Modeling Guidance and Examples for Commonly Asked Questions

(Part 1)

Rachel Melton and Matthew KovarAir Permits Division

Texas Commission on Environmental QualityAdvanced Air Permitting Seminar 2014

Treatment of Intermittent Emissions

• Modeling is a challenge for 1-hour NO2 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 §117.2030(c)

– DFW Ozone Nonattainment Area – 30 TAC §117.2130(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 NOx-to-NO2 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 NOx to NO2 through the chemical mechanism of ozone titration:

NO + O3 → NO2 + O2

• Requires several inputs in AERMOD:

– In-stack NO2/NOx ratios

– Background ozone concentrations

– Ambient NO2/NOx equilibrium ratio

In-stack NO2/NOx 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 NO2/NOx 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 NOx converted to NO2 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 NO2 National Ambient Air Quality Standard.

• Default in-stack NO2/NOx ratio of 0.2 for “distant nearby” sources.

• Ambient Ratio Method 2 (ARM2).

Questions?

Contact Information

• Matthew Kovar– Air Dispersion Modeling Team

– (512) 239-0180

– matthew.kovar@tceq.texas.gov

• Rachel Melton– Air Dispersion Modeling Team

– (512) 239-2358

– rachel.melton@tceq.texas.gov

Air Permits Division

(512) 239-0180matthew.kovar@tceq.texas.gov

Matthew Kovar

Air Permits Division

(512) 239-2358rachel.melton@tceq.texas.gov

Rachel Melton