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 NO x 

### 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 (NO x ) 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 repr esentative of longer te rm 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 NO x limited. Therefore, the focus here is on the effect of NO x reductions from various sources/regions within GA on ozone levels in Macon. Two main NO x emissions reductions scenarios/cases have been modeled, and thei r effect on ozone concentrations in Macon analyzed. Below is a summary of the cases modeled (also see Table 1): Case 1: NO x reductions from the existing SCRs at GA-Power Plants Bowen (all four units), Wansley (both units) and Hamm ond (unit #4), compared to the preinstallation conditions. These SCRs reduce total combined NO x emissions by 184 tons per day (TPD). Case 2: Reductions in Atlanta NO x emissions from 2002 to 2006, totaling 96.4 TPD (GA-DNR, 2006) .Table 1 : Summary of emissions reduction scenarios modeled
Case # Plant/Region modeled NO x TPD reduction
1 Bowen, Hammond and Wansley (benefits of existing SCRs) 184

2 Reduction in Atlanta NO x 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 concentr ations (based on both 2002 and 2009 VISTAS Base D emissions), are shown in Figure 1. Case 2 (modeled benefits of lower NO x emissions from Atlanta) shows the highest be nefits in terms of ppb reduction in 8-hour ozone concentrations in Macon, with reduc tions of up to 4.5 ppb. Case 1 (modeled benefits pf existing SCRs at Bowen, Wansle y 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 Atlant a 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 concen tration was above 75 ppb (8 days total), are given in Table 2. Also evident in Figure 1 are the impacts of NO x 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 NO x emissions will continue to lower concentrations of ozone in Macon.-0.50 0.50 1.50 2.50 3.50 4.50 5.50

5 / 2 3 5/25 5/27 5 / 2 9 5/31 6 / 2 6/4 6 /6 6/8 6/10 6 / 1 2 6/14 6 /16 6 / 1 8 6/20 6 / 2 2 6/24Reduction in daily maximum 8-hr O3 (Case 1,2; ppb)

0.0 20.0 40.0 60.0 80.0 100.0 120.0Daily 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 oz one concentrations corresponding to cases 1,2 (Table 1), along with base-case oz one concentrations in Macon (2002 and 2009 inventories). Green rectangles indicate days in which th e 2002 base case maximum daily 8-hr ozone concentration was above 75 ppb (use d for calculations presented in Table 2).

Table 2 : Average reductions in 8-hour ozone concentrations (ppb) and normalized reductions (ppt O3 / TPD NO x ) for days in which the 2002 base case concentration was above 75 ppb (8 days total)
Case # Average reduction (ppb) Norm alized reduction (ppt O3 / TPD NO x )
1 1.06 5.73
2 2.33 24.2Meteorological 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” te mperature (T) and “low” relative-humidity (RH) and wind-speed (WS), since such meteorol ogical 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 90 th percentile (for 1998-2004), and RH and WS below the 50 th percentile; 2) T above the 75 th percentile, and RH and WS below the 50 th 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 2004Number of "ozone conducive days"

70 80 90 100 110 1204th 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 cl assifications of temperature, relativehumidity and wind-speed, along with trends in observed ozone concentrations in Macon (4 th highest 8-hr ozone valu e (FHV) per ozone season)Summary and conclusions This analysis has shown that reductions in NO x emissions from the Atlanta area, along with reduced EGU NO x from existing SCRs in several power-plants throughout northern Georgia have lead to reduced ozone concentr ations in Macon, regardless/despite of the frequency of occurrence of ozone conduciv e metrological conditions. Since further reductions in Atlanta emissions are expected (f leet 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 Atla nta 8-Hour Ozone Non-attainment Area, Environmental Protectio n Division, Air Protecti on Branch, October 26, 2006 ( http:// .gaepd.org/Files_PDF/plans/sip/draft_early_progress_sip_narrative4_clean.pdf www ).

Marmur, A., Boylan, J., Khan, M., and Cohan, D. (2005), 8-Hour Ozone and PM 2.5 Modeling to Support the Georgia SIP, 4th A nnual CMAS Models-3 Users' Conference, Chapel Hill, NC ( http:// .cmascenter.org/conference/2005/ppt/7_6.pdf www )