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).
Contact Information
• Matthew Kovar– Air Dispersion Modeling Team
– (512) 239-0180
• Rachel Melton– Air Dispersion Modeling Team
– (512) 239-2358
Air Permits Division
(512) [email protected]
Matthew Kovar
Air Permits Division
(512) [email protected]
Rachel Melton