be different, representativeness should be judged for each 

variable separately. For example, for a variable such as wind direction, the data may need to be 

collected very near plume height to be adequately representative, whereas, for a variable such as 

tenlperature, data from a station several kilometers away from the source may in some cases be 

considered to be adequately representative. 

Comment 8: Avenal's May 13,2010 I-hour N02 submittal did not include any analysis showing the 

representativeness of air quality data at the Hanford site with respect to the conditions at the 

project. The submittal should include the appropriate analysis, and other candidate ambient air 

quality monitoring stations should be discussed. 

Comment 9: The method used to substitute for missing hourly ambient 03 concentrations for use 

with the PVMRM option is generally of greater importance for the hourly standard than for the 

annual standard. The approach for replacing missing hourly ambient 03 concentrations from the 

Hanford monitoring station with hour-appropriate values from the previous or next day (discussed 

in Avenal's submission at section 2.5.1 Data Substitution-Ozone) is not fully defined and may 

overstate the ozone limiting potential in some cases, which could underestimate ambient N02 

concentrations. This method for substituting missing 03 concentrations should be replaced with a 

clearly defined approach that includes a reasonable assurance that substituted values will not result 

in underestimates of ambient N02 concentrations.