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Lead in Air

SIP Toolkit - Attainment Demonstrations and Air Quality Modeling

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The lead SIP regulations found at 40 CFR 51.117 require states to employ atmospheric dispersion modeling for the demonstration of attainment for areas in the vicinity of point sources listed in 40 CFR 51.117(a)(1). To complete the necessary dispersion modeling, meteorological, and other data are necessary. Dispersion modeling should follow the procedures outlined in EPA’s latest guidance document entitled ‘‘Guideline on Air Quality Models’’. This guideline indicates the types and historical records for data necessary for modeling demonstrations (e.g., on-site meteorological stations, 12 months of meteorological data are required in order to demonstrate attainment for the affected area).

Requirements concerning control strategy demonstration are stated in 40 CFR 51.117(a). Lead nonattainment area SIPs must provide for the implementation of control measures for point and area source emissions of lead in order to demonstrate attainment of the lead NAAQS as expeditiously as practicable, but no later than the attainment date for the affected area. If a state adopts less than all available measures in an area but demonstrates, adequately, that reasonable further progress (RFP), and attainment of the lead NAAQS are assured, and the application of all such available measures would not result in attainment any faster, then a plan which requires implementation of less than all technologically and economically available measures may be approved (see 44 FR 20375 (April 4, 1979) and 56 FR 5460 (February 11, 1991)).

EPA notes that 40 CFR 51.117(a) provides that states must include, as a part of their attainment modeling demonstration, an analysis showing that the SIP will attain and maintain the standard in areas in the vicinity of certain point sources that are emitting at the level of 25 tpy, and also in ‘‘any other area that has lead air concentrations in excess of the national ambient air quality standard concentration.’ In doing the analysis, required under 40 CFR 51.117(a)(2), EPA expects the state will take into consideration all sources of lead emissions within the nonattainment area that may be required to be controlled, taking into consideration the attainment needs of the area.

Modeling analyses should conform with EPA’s guidelines on air quality models contained in Appendix W to 40 CFR part 51. The Guideline is used by EPA, States, and industry to prepare and review new source permits and State Implementation Plan revisions. The Guideline is intended to ensure consistent air quality analyses for activities regulated at 40 CFR 51.112, 51.117, 51.150, 51.160, 51.166, and 52.21. EPA originally published the Guideline in April 1978 and it was incorporated by reference in the regulations for the Prevention of Significant Deterioration (PSD) of Air Quality in June 1978. EPA revised the Guideline in 1986, and updated it with supplement A in 1987, supplement B in July 1993, and supplement C in August 1995. We published the Guideline as appendix W to 40 CFR part 51 when we issued supplement B. Modeling input data, including emission rates, are addressed in Section 8.0 of Appendix W.

Additional modeling guidance for lead SIPs was issued on July 8, 2011 and addressed the following issues.

11 Q. What is the appropriate emissions rate (peak seasonal, monthly, or annual rate) to use for the three-month rolling average?
A. Modeling analyses should conform with EPA’s guidelines on air quality models contained in Appendix W to 40 CFR part 51. Modeling input data, including emission rates, are addressed in Section 8.0 of Appendix W. The averaging period for the 2008 Lead NAAQS is a rolling 3-month average evaluated over a 3-year period. Accordingly, emissions limits should be based on concentration estimates for this same period (3-month average) as described in Section 10.2.3 of Appendix W. The emissions rate to input into AERMOD for attainment demonstrations is based on the maximum allowable or permit limit emissions, often 1-hour limits. Table 8-1 of Appendix W (see below) gives the calculation methodology to use to calculate the emissions rate to input into AERMOD. The input emissions rate for a source subject to SIP limits is a product of the maximum allowable or permit limit emissions, operating level (actual or design capacity, whichever is greater, or federally enforceable permit condition) and operating factor. This same calculation is also used for nearby sources. For “other” sources, the operating level is the annual level when actually operating averaged over the most recent two years. For definitions of nearby and other sources, see Section 8.2.3 of Appendix W.

For the purpose of deriving permit limits for Lead based on modeling, we interpret the above procedures as follows. In general, the maximum hourly emission rate (PTE) should be used as the basis for establishing emission limits and for model input. This approach is appropriately conservative for emissions units that: 1) could be operated at a relatively high capacity factor (% of available capacity) over the applicable averaging period, 2) are associated with noncontinuous compliance monitoring methods (e.g., periodic source testing), and 3) have emissions that are not well correlated with production or other measureable surrogate monitoring parameters. Additionally, where significant uncertainty exists with respect to estimated emissions, modeling peak hourly rates may be necessary to account for this uncertainty.

In certain cases, longer term average emission rates or emissions representative of actual operating schedules may be approved for use in modeling demonstrations and corresponding permit limitations. Consistent with Appendix W, where a source is willing to accept an enforceable limitation on operating schedule, emissions need only be modeled during allowable periods of operation. Longer-term average emission limits (e.g., monthly average, 3-month average, or 3-month total) may be approved for qualifying emissions units. To be approvable, such limitations must be enforceable as a practical matter. We emphasize that approval of model input data and proposed emissions limits must be granted by the reviewing authority on a case-by-case basis, taking into account source and emissions unit-specific factors. Modeled emission rates, including any proposed limitations on emissions or source operation, should be documented in the modeling protocol, and any associated permit application materials submitted to the reviewing agency for approval.

12 Q. What is required for modeling for attainment demonstrations? When should allowable emissions be used and when should permits be used?
A. Modeling for attainment demonstrations is used to show that a nonattainment area will be in attainment by the attainment date. The modeling is used to show the effectiveness of control measures on the sources. For attainment modeling, maximum allowable or federally enforceable permit limits should be the basis of the model input emissions, as described in Section 8.1 and Table 8-1 of Appendix W and the Guideline for Air Quality Models.

13 Q. What is EPA’s policy on conducting model evaluations to avoid under or over predictions compared to monitoring results?
A. As part of the model promulgation process, AERMOD has been evaluated in several studies and showed excellent performance. For details about the AERMOD evaluation results see the AERMOD Evaluation Paper (PDF) (41pp, 1.4 MB). Any potential bias in modeled results versus monitored concentrations would most likely be introduced by the user (inaccurate characterization of emissions or use of non-representative meteorological data in the modeling) and if found should be explained by the user.

Depending on the level of emissions used (allowable versus actual) and the number of monitors being used in the evaluation, it may be possible to conduct a model evaluation for the specific case being modeled. In terms of the number of monitors, comparison to a single monitor is not considered sufficient to indicate a model bias. If maximum allowable emissions or permitted emissions are used as the model input emissions (either for the SIP modeling or use in designations modeling), it would be expected that modeled concentrations would not be comparable to monitored concentrations because of the nature of the emissions level.

For designations modeling using actual emissions, if emissions have been characterized accurately and input meteorological data is representative of the area being modeled, one way to measure the performance of the modeled results against monitored values is to use quantile-quantile plots (QQ plots) by plotting the observed and modeled values in ranked order, i.e. highest monitored value paired with highest modeled value at monitor receptor locations. This creates concentration pairs of monitored and modeled concentrations that are no longer paired in time space and is considered a pragmatic procedure to evaluate model performance as noted in the AERMOD Evaluation Paper.

14 Q. What is EPA’s policy on modeling for contingency measures?
A. It is not necessary to model the effect of implementing contingency measures. However, modeling is one way to help gauge the potential effectiveness of backstop measures and may demonstrate that any contingency measures that are adopted in the SIP are sufficient to be approved by EPA. Modeling of contingency measures should follow the guidelines in Appendix W.

15 Q. When will the new AERMOD, AERMET, and AERMINUTE updates be released?
A. The latest updates to AERMET, and AERMINUTE, version 11059, were released on March 8, 2011. The latest updates to AERMOD and AERMAP were released on April 14, 2011 (version 11103).

16 Q. Will AERMOD calculate the Lead NAAQS design value?
A. AERMOD does not calculate the Lead NAAQS design value. A post-processor called LEADPOST (zip file) (38.5 MB) will calculate the Lead NAAQS design values from monthly modeled output.

17 Q. If the modeling uses 5 years of National Weather Service data do all years have to be modeled in one run or can individual years be modeled?
A. The modeled design value is calculated as the rolling 3-month average concentration at each receptor across the five years. AERMOD does not calculate the design value, so post-processing is required. The EPA post-processor, LEADPOST, can be used to calculate the design values. The five years of model output do not have to be in one AERMOD run. Each individual year can be run separately and the output for each year can be input into LEADPOST. LEADPOST will read the individual files and calculate the design values across the five years provided that each year’s runs have the same receptors and source group contributions.

Modeling Questions and Answers (PDF) (6pp, 56k) - August 10, 2012

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