Ozone Sensitivity in Atlanta in Relation to the SEMAP Sensitivity Study

As part of the SouthEastern Modeling, Analysis, and Planning (SEMAP) project, Georgia Tech 

performed an analysis of the sensitivity of ozone concentrations in the Eastern U.S. to reductions in 

emissions of both nitrogen oxides (NOx) and volatile organic compounds (VOCs). This analysis was 

based off of the 2007 and 2018 SEMAP modeling which used CMAQ version 5.01 with updates to the 

vertical mixing coefficients and land-water interface. The entire "ozone season" was modeled (May 1 –

September 30) using a 12-km modeling grid that covered the Eastern U.S. Details of the modeling 

platform set-up can be found in Appendix E.

Sensitivities were modeled relative to 2018 emissions to evaluate the impact of NOx and VOC reductions 

on daily 8-hour maximum ozone concentrations. Each emission sensitivity run reduced the 2018 

anthropogenic NOx or VOC emissions (point, area, mobile, NONROAD, marine/aircraft/rail) within a 

specific geographic region by 30%. The 14 geographic regions included Alabama, Florida, Georgia, 

Kentucky, Mississippi, North Carolina, South Carolina, Tennessee, Virginia, West Virginia, Maryland, 

MANE-VU (minus MD), LADCO, and CENRAP. This resulted in a total of 28 model runs (2 precursors 

x 14 regions). The NOx and VOC sensitivities were evaluated at every ozone monitor in the domain.

GA EPD used the SEMAP NOx and VOC sensitivity modeling to examine the normalized sensitivities of

NOx and VOC emissions on 8-hour daily maximum ozone concentrations (part per trillion ozone/ton per 

day, ppt/TPD) at 10 ozone monitors in Atlanta. This analysis started with the day-by-day NOx and VOC 

emission sensitivities (ppb) for May 1 – September 30. Not all modeled days were used in the 

calculations. The criteria for selecting days to include in the calculation generally follows the approach 

used by EPA to select days to include in the relative response factor (RRF) calculation as described in 

EPA’s “Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of Air Quality 

Goals for Ozone, PM2.5, and Regional Haze”. For our analysis, the following criteria were used to select

the days that would be included in the average sensitivity calculation:

 To address the 1997 ozone NAAQS, an initial threshold value of 85 ppb was used. If the 2018 

modeled 8-hour daily maximum ozone concentration was at or above the threshold, then those 

days were included in the calculation. If at least 10 modeled days were at or above the initial 

threshold, then an average sensitivity was calculated based on those days. If fewer than 10 days 

were available, the threshold was dropped until 10 days were available or the minimum allowable 

threshold value of 80 ppb was reached. If there were fewer than four days available when the 

minimum allowable threshold value was reached, the minimum allowable threshold was lowered 

until at least four days were available to include in the average sensitivity calculation.

 To address the 2008 ozone NAAQS, an initial threshold value of 75 ppb was used. If the 2018 

modeled 8-hour daily maximum ozone concentration was at or above the threshold, then those 

days were included in the calculation. If at least 10 modeled days were at or above the initial 

threshold, then an average sensitivity was calculated based on those days. If fewer than 10 days 

were available, the threshold was dropped until 10 days were available or the minimum allowable 

threshold value of 70 ppb was reached. If there were fewer than four days available when the 

minimum allowable threshold value was reached, the minimum allowable threshold was lowered 

until at least four days were available to include in the average sensitivity calculation.

The average absolute sensitivity was calculated for NOx (85 ppb threshold), NOx (75 ppb threshold), 

VOCs (85 ppb threshold), and VOCs (75 ppb threshold) at each Atlanta ozone monitor location (Table 1). 

Next, the average absolute sensitivity was normalized by the emission reduction to give the normalized 

sensitivity (ppb/TPD). The SEMAP 30% emission reductions were statewide, but the ozone impacts at 
the Atlanta monitors will mostly results from the local NOx and VOC emission reductions in the nearby 

15 ozone nonattainment counties. Therefore, it was not appropriate to normalize the local NOx and VOC 

sensitivity results by the statewide emission reduction. Instead, a conservative approach would be to 

assume the ozone impacts at the 10 Atlanta monitors resulted solely from the local NOx and VOC 

emission reductions in the nearby 15 ozone nonattainment counties. Therefore, the average absolute 

sensitivity was normalized by the emission reductions from NOx and VOC reductions in the nearby 15 

ozone nonattainment counties. The anthropogenic NOx emissions in the 15 ozone nonattainment counties

are 281.5 TPD, so a 30% reduction is 84.5 TPD. The anthropogenic VOC emissions in the 15 ozone 

nonattainment counties are 280.0 TPD, so a 30% reduction is 84.0 TPD. Finally, the normalized 

sensitivities (ppb/TPD) were multiplied by 1000 to convert the units to ppt/TPD (Table 2).

Table 1. Absolute NOx and VOC sensitivity at 10 Atlanta ozone monitors.

AIRS ID County Site Name

30% NOx w/ 75 

ppb threshold 

(ppb)

30% NOx w/ 85 

ppb threshold 

(ppb)

30% VOC w/ 75 

ppb threshold 

(ppb)

30% VOC w/ 85 

ppb threshold 

(ppb)

13-067-0003 Cobb, GA Kennesaw -6.272 -6.733 -0.380 -0.407

13-077-0002 Coweta, GA Newnan -6.807 -8.087 -0.148 -0.131

13-085-0001 Dawson, GA Dawsonville -5.252 -5.252 -0.059 -0.059

13-089-0002 DeKalb, GA South DeKalb -6.515 -7.385 -0.487 -0.576

13-097-0004 Douglas, GA Douglasville -6.732 -7.073 -0.350 -0.607

13-121-0055 Fulton, GA Confederate Ave. -5.167 -5.653 -0.644 -0.888

13-135-0002 Gwinnett, GA Gwinnett -6.440 -7.608 -0.222 -0.246

13-151-0002 Henry, GA McDonough -7.341 -7.645 -0.282 -0.388

13-223-0003 Paulding, GA Dallas /Yorkville -5.849 -5.849 -0.096 -0.096

13-247-0001 Rockdale, GA Conyers -7.580 -8.461 -0.262 -0.270

Table 2. Normalized NOx and VOC sensitivity at 10 Atlanta ozone monitors.

AIRS ID County Site Name

NOx w/ 75 ppb 

threshold 

(ppt/TPD)

NOx w/ 85 ppb 

threshold 

(ppt/TPD)

VOC w/ 75 ppb 

threshold 

(ppt/TPD)

VOC w/ 85 ppb 

threshold 

(ppt/TPD)

13-067-0003 Cobb, GA Kennesaw -74.265 -79.719 -4.530 -4.851

13-077-0002 Coweta, GA Newnan -80.595 -95.757 -1.762 -1.563

13-085-0001 Dawson, GA Dawsonville -62.190 -62.190 -0.702 -0.702

13-089-0002 DeKalb, GA South DeKalb -77.139 -87.440 -5.796 -6.854

13-097-0004 Douglas, GA Douglasville -79.705 -83.744 -4.165 -7.225

13-121-0055 Fulton, GA Confederate Ave. -61.173 -66.934 -7.664 -10.577

13-135-0002 Gwinnett, GA Gwinnett -76.251 -90.082 -2.638 -2.926

13-151-0002 Henry, GA McDonough -86.918 -90.519 -3.354 -4.624

13-223-0003 Paulding, GA Dallas /Yorkville -69.252 -69.252 -1.145 -1.145

13-247-0001 Rockdale, GA Conyers -89.754 -100.186 -3.121 -3.210

These results show that NOx emission reductions are generally 10-20 times more effective than VOC 

emission reductions at reducing ozone concentrations. In order to look at the impact of removing NOx or 

VOC controls on the 1997 and 2008 ozone NAAQS, the most conservative approach would be to use the 

highest normalized NOx and VOC sensitivity value across the 10 Atlanta ozone monitors. The highest 

normalized NOx sensitivity is 100.2 ppt/TPD and the highest normalized VOC sensitivity is 10.58 

ppt/TPD.