Modeling Guidance and Examples for Commonly Asked Questions (Part 1) Rachel Melton and Matthew Kovar Air Permits Division Texas Commission on Environmental Quality Advanced Air Permitting Seminar 2014

Treatment of Intermittent Emissions Modeling is a challenge for 1-hour NO 2 NAAQS compliance demonstrations. Examples include: – Firewater pump engines – Emergency generator engines – Startup/shutdown operations

Modeling Intermittent Emissions Operate infrequently or on a random operating schedule. Representing these types of sources as operating continuously throughout the year often results in the sources becoming the culpable emission scenario. Modeled with an adjusted hourly emission rate rather than the maximum hourly emission rate.

Adjusted Hourly Emission Rate For a firewater pump tested 52 hours per year: – Modeled emission rate = maximum hourly emission rate * 52/8760 The calculation of the modeled hourly emission rate will be different for intermittent emissions with operational restrictions. 30 TAC Chapter 117 includes operational restrictions for emergency engines in ozone non-attainment areas: – HGB Ozone Nonattainment Area – 30 TAC § (c) – DFW Ozone Nonattainment Area – 30 TAC § (c)

30 TAC Chapter 117 With some exceptions, the operation of any stationary diesel or dual-fired engine for testing or maintenance cannot occur between the hours of 6 a.m. and 12 p.m. For a firewater pump tested 52 hours per year in the HGB Ozone Nonattainment Area: – 18 hours/day *365 days/year = 6570 hours/year – Modeled emission rate = maximum hourly emission rate * 52/6570

Intermittent Source Determination There are several factors to determine if a source is intermittent: – How many sources are there? – How often does the source operate per year? – What is the duration of operation once the source is operating? – Does the source operate on a known schedule, or does it operate randomly? – Does the source operate simultaneously with other sources?

Example 1 An applicant has seven firewater pump engines on site: – Each engine tested once per week for 52 hours per year. – Engine testing can occur over multiple days of the week. The engines would not be considered intermittent in this scenario: – The engine emissions could affect up to seven days per week.

Example 2 An applicant has seven firewater pump engines on site: – Each engine tested once per week for 52 hours per year. – All engines are tested on the same day of the week. The engines could be considered intermittent in this scenario: – The engine emissions would only affect one day per week.

Tier 3 Options for NO x -to-NO 2 Conversion Two options available in AERMOD: – Plume Volume Molar Ratio Method (PVMRM) – Ozone Limiting Method (OLM) Treated as non-default model options. Must submit protocol documenting modeling approach before submitting modeling with these options.

PVMRM & OLM Accounts for conversion of NO x to NO 2 through the chemical mechanism of ozone titration: NO + O 3 → NO 2 + O 2 Requires several inputs in AERMOD: – In-stack NO 2 /NO x ratios – Background ozone concentrations – Ambient NO 2 /NO x equilibrium ratio

In-stack NO 2 /NO x Ratio Default in-stack ratio is 0.5: – Can enter a single value for all sources or source-specific values. Must provide justification for the use of non-default in-stack ratios: – In-stack ratios are verified by the permit engineer. It is in the applicant’s best interests to use in-stack ratios specific to the modeled sources: – Using the default in-stack ratio may result in predicted concentrations similar to the Tier 2 ambient ratio method.

Background Ozone Concentrations Several ways to input background concentrations: – Single ozone concentration (OZONEVAL) – Temporally-varying ozone concentrations (O3VALUES) – Hour-by-hour ozone concentrations (OZONEFIL)

Background Ozone Concentrations (continued) Must provide justification to show selected ozone monitor is representative. When an hourly ozone concentration file is used, the meteorological data used in the modeling analysis must correspond to the years of the ozone concentration data.

Ambient NO 2 /NO x Equilibrium Ratio Default value is 0.9. The equilibrium ratio is typically reached far beyond the point of maximum predicted ground level concentrations.

PVMRM or OLM? EPA currently does not have a preference for one option over the other. PVMRM performs well for isolated elevated point sources. OLM performs well for low level releases and area sources.

Additional Considerations EPA intermittent guidance should not be applied in conjunction with PVMRM or OLM. AERMOD may not appropriately account for the amount of NO x converted to NO 2 when the emission rate for a source is adjusted per the intermittent guidance.

Recent Guidance Clarification on the Use of AERMOD Dispersion Modeling for Demonstrating Compliance with the NO 2 National Ambient Air Quality Standard. Default in-stack NO 2 /NO x ratio of 0.2 for “distant nearby” sources. Ambient Ratio Method 2 (ARM2).

Questions?

Contact Information Matthew Kovar – Air Dispersion Modeling Team – (512) – Rachel Melton – Air Dispersion Modeling Team – (512) – Air Permits Division (512) Matthew Kovar Air Permits Division (512) Rachel Melton