Appendix F

Background Concentration Determination for Non-Attainment Area

January 23, 2015

U.S.EPA issued the “Guidance for 1-Hour SO2 Nonattainment Area SIP Submissions”

on April 23, 2014. In Appendix A of the guidance, several approaches for determining 

the SO2 background concentrations are suggested. In this appendix, Ohio EPA, working 

with the Kentucky DAQ, has applied the methods suggested in U.S. EPA’s guidance to 

identify the potential SO2 background concentration in the Campbell County KY-OH,

nonattainment area for the 1-hour SO2 National Ambient Air Quality Standard (NAAQS).

According to the guidance, a “first tier” approach uses a uniform monitored background 

contribution based on the overall highest hourly background SO2 concentration from a 

representative monitor. However, this approach is identified as “... conservative in many 

cases and may also be prone to reflecting source-oriented impacts, increasing the 

potential for double-counting of monitored contribution.” Therefore, U.S.EPA 

recommends a less conservation “first tier” method based on the monitored design 

values for the latest 3-year period. From the design value report (See Appendix A-2 of 

the Redesignation Request and Maintenance Plan) at Monitor 21-037-3002 for 2012-

2014, the 3-year design value is 72 ppb, which is slightly less than the 2010 1-hour SO2

NAAQS. Thus, both of the “first tier” methods are overly conservative and are not 

appropriate for determining the background concentration of this area.

As discussed in U.S. EPA’s guidance, Section 8.2.2.b of Appendix W provides another 

option:

“Use air quality data in the vicinity of the source to determine the background 

concentration for the averaging times of concern. Determine the mean 

background concentration at each monitor by excluding concentrations when the 

source in question is impacting the monitor...For shorter time periods, the 

meteorological conditions accompanying concentrations for concern should be 

identified. Concentrations for meteorological conditions of concern, at monitors, 

not impacted by the source in question, should be averaged for separate 

averaging time to determine the average background value. Monitoring sites 

inside a 90 degree sector downwind of the source may be used to determine the 

area of impact.”

The Kentucky monitor 21-037-3002 is the only monitor located in the vicinity of our 

study area suitable for this analysis. Two major SO2 emission sources, Duke Energy’s 

W.C. Beckjord (Beckjord) and W. H. Zimmer (Zimmer) facilities were considered as a 

part of this analysis and concentrations were excluded when these sources in question 

were impacting the monitor.

Google Earth tools were used to determine an arc of 90 degrees when these two

sources in question are impacting the monitor, and the hourly meteorological data 

(WBAN: 93814) was downloaded via ftp://ftp.ncdc.noaa.gov/pub/data/noaa/isd-lite. As 

can be seen in the Figure 1 below, these two sources would impact the monitor when 
the wind blows from 74.81° to 164.81°. Therefore, the hourly monitoring data with the 

recorded wind directions within the range were eliminated. 

Figure 1

The average SO2 concentration was then calculated for each year in the 2010-2014 

dataset which was retrieved from U.S. EPA’s Air Quality System (AQS). Data for 2014 

was only available through August 31, 2014 at the time of this analysis. In addition, to 

be more conservative, the annual average SO2 concentrations for non-zero values 

were calculated. The results show the highest value, 4.40 ppb, is the background 

concentration for this area using this approach (see Table 1).

Table 1

Year Background Concentration (all data) 

(unit: ppb)

Background Concentration (nonzero) (unit: ppb)

2010 3.82 4.36

2011 2.12 4.10

2012 1.16 3.91

2013 1.01 4.40

201411.86 2.89

 

1

Through August 31, 2014
Because it has been determined that Beckjord is the only source impacting this monitor, 

and given their proximity to the monitor and the predominant wind directions potentially 

impacting the monitor, the same analysis was also conducted and concentrations were 

excluded only when Beckjord was impacting the monitor. Again, Google Earth tools 

were used to determine an arc of wind directions to exclude from the background 

determination. In this instance, a reduced arc of 45 degrees was used instead of 90 

degrees. It was determined that an exclusion arc of 90 degrees relative to the monitor 

would potentially eliminate any potential contributions of Zimmer emissions to monitor 

values. The reduced arc allows for the exclusion of Beckjord emission impacts, the 

inclusion of Zimmer impacts, and provides an additional measure of conservatism to the 

background determination. The hourly meteorological data (WBAN: 93814) was

downloaded via ftp://ftp.ncdc.noaa.gov/pub/data/noaa/isd-lite. As can be seen in the 

Figure 2 below, Beckjord would impact the monitor when the wind blows from 81° to 

126°. Therefore, the hourly monitoring data with the recorded wind directions within the 

range were eliminated. 

Figure 2

The average SO2 concentration was then calculated for each year in the 2010-2014 

dataset. Again, to be more conservative, the annual average SO2 concentrations for 

non-zero values were calculated. The results show the highest value, 4.76 ppb, is the 

background concentration for this area using this approach (see Table 2).
Table 2

Year Background Concentration (all data) 

(unit: ppb)

Background Concentration (nonzero) (unit: ppb)

2010 3.82 4.49

2011 1.94 4.28

2012 1.03 4.29

2013 0.8 4.76

201421.66 2.91

In summary, the background concentration for this area can be conservatively 

estimated at 4.76 ppb without double-counting emissions from Beckjord based on the 

above analysis.

To preserve conservatism in the background and to demonstrate further that the 

shutdown of the Walter C. Beckjord facilities provides for attainment, Ohio EPA 

conducted an analysis of hourly SO2 concentrations recorded at monitor 21-037-3002, 

wind direction data from the Cincinnati weather station located at the Cincinnati 

Northern Kentucky Airport, and emissions from both the Walter C. Beckjord and William 

H. Zimmer facilities for years 2012 through February 28, 2015. The full analysis is 

presented in detail in Appendix K of the redesignation request. Briefly, Ohio EPA 

compiled 2,939 non-zero monitor values for which SO2 emissions from the Walter C. 

Beckjord facility were zero. The 99th percentile value of these hourly data is the 29th

highest hourly value, or 11 ppb. Note that 11 ppb represents the 99th percentile of all 

hours, not of maximum daily values and is therefore more conservative than an actual 

design value for this period. Additionally, by not attempting to account for plume travel 

time from the Walter C. Beckjord facility to the monitor, it is likely that some hours in this 

dataset represent impacts of emissions from the Walter C. Beckjord facility, and 

therefore carry an additional measure of conservatism. As noted in Appendix K of the 

redesignation request, no exceedances of the standard were recorded at any hour for 

which emissions from Walter C. Beckjord were zero. 

 

2

Through August 31, 2014