12
L. R. F. Henneman et al.
2.3 Results
Comparisons between brute-force OPEs (which do not account for differences in
O 3 and NO Z deposition rates) calculated using differences between three scenarios
(2011 B ASE − 2011 50%N O X and 2011 50%N O X − 2011 10%N O X ) and O P E
N O DE P yield
a slope near one, establishing credibility in the sensitivities approach for estimating
OPE.
Nationwide OPE corrected for differing deposition rates is slightly higher than
O P E
N O DE P (Table 2.1), but the difference decreases as NO X emissions decrease.
Spatially, OPE is elevated in urban areas and near highways, and decreases with
increasing latitude (Fig. 2.1). OPE increase with decreasing NO X emissions; however,
the rate of increase in OPE decreases as NO X emissions approach zero.
Average domain-wide mobile (EGU) OPEs are slightly lower (higher) than total
OPEs in 2001 and 2011 (Table 2.1). Spatially, O P E M O B generally mirrors the distribution of total OPE, whereas O P E EGU is elevated across the domain except for
areas near electricity generating units.
At SEARCH sites, empirically-derived OPE behaves similarly to CMAQsimulated OPE, i.e., OPEs increase with decreasing NO Z concentration (Fig. 2.2).
The empirical estimates find slightly lower OPEs than CMAQ, with a more pronounced difference in rural areas at high NO Z concentrations. Both the CMAQestimated and empirical OPEs, however, reach location-specific maxima as NO Z
concentrations approach zero.
2.4 Discussion
Increasing OPEs with decreasing NO X emissions suggests that further emissions
reductions will be increasingly effective at reducing the highest levels of O 3 . As
OPE approaches site-specific maxima, however, the increasing benefits of decreasing
NO X emissions wanes. These site-specific maxima reflect site-specific meteorological conditions and differing availability of other atmospheric species. That O P E EGU
is greater than O P E M O B in most areas suggests that further NO X emissions reductions from EGUs will be more efficient at reducing O 3 than mobile NO X emissions
reductions on a per mass emitted basis. These effects, however, vary by location.
In future work, there is potential to investigate OPE on days that have low photochemical state. Current policies have been effective at reducing the highest O 3
concentrations, but wintertime and night time O 3 concentrations have increased as
near-source NO X inhibition of O 3 formation has decreased with deceasing emissions.
Questions and Answers
QUESTIONER: Randall Martin
QUESTION: It is interesting that the OPE for EGUs exceeded the OPE for more
diffuse traffic sources. Could this reflect stack heights, or numerical diffusion of EGU
emissions within the grid box, or another explanation?
L. R. F. Henneman et al.
2.3 Results
Comparisons between brute-force OPEs (which do not account for differences in
O 3 and NO Z deposition rates) calculated using differences between three scenarios
(2011 B ASE − 2011 50%N O X and 2011 50%N O X − 2011 10%N O X ) and O P E
N O DE P yield
a slope near one, establishing credibility in the sensitivities approach for estimating
OPE.
Nationwide OPE corrected for differing deposition rates is slightly higher than
O P E
N O DE P (Table 2.1), but the difference decreases as NO X emissions decrease.
Spatially, OPE is elevated in urban areas and near highways, and decreases with
increasing latitude (Fig. 2.1). OPE increase with decreasing NO X emissions; however,
the rate of increase in OPE decreases as NO X emissions approach zero.
Average domain-wide mobile (EGU) OPEs are slightly lower (higher) than total
OPEs in 2001 and 2011 (Table 2.1). Spatially, O P E M O B generally mirrors the distribution of total OPE, whereas O P E EGU is elevated across the domain except for
areas near electricity generating units.
At SEARCH sites, empirically-derived OPE behaves similarly to CMAQsimulated OPE, i.e., OPEs increase with decreasing NO Z concentration (Fig. 2.2).
The empirical estimates find slightly lower OPEs than CMAQ, with a more pronounced difference in rural areas at high NO Z concentrations. Both the CMAQestimated and empirical OPEs, however, reach location-specific maxima as NO Z
concentrations approach zero.
2.4 Discussion
Increasing OPEs with decreasing NO X emissions suggests that further emissions
reductions will be increasingly effective at reducing the highest levels of O 3 . As
OPE approaches site-specific maxima, however, the increasing benefits of decreasing
NO X emissions wanes. These site-specific maxima reflect site-specific meteorological conditions and differing availability of other atmospheric species. That O P E EGU
is greater than O P E M O B in most areas suggests that further NO X emissions reductions from EGUs will be more efficient at reducing O 3 than mobile NO X emissions
reductions on a per mass emitted basis. These effects, however, vary by location.
In future work, there is potential to investigate OPE on days that have low photochemical state. Current policies have been effective at reducing the highest O 3
concentrations, but wintertime and night time O 3 concentrations have increased as
near-source NO X inhibition of O 3 formation has decreased with deceasing emissions.
Questions and Answers
QUESTIONER: Randall Martin
QUESTION: It is interesting that the OPE for EGUs exceeded the OPE for more
diffuse traffic sources. Could this reflect stack heights, or numerical diffusion of EGU
emissions within the grid box, or another explanation?