## APPEN DI X C

- SENSITIVITY OF OZONE IN ATLANTA TO NOx AND VOC EMISSIONS

As part of the S outh E astern M odeling, A nalysis, and P lanning (SEMAP) p roject , Georgia Tech performed an analysis of the sensitivity of ozone concentrations in the Eastern U.S. t o 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

.

Sensitiviti es were mo deled 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 emission s (p oint, 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 sens itivities of NOx and VOC emission s 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

– S eptember 30. Not all modeled days were used in the calculation s

. 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 “ Gu idance on the Use of Models and O ther Analyses for Demonstrating Atta inment of Air Quality Goals for Ozone, PM2.5, and Regional Haze ”. For our analysis , t he following criteria were used to select the days that would be included in th e average sensitivity calculation t o address the 2008 ozone NAAQS :

• A n initial thres hold 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 wer e included in the calculation.

• If at least 10 modeled days were at or above the initial threshold, then an average sensitivity was

c alculated based on those days.

• If fewer than 10 days were available, the threshold was dropped until 10 days were available or the minimum allowable thres hold v alue 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 norm alize 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 nea rby 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 nonattai nment 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 equi val ent 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.