## VOC Insensitivity To Ozone Production In North Carolina

Introduction
North Carolina has made significant strides over the past two decade s in reducing the ambient levels of ground

- level ozone in response to ever strengthening National Ambient Air Quality S tandards (NAAQS).
North Carolina was able to fully de monstrate attainment and continued maintenance of the previous one

hour average ozone standard resulting in that standard’s revocation in 2005. North Carolina has also been very successful in demonstra ting 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

– A nnual 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

con ducted 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 afte rnoon 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 d uring 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 pote ntial 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 Unit ed States. This study , titled “The S ensitivity Of U.S. S urface
O zone F ormation To NOx And VOCs As V iewed F rom S pace,” found that the majority of the Southeastern United States was vastly NOx limited. In North Carolina, the entire state was shown to be
NO x 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 S ensitivities

In th e 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 improv ements 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. Fi gure 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 Nor th Carolina, respectively. This reduction is greater than twice the total anthropogenic emissions.

Figure 2

– 2009 Summertime VOC emissions (tons/day) for EPA Region 4 States (left) and North Carolina (right).

In the Southeast, neither the thirty per cent 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

concent rations, 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 thetotal amount of VOC emission in the atmosphere. A sa mpling 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 11 th

.

Figure 3

– Ozone response from a 30% r eduction in Biogenic VOC (l eft)

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% reduc tion in Biogenic VOC (left)

and Anthropogenic VOC (right) for June 12, 20 02. (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 in significant 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 act ually 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 n ot experiencing any ozone conditions that threaten the NAAQS.

Table 1

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

to O 3 change (in ppb) at various O 3 non - attainment areas.



## Conclusions
The result of ongoing ambient monito ring 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 indica tions 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 cont inued best policy in the state.