RE: Palmdale NO2, etc. follow-up

Head, Sara to: Scott Bohning 12/14/2010 09:43 PM

Cc:

"Hamel, Richard", Shirley Rivera, Carol Bohnenkamp, Julie Walters, 

MICHAEL.CARROLL, Tom Barnett

1 attachment

Response to 2nd set of EPA Comments on NO2 Modeling.pdf

Scott –

Attached are our responses to several of the issues you raised in the email below. We thought it would 

be helpful to get these responses to you before our 10 am call on Wednesday (12/15) morning, so you 

could see what our status is on addressing them. Hopefully you will have had a chance to look through 

this before the call so that we can discuss what is still needed.

Sara

Sara J. Head, QEP

AECOM Environment

T 805-388-3775 ext 227 

C 805-320-8059 

sara.head@aecom.com

From: Bohning.Scott@epamail.epa.gov [mailto:Bohning.Scott@epamail.epa.gov] 

Sent: Thursday, October 28, 2010 1:11 PM

To: Head, Sara

Cc: Hamel, Richard; Rivera.Shirley@epamail.epa.gov; Bohnenkamp.Carol@epamail.epa.gov;

Walters.Julie@epamail.epa.gov

Subject: Palmdale NO2, etc. follow-up

Sara -

This is a follow-up to our 10/25 phone call, and AECOM's 10/7 memo "Response to EPA Comments on 

AECOM 1-hour NO2 NAAQS Analysis for PHPP".

Below are paraphrases of the discussion that Region 9 recently had with OAQPS about 1-hour NO2 

analyses for PSD permits. At the end of each topic, I have included a numbered recommendation for 

the analysis of the Palmdale Hybrid Power Project. At the very end I also address the 3 year vs. 5 years 

of meteorological data issue, and an additional issue on the cumulative inventory. 

While there is not a straight "yes/no" answer on some of these, there are some recommendations for 

additional documentation on several of the issues that would help us ensure adequate justification for 

various approaches, as envisaged in the "Applicability of Appendix W Modeling Guidance for the 1-hour 

NO2 NAAQS", memorandum from Tyler Fox, 2010-06-28.
We appreciate the substantial work that has already gone into these issues, and your patience as we 

apply existing guidance for the new 1-hour NO2 standard to particular cases.

I would be available to discuss this most times during next week (though not Tues. 11/2 from 11-3, nor 

on Thurs. 11/4).

- Scott B.

(P.S. out on Friday 10/22)

Notes on 1-hr NO2 from 2010-10-27 call with OAQPS, with recommendation to permit applicant

IN-STACK NO2/NOX RATIOS

Should not just cite conversation with equipment vendor on ratios; provide information and/or data that 

the ratio conclusion is based on.

Should not just cite line of a table provided by e.g. a local District; provide documentation of data it is 

based on, argument for similarity of tested source equipment to project being modeled.

Note that the importance of the in-stack ratio varies by situation. For a short stack having peak NO2 

impact at fence line at night, with limited mixing and low O3, there will be limited NO to NO2 conversion 

beyond what is in the stack, so the in-stack ratio will be crucial, and can determine the overall answer; 

stronger justification needed for a low ratio. For a taller stack with good mixing and higher O3, ratio is 

less important (and conversely, for PVMRM the equilibrium ratio will be more important). Applicant 

should assess where a particular project lies between these two extremes, consider how this will affect 

the argument for the ratio's adequacy.

1. PALMDALE RECOMMENDATION: Please supply any available documentation, such as stack test 

data, substantiating the in-stack ratios from vendor GE. Given characteristics of the source and the 

conditions under which high impacts occur, as mentioned above, assess how this affects the adequacy of 

the ratio used.

NO2 AND O3 BACKGROUND MONITORS

OK to show NO2 conservativeness based on monitor proximity to roadways, and monitor inclusion of 

some impacts from nearby sources. Should be quantitative where possible, e.g. compare monitor and 

project site on their distances from nearby point sources, roadways, amount of vehicle traffic.

In general, choice of background cannot be determined in isolation; it depends on how it is added to 

model, and on what conditions that background represents. (What background sources does it capture 

under various conditions?) Consider what is upwind of monitor and of project.

Background concentration can vary by direction (and by meteorological condition...)

Project and monitor similar distance to urban area may add to argument for representativeness.
2. PALMDALE RECOMMENDATION: The documentation supplied about the 

representativeness/conservativeness of the Lancaster Division Street NO2 monitor is very useful, but it 

should be supplemented with a comparison to the features of the project site, such as distance to nearby 

point sources, roadways and the amount of traffic on them.

HOUR-BY-HOUR ADDITION OF MONITORED BACKGROUND

Expect a very strong justification for this degree of temporal pairing. A high monitored value may not 

happen to coincide with a high project impact for the data years used, but it still might occur in future 

years. Ideally, aim to show why high background and high source impact would not be expected to occur 

together, based on source characteristics and meteorological conditions.

Longer temporal basis preferable, and/or base on tie to meteorological conditions.

Focus on specifics of the case rather than generic arguments about conservativeness. Examine 

specifics of hour by hour (and seasonal) background concentration, impact of project, impact of nearby 

sources; use this in arguing for a temporal pairing scheme.

3. PALMDALE RECOMMENDATION: Temporal pairing of modeled and monitored concentrations based 

on months or tied to meteorological conditions would be far preferable than hour-by-hour pairing, as the 

latter would require more justification to demonstrate that it is representative and adequately 

conservative. The argument that the overall modeling approach used is conservative should be 

strengthened. Ideally this would show why high background and high source impacts would not be 

expected to occur together, based on characteristics of the project source, what sources the monitor

represents at different times, and the meteorological conditions. Information such as statistics and time 

series plots of impacts and background concentrations would be useful in developing the argument.

EQUILIBRIUM NO2/NOX RATIO FOR PVMRM

Not appropriate to use an annual average. (Also note that equilibrium NO2/NOx ratio for PVMRM is an 

hourly upper bound used in the algorithm, is functionally different than ARM ratio for "area wide 

quasi-equilibrium conditions".)

May be OK to use highest monitored ratio as the equilibrium ratio. The ratio may vary by hour or 

condition; consider what the particular ratio represents. Is the ratio used for a given hour conservative 

for the meteorological condition in which it is used?

4. PALMDALE RECOMMENDATION: Not applicable.

MISSING O3 DATA PROCEDURE

Not appropriate to use an annual average to fill in missing hourly ozone values: misses high values that 

can occur during the day.
Using the higher of the missing interval endpoints is more conservative than interpolation, but for longer 

intervals may miss an ozone peak. Even if it is OK for a particular data set, it's not a good general rule 

to approve. Better to start with an approach guaranteed to be conservative, then make special case 

arguments for problematic hours. (Instead of starting with a usually-conservative approach and then 

arguing that it's probably OK.)

Example approach: for each of the 24 hours of the day, use maximum value that occurs for that hour (24 

separate default values); or do this for each month (288 separate default values). Requires a bunch of 

post-processing.

5. PALMDALE RECOMMENDATION: It would be preferable to start with a conservative approach, such 

as substitution of a value from the corresponding seasonal or monthly high hour (as mentioned in the 

example). Professional judgement could then be applied to problematic hours, using an alternative 

approach that continues to insure that no underprediction bias is introduced. However, the approach 

proposed during our 10/25 phone call, using interval endpoint highs for intervals longer than 3 hours 

would still be acceptable for this particular data set, if it is one part of an overall substitution procedure, 

and is accompanied by a documented assessment of the reasonableness of substitutions of 3 or more 

consecutive daytime hours in comparison with neighboring days.

ADDITIONAL PALMDALE ISSUES

6. As discussed in the "Applicability of Appendix W Modeling Guidance for the 1-hour NO2 NAAQS", 

memorandum from Tyler Fox, 2010-06-28, as well as in Guideline on Air Quality Models §8.3.1.2, five 

years of representative meteorological data (or one year of on-site data), are required in air quality 

impact analyses. However, the modeling supplied for NO2 as well as for the other pollutants evaluated 

used only three years of data. While three years of data may have been accepted for past applications 

by other permitting authorities, or even by EPA in particular circumstances, we do not believe that is a

sufficient rationale for using less than the 5 years specified in EPA's regulatory guidance. Additional 

justification would be required for using a shorter data set.

7. The October 7th AECOM memo cites Chris Anderson of the Antelope Valley Air Quality Management 

District (AVAQMD) as the source of information about nearby sources to use in a cumulative modeling 

assessment. However, there should be documentation of the selection criteria were used, and whether 

they meet the requirements in the Guideline on Air Quality Models §8.2.3 on background concentrations 

in multi-source areas.

- Scott B.

-- 

=+=+=+=+=+=+=+=+=+=+=+=+=+=+=

"Scott Bohning" <bohning.scott@epa.gov>

U.S. EPA Region 9, AIR-7

415/947-4127 fax-3579

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