4 The record articles

North Carolina Modeling Considerations

Posted: February 27th, 2019

Author: All4 Staff 

In May and June 2018, North Carolina Department of Environmental Quality (NC DEQ) released various updates regarding air quality modeling for facilities located in North Carolina.  The updates address the following topics.

♦ Modeling guidance for toxics

-Guidance for quarries
-Guidance for on-site meteorology

♦ Available North Carolina modeling data

In May 2018 NC DEQ released an update for toxics modeling guidance, titled Guidelines for Evaluating the Air Quality Impacts of Toxic Pollutants in North Carolina.  In this guidance, NC DEQ has removed Section 1.2.3, which discussed specific modeling guidelines for quarries.  The guidance addressed dispersion modeling to demonstrate compliance with the National Ambient Air Quality Standards (NAAQS) for particulate matter with a diameter of less than 10 micrometers (PM10) which has instead been published in a new guidance document specifically addressing quarries titled Quarry Guidance for Refined Modeling.

If NC DEQ requires modeling for a quarry, typical sources will include crushers, screens, conveyor dumps, truck dumps, open pit, and haul roads. The previous NC DEQ modeling guidance required all new quarries and quarries that proposed modifications to their primary crusher, to undergo dispersion modeling to demonstrate compliance with the NAAQS for PM10.  With the new guidance, NC DEQ no longer requires this analysis. However, NC DEQ may still require an air quality dispersion modeling analysis if there is a concern that the new quarry or modification will cause an exceedance of the NAAQS or another applicable state or federal regulation. The new guidance no longer limits modeling to project modifications to the primary crusher and NC DEQ may require modeling for modifications involving other emissions units at the quarry.  Specific guidelines from NC DEQ for conducting the analysis remain unchanged. For instance, 24-hour PM10 remains the primary pollutant to evaluate, and 24-hour and annual concentrations of total suspended particles (TSP) will also be required in the analysis.

Additionally, the May 2018 NC DEQ toxics modeling guidance update revised Section 5.6, which addresses the comparison of applicable air toxics modeling results to the acceptable ambient levels (AAL) for toxic air pollutants listed in 15A NCAC 02D Section .1104. The guidance has been updated to reflect requirements in Appendix W to 40 CFR Part 51 around the use of onsite meteorological data. Consistent with previous guidance, if the most recent year of meteorological data is used and modeling results are below 50% of the applicable AAL, no further modeling is needed. This holds true whether airport or onsite meteorological data is used in the modeling analysis. Under the new guidance, if airport meteorological data is used, and the modeling results are over 50% but under 100% of the applicable AAL, an analysis using five years of met data is required. If onsite meteorological data is used, the model results using one year of met data are acceptable as long as they are under 100% of the applicable AAL. In all cases, if modeling results indicate an exceedance of an AAL, an evaluation of permit restrictions and/or source modifications will be necessary, along with remodeling to demonstrate compliance.

In a separate update, NC DEQ has specified that AERMOD version 18081 should be used in future modeling submittals.  NC DEQ has updated North Carolina surface profile elevations and meteorological data sets to reflect data from 2013 to 2017. These data sets should be used in any future modeling.

If you have questions about how these updates could affect your facility, or if you need assistance in conducting an air quality modeling analysis, please contact us.

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