VOC Insensitivity To Ozone Production In North Carolina

Introduction

North Carolina has made significant strides over the past two decades in reducing the ambient levels of 

ground-level ozone in response to ever strengthening National Ambient Air Quality Standards (NAAQS). 

North Carolina was able to fully demonstrate attainment and continued maintenance of the previous onehour average ozone standard resulting in that standard’s revocation in 2005. North Carolina has also been 

very successful in demonstrating attainment of the 1997 eight-hour average ozone standard at all but one 

of the ozone monitoring sites in the state. Even with the recent 2008 eight-hour average ozone standard, 

the majority of North Carolina is currently attaining this standard with exceptions only in the three largest 

metropolitan regions. Figure 1 demonstrates the trend in eight-hour average ozone exceedances since 

1997 statewide and at the aforementioned three largest metropolitan regions.

Figure 1 – Annual Trend In Eight-Hour Average Ozone Exceedances

Nitrogen Oxide (NOx) emissions reductions have been the primary strategy for accomplishing the 

significant reductions in ozone concentrations in North Carolina over the span discussed above. A study 

conducted by Sonoma Technology, Inc. for North Carolina in 1998 determined that there was no clear 

signal of Volatile Organic Compound (VOC) limitation anywhere in the state. So, reductions in 

manmade VOCs would not lead to any significant reductions in afternoon ozone concentrations. Rather, 

it was determined that the majority of North Carolina was transitionally NOx limited and that reductions 

in NOx emissions would have the greatest benefit on improving ozone concentrations. This report found 

that only during the early morning hours would decreases in NOx emissions have an ozone concentration 

disbenefit due to the reduction in NOx titration of ozone. However, ozone concentrations are typically at 

their lowest levels during the early morning and this potential disbenefit would not have any impact on 

North Carolina’s ability to demonstrate attainment of the ozone NAAQS.
A more recent study performed by Duncan et al. in 2009 investigated satellite based NOx, VOC, and 

ozone measurements across the entire United States. This study, titled “The Sensitivity Of U.S. Surface

Ozone Formation To NOx And VOCs As Viewed From Space,” found that the majority of the 

Southeastern United States was vastly NOx limited. In North Carolina, the entire state was shown to be 

NOx limited for ozone production with the rural areas extremely NOx limited. This study further 

confirms the previous Sonoma Technology, Inc. report and North Carolina’s continuing ozone reduction 

strategy of NOx emission reductions.

VOC Sensitivities

In the mid-2000’s a series of summertime and wintertime VOC sensitivity evaluations were performed 

through the Visibility Improvement State and Tribal Association of the Southeast (VISTAS) and the 

Association for Southeastern Integrated Planning (ASIP) regional modeling initiatives. These VOC 

sensitivities along with numerous other chemical species sensitivities helped the VISTAS and ASIP 

organizations to make appropriate recommendations for regional haze, fine particulate matter, and ozone 

concentration improvements throughout the Southeast.

In these VOC sensitivities, model simulations were performed to understand the ozone response to natural 

and manmade VOC emissions. Biogenic and anthropogenic VOC emissions were reduced by thirty 

percent, respectively. Figure 2 displays the VOC emissions for the Region 4 states and North Carolina. 

A thirty percent reduction in the biogenic emissions would result in approximately 23,100 tons/day and 

2,700 tons/day reductions in VOC emissions for the Region 4 states and North Carolina, respectively. 

This reduction is greater than twice the total anthropogenic emissions.

Point

1%

Area

5%

Nonroad Mobile

2%

Highway Mobile

3%

Biogenic

89%

Region 4 States VOC Emissions (tons/summer day)

Biogenic Emissions = 77,010 tons/day Anthropogenic Emissions = 9,554 tons/day Total Emissions = 86,564 tons/day

Point

2%

Area

4%

Nonroad Mobile

2%

Highway Mobile

4%

Biogenic

88%

North Carolina VOC Emissions (tons/summer day)

Biogenic Emissions = 9,143 tons/day Anthropogenic Emissions = 1,219 tons/day Total Emissions = 10,362 tons/day

Figure 2 – 2009 Summertime VOC emissions (tons/day) for EPA Region 4 States (left) 

and North Carolina (right).

In the Southeast, neither the thirty percent reduction in biogenic VOC emissions nor the thirty percent 

reduction in the anthropogenic VOC emissions resulted in any significant reductions in ozone 

concentrations. The biogenic VOC emission reductions demonstrated the most change in ozone 

concentrations, but the changes in ozone concentrations were still only in the range of 1 to 2 parts per 

billion (ppb). This can be attributed to the overwhelming contribution of biogenic VOC emissions to the 
total amount of VOC emission in the atmosphere. A sampling of these VOC sensitivities is provided in 

Figures 3 and 4. Of note, the highest ozone concentrations in North Carolina during the summertime 

VOC sensitivity occurred on June 11th.

Figure 3 – Ozone response from a 30% reduction in Biogenic VOC (left)

and Anthropogenic VOC (right) for June 11, 2002.

(Cool colors represent ozone benefits and warm colors represent ozone disbenefits.)

Figure 4 – Ozone response from a 30% reduction in Biogenic VOC (left)

and Anthropogenic VOC (right) for June 12, 2002.

(Cool colors represent ozone benefits and warm colors represent ozone disbenefits.)
A closer evaluation of the VOC sensitivity modeling data in North Carolina during both the summertime 

and wintertime simulations further demonstrates the relatively insignificant change in ozone 

concentrations due to a thirty percent reduction in biogenic and anthropogenic VOC emissions. Table 1 

clearly shows that the anthropogenic VOC emission reductions do not reduce ozone concentrations during 

the summertime and actually may cause a slight disbenefit or increase in ozone. Reducing anthropogenic 

VOC emissions during the wintertime does cause some improvement of ozone concentrations, but the 

change is less than 0.5ppb and would occur at a time that North Carolina is not experiencing any ozone 

conditions that threaten the NAAQS.

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

DO3 (ppb)

North Carolina

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

DO3 (ppb)

North Carolina

0

2000

4000

6000

8000

10000

12000

Anthropogenic or Biogenic

DVOC (Mg/day)

Emission Reductions

AVOC BVOC

Table 1 – Winter (left) and Summer (right) 30% VOC emission reduction contributions

to O3 change (in ppb) at various O3 non-attainment areas.

Conclusions

The result of ongoing ambient monitoring and satellite measurements of NOx, VOC, ozone, and other 

chemical species continue to indicate that the atmosphere over North Carolina is NOx limited with 

respect to afternoon ozone formation. This would indicate that NOx emissions reductions are the best 

strategy for reducing ozone concentrations. VOC sensitivity modeling also demonstrates that significant

reductions in biogenic or anthropogenic VOC emission do not amount to any significant improvement to 

ozone concentrations. There are even indications that anthropogenic VOC emission reductions could 

have a slightly disbenefit in ozone concentrations during the summertime. Additionally, two decades of 

NOx emission reduction strategies in North Carolina have yielded significant ozone concentrations

reductions, near statewide attainment of the ozone NAAQS, and validation that NOx emissions reduction 

rather than VOC emission reductions are the continued best policy in the state.