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McLennan County (Monitor Number: 48-309-1037) was chosen as the most representative of the available monitors for SO₂ and NO₂. With a population of 213,500, the concentrations from this monitor should be a very conservative estimate for Shackelford County.

Since McLennan County PM_2.5 data does not meet completeness requirements, Potter County (Monitor Number: 48-375-0320) with a population of 121,000 was chosen as a conservative and representative monitor for PM_2.5. Potter County is relatively close to Shackelford County geographically, and it has a higher population. Potter County data will provide conservative estimates of the background concentrations in Shackelford County.

Since McLennan and Potter County do not have complete data for PM₁₀, Tarrant County (Monitor Number: 48-439-3010), was used for PM₁₀ data. With a population of 650,000, Tarrant County is more populous than Shackelford County, and it is relatively close geographically. Concentration data from Tarrant County will provide conservative estimates of concentrations in Shackelford County.

The table below shows 2008 emission rates for each pollutant in the four counties discussed above. The emissions data comes from the EPA’s National Emissions Inventory (http://www.epa.gov/air/emissions/index.htm). The emission density is the emission total divided by the area of the county. On both an overall basis and a per-square-mile basis, the emissions in Shackelford County are significantly less than the emissions in the other counties.  Therefore, this supports the use of the data from the counties discussed in obtaining a conservative estimate for background concentrations of the various pollutants for the facility location.

Method of Obtaining Percentile Values

The tables below give guidelines for finding both 98^th and 99^th percentile values. To find percentile values, the data are arranged from largest to smallest. As an example, the 99^th percentile value of a data set with 250 data points would be the 3^rd value in the sorted list. Similarly, the 98^th percentile value of a data set with 360 data points would be the 8^th value in the sorted list. These tables and the full procedure can be found in Appendix T - Interpretation of the Primary National Ambient Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide) to 40 CFR Part 50 National Primary and Secondary Ambient Air Quality Standard  and Appendix N - Interpretation of the National Ambient Air Quality Standards for PM2.5 to 40 CFR Part 50 National Primary and Secondary Ambient Air Quality Standards.

NO₂ Background Concentration

Using concentration data from McLennan County (Monitor Number: 48-309-1037), the annual 98^th percentile values for 2008, 2009 and 2010 are 29.0, 31.3 and 27.0 ppb, respectively.  The 1-hour design value for the NO₂ background concentration (which is the average of the three 98^th percentile values) is 29 ppb. This value is equivalent to 55 μg/m³.

The EPA’s data completeness rule requires that, for each quarter, 75% of the scheduled sampling days must have data. The data for each year meets this requirement (i.e. each year has data for over 330 days.)  Methods used to obtain these values are in accordance with those given in Appendix S - Interpretation of the Primary National Ambient Air Quality Standards for Oxides of Nitrogen (Nitrogen Dioxide) to 40 CFR Part 50 National Primary and Secondary Ambient Air Quality Standards.

SO₂ Background Concentration

Using concentration data from McLennan County (Monitor Number: 48-309-1037), the annual 99^th percentile values for 2008, 2009 and 2010 are 5.0, 6.7 and 7.0 ppb, respectively. The 1-hour design value for SO₂ (which is the average of the three 99^th percentile values) is 6 ppb. This value is equivalent to 16 μg/m³.

The EPA’s data completeness rule requires that, for each quarter, 75% of the scheduled sampling days must have data. The data for each year meets this requirement. Each year has data for over 340 days. Methods used to obtain these values are in accordance with those given in Appendix T - Interpretation of the Primary National Ambient Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide) to 40 CFR Part 50 National Primary and Secondary Ambient Air Quality Standards.

PM_2.5 Background Concentration

PM_2.5 modeling will use background concentration data from the EPA for both the 24-hour and annual standards. Data from Potter County (Monitor Number: 48-375-0320) was used in obtaining background concentration data.  For the 24-hour standard, the 98^th percentile values are 15.9, 13.1 and 11.6 μg/m³ for the years 2008, 2009 and 2010, respectively. The 24-hour design value for PM_2.5 (the average of the three 98^th percentile values) is 14 μg/m³.

For the annual standard, the annual averages (calculated as the mean of the quarterly averages) are 6.7, 6.0 and 5.9 μg/m³ for the years 2008, 2009 and 2010, respectively. The annual design value (the mean of the three annual averages) is 6.2 μg/m³.  Quarterly averages were calculated by averaging the data from January 1^st through March 31^st for Quarter 1, April 1^st through June 30^th for Quarter 2, July 1^st through September 30^th for Quarter 3 and October 1^st through December 31^st for Quarter 4. 

The EPA’s data completeness rule requires that, for each quarter, 75% of the scheduled sampling days must have data. The data for each year meets this requirement. Each year has data for over 350 days. Methods used to obtain these values are in accordance with those given in Appendix N - Interpretation of the National Ambient Air Quality Standards for PM2.5 to 40 CFR Part 50 National Primary and Secondary Ambient Air Quality Standards.

PM₁₀ Background Concentration

PM₁₀ modeling will use background concentration data from the EPA for the 24-hour standard. Historical data from Tarrant County (Monitor Number: 48-439-3010), was used in obtaining background concentration data. Using the high, second high method for the 24-hour standard, the second highest monitored concentration in 2010 is 44 μg/m³.

The EPA’s data completeness rule requires that, for each quarter, 75% of the scheduled sampling days must have data. The guidelines in 40 CFR 58.12 – Operating Schedules require a PM₁₀ sampling frequency of once every six days.  The data for each year meets this requirement. For a full discussion of PM₁₀ data requirements, see Appendix K to Part 50—Interpretation of the National Ambient Air Quality Standards for Particulate Matter to 40 CFR Part 50 National Primary and Secondary Ambient Air Quality Standards.

Biography

Karen M. Bullard, P.E. is an Engineering Partner and the President of Bullard Environmental Consulting, Inc. She has over 19 years experience in environmental engineering, compliance, and permitting. She worked for the Texas Commission on Environmental Quality (TCEQ) for four years as an Air Permit Specialist in the Coatings and Combustion Section, where she developed a thorough understanding of the governmental procedures and policies in Texas. Karen has a Bachelors of Science Degree in Chemical Engineering from the University of Texas at Austin.          

Karen M. Bullard, P.E. No. 88449

Article # 0061 

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