APPENDIX C - SENSITIVITY OF OZONE IN ATLANTA TO NOx AND VOC EMISSIONS 

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 and the detailed modeling results 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 billion 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 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 and VOCs at each Atlanta ozone monitor 

location (Table 1). The average absolute NOx sensitivity across Atlanta is 6.396 ppb for a 30% reduction 

in NOx emissions across Georgia and the average absolute VOC sensitivity across Atlanta is 0.293 ppb 

for a 30% reduction in VOC emissions across Georgia. 

Next, the average absolute sensitivity at each monitor 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. The 

normalized sensitivity was calculated for NOx and VOCs at each Atlanta ozone monitor location 

(Table 2). The average normalized NOx sensitivity across Atlanta is 0.0757 ppb/TPD and the average 

normalized VOC sensitivity across Atlanta is 0.0035 ppb/TPD.

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% VOC w/ 75 

ppb threshold 

(ppb)

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

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

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

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

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

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

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

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

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

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

AVERAGE (ppb) -6.396 -0.293

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

AIRS ID County Site Name

30% NOx w/ 75 

ppb threshold 

(ppb/TPD)

30% VOC w/ 75 

ppb threshold 

(ppb/TPD)

13-067-0003 Cobb, GA Kennesaw -0.0743 -0.0045

13-077-0002 Coweta, GA Newnan -0.0806 -0.0018

13-085-0001 Dawson, GA Dawsonville -0.0622 -0.0007

13-089-0002 DeKalb, GA South DeKalb -0.0771 -0.0058

13-097-0004 Douglas, GA Douglasville -0.0797 -0.0042

13-121-0055 Fulton, GA Confederate Ave. -0.0612 -0.0077

13-135-0002 Gwinnett, GA Gwinnett -0.0763 -0.0026

13-151-0002 Henry, GA McDonough -0.0869 -0.0034

13-223-0003 Paulding, GA Dallas /Yorkville -0.0693 -0.0011

13-247-0001 Rockdale, GA Conyers -0.0898 -0.0031

AVERAGE (ppb/TPD) -0.0757 -0.0035

These results show that NOx emission reductions are generally 15-25 times more effective than VOC 

emission reductions at reducing ozone concentrations. VOC emission increases can be converted into 

equivalent NOx emission reductions by taking the ratio of the Atlanta average normalized sensitivity to 

NOx emissions divided by the Atlanta average normalized sensitivity to VOC emissions:

• (0.0757 ppb/TPD NOx)/( 0.0035 ppb/TPD VOC) = 21.7 TPD VOC/TPD NOx

In other words, a 21.7 TPD increase in VOC emissions is equivalent to a 1.0 TPD increase in NOx 

emissions. Hence, a 21.7 TPD increase in VOC emissions can be offset with a 1.0 TPD reduction in NOx 

emissions.