The Atlanta Eight-Hour Ozone Attainment Demonstration 

Appendix H 

Sensitivity of Ozone in Macon to 

Reductions in NOx Emissions from 

Various Sources in Georgia 
Sensitivity of ozone concentrations in Macon to reductions in NOx

emissions from various sources in Georgia 

Introduction 

GA-EPD has performed an analysis of the sensitivity of ozone concentrations to 

reductions in emissions of both nitrogen oxides (NOx) and volatile organic compounds 

(VOCs). The analysis was based off of the Visibility Improvement State and Tribal 

Association of the Southeast (VISTAS) modeling, using the “Base D” emissions 

inventory (for both 2002 and 2009), and CMAQ version 4.4 with the CB-IV chemical 

mechanism. A 12-km modeling grid was used, covering most of the southeastern USA. 

The episode modeled was May 23 – June 25, 2002, as it contained many days of elevated 

ozone concentrations and has been found representative of longer term trends in ozone 

concentrations. More details regarding the model setup and sensitivity analysis can be 

found in Marmur et al., 2005. 

Cases modeled 

Results from the sensitivity analysis indicated that the region modeled is NOx limited. 

Therefore, the focus here is on the effect of NOx reductions from various sources/regions 

within GA on ozone levels in Macon. Two main NOx emissions reductions 

scenarios/cases have been modeled, and their effect on ozone concentrations in Macon 

analyzed. Below is a summary of the cases modeled (also see Table 1): 

Case 1: NOx reductions from the existing SCRs at GA-Power Plants Bowen (all four 

units), Wansley (both units) and Hammond (unit #4), compared to the preinstallation conditions. These SCRs reduce total combined NOx emissions by 

184 tons per day (TPD). 

Case 2: Reductions in Atlanta NOx emissions from 2002 to 2006, totaling 96.4 TPD 

(GA-DNR, 2006) . 
Table 1: Summary of emissions reduction scenarios modeled 

Case # Plant/Region modeled NOx TPD reduction 

1 Bowen, Hammond and Wansley (benefits of existing 

SCRs) 

184 

2 Reduction in Atlanta NOx emissions from 2002 to 2006 96.4 

Modeling results 

Reductions in daily maximum 8-hour ozone concentrations for each of the above cases, 

along with the modeled base-case concentrations (based on both 2002 and 2009 VISTAS 

Base D emissions), are shown in Figure 1. Case 2 (modeled benefits of lower NOx

emissions from Atlanta) shows the highest benefits in terms of ppb reduction in 8-hour 

ozone concentrations in Macon, with reductions of up to 4.5 ppb. Case 1 (modeled 

benefits pf existing SCRs at Bowen, Wansley and Hammond) shows reductions of up to 

3.5 ppb. Both these reductions are the highest on days of high ozone concentrations, 

which demonstrates the contribution of Atlanta and the modeled power plants to elevated 

ozone concentrations (and to the design-value) in Macon. The average and normalized 

reductions in ozone, for days in which the 2002 base case concentration was above 75 

ppb (8 days total), are given in Table 2. Also evident in Figure 1 are the impacts of NOx

emissions reductions expected by 2009, compared to the 2002 case (most of the 

reductions are from the mobile-source and EGU sectors; for more details see Marmur et 

al., 2005). This further demonstrates that ongoing reductions in NOx emissions will 

continue to lower concentrations of ozone in Macon. 
-0.50

0.50

1.50

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5/23

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Reduction in daily maximum 8-hr O3 (Case 1,2; ppb)

0.0

20.0

40.0

60.0

80.0

100.0

120.0

Daily maximum 8-hr O3 (2002,2009; ppb)

Existing SCRs (Case1) Atlanta NOx reductions (Case 2)

max 8-hr O3 (2002 emissions) max 8-hr O3 (2009 emissions)

Figure 1: Daily reductions in maximum 8-hour ozone concentrations corresponding to 

cases 1,2 (Table 1), along with base-case ozone concentrations in Macon (2002 and 2009 

inventories). Green rectangles indicate days in which the 2002 base case maximum daily 

8-hr ozone concentration was above 75 ppb (used for calculations presented in Table 2). 

Table 2: Average reductions in 8-hour ozone concentrations (ppb) and normalized 

reductions (ppt O3 / TPD NOx) for days in which the 2002 base case concentration was 

above 75 ppb (8 days total) 

Case # Average reduction (ppb) Normalized reduction (ppt O3 / TPD NOx) 

1 1.06 5.73 

2 2.33 24.2 
Meteorological patterns 

To assess whether the observed reduction in ozone concentrations in Macon is due to 

favorable meteorology, rather than emissions reductions, we analyzed the number of 

“ozone conducive days” during the ozone seasons of 1998-2004. An “ozone conducive 

day” is defined here as a day of “high” temperature (T) and “low” relative-humidity (RH) 

and wind-speed (WS), since such meteorological conditions are favorable to the 

formation of ozone. We defined “high” and “low” in a relative manner, and report the 

count of days based on two sets of definitions (Figure 2): 1) T above the 90th percentile 

(for 1998-2004), and RH and WS below the 50th percentile; 2) T above the 75th percentile, 

and RH and WS below the 50th percentile. These data clearly show (Figure 2) that ozone 

concentrations have decreased regardless (and in some cases despite) of the frequency of 

occurrence of ozone-conducive meteorological conditions. 

0

5

10

15

20

25

1998 1999 2000 2001 2002 2003 2004

Number of "ozone conducive days"

70

80

90

100

110

120

4th highest 8-hr O3 value (ppb)

T>90th%ile, RH/WS<50th%ile T>75th%ile, RH/WS<50th%ile O3 FHV

Figure 2: Count of “ozone conducive days” based classifications of temperature, relativehumidity and wind-speed, along with trends in observed ozone concentrations in Macon 

(4th highest 8-hr ozone value (FHV) per ozone season) 
Summary and conclusions 

This analysis has shown that reductions in NOx emissions from the Atlanta area, along 

with reduced EGU NOx from existing SCRs in several power-plants throughout northern 

Georgia have lead to reduced ozone concentrations in Macon, regardless/despite of the 

frequency of occurrence of ozone conducive metrological conditions. Since further 

reductions in Atlanta emissions are expected (fleet turnover, as indicated by data from the 

emissions inventory), ozone concentrations in Macon are expected to decrease further. 

References 

Georgia Department of Natural Resources (2006), Georgia’s Draft Early Progress State 

Implementation Plan Revision for the Atlanta 8-Hour Ozone Non-attainment Area, 

Environmental Protection Division, Air Protection Branch, October 26, 2006 

(http://www.gaepd.org/Files_PDF/plans/sip/draft_early_progress_sip_narrative4_clean.pdf). 

 Marmur, A., Boylan, J., Khan, M., and Cohan, D. (2005), 8-Hour Ozone and PM2.5

Modeling to Support the Georgia SIP, 4th Annual CMAS Models-3 Users' Conference, 

Chapel Hill, NC (http://www.cmascenter.org/conference/2005/ppt/7_6.pdf)