Take a Test    Article Library    CEEJ Home    Submit an Article     Contact CEEJ


Article # 0061

Procedures for Processing Air Quality Monitoring Data to Obtain Background Concentrations for Use in NAAQS Analyses

 

This document provides an example of how to obtain NO2 1-hour, SO2 1-hour, PM10 24-hour, PM2.5 24-hour and PM2.5 annual backgrounds concentrations in Shackelford County, Texas, according to procedures accepted by the Texas Commission on Environmental Quality (TCEQ).  This data is used during an air dispersion modeling analysis to show that air emissions from a process are in compliance with the National Ambient Air Quality Standards (NAAQS). For the analysis of compliance with these particular standards and averaging periods, the TCEQ does not have set background concentrations to use in each county and background concentrations must be developed from monitoring data.  In this example, data from 2008, 2009 and 2010 was obtained from the EPA website at http://www.epa.gov/airexplorer/index.htm.  As instructed by TCEQ modeling staff, the following procedure was followed:

 

Description of Location and Monitors Used

 

The example facility is located in Shackelford County, a rural county with a population of approximately 3300. To estimate the PM10, PM2.5, NO2 hourly and SO2 hourly background concentrations, an attempt was made to find monitors for these pollutants in nearby areas with a similar population. There are very few SO2 and NO2 monitors in the state, especially in less populous areas.

 


 

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 PM2.5 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 PM2.5. 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.

 

 

2008 Emissions

(tons)

2008 Emission Density

(tons/sq. mi.)

County

County Area

(sq. mi.)

SO2

NOx

PM2.5

PM10

SO2

NOx

PM2.5

PM10

Shackelford

916

9

1410

206

1573

0.01

1.54

0.22

1.72

McLennan

1060

1120

11503

3040

21601

1.06

10.85

2.87

20.38

Potter

922

19266

17382

1120

7407

20.90

18.85

1.21

8.03

Tarrant

897

1373

56409

5190

33470

1.53

62.89

5.79

37.31

 

Method of Obtaining Percentile Values

 

The tables below give guidelines for finding both 98th and 99th percentile values. To find percentile values, the data are arranged from largest to smallest. As an example, the 99th percentile value of a data set with 250 data points would be the 3rd value in the sorted list. Similarly, the 98th percentile value of a data set with 360 data points would be the 8th 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 98th 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 98th 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 99th 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 99th 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.

 

PM2.5 Background Concentration

 

PM2.5 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 98th 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 PM2.5 (the average of the three 98th 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 1st through March 31st for Quarter 1, April 1st through June 30th for Quarter 2, July 1st through September 30th for Quarter 3 and October 1st through December 31st 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 


 

Take a Test    Article Library    CEEJ Home    Submit an Article     Contact CEEJ