Voglio sapere a riguardo di…

lotter on DSK11XQN23PROD with RULES1

33532

Federal Register / Vol. 86, No. 120 / Friday, June 25, 2021 / Rules and Regulations
effectiveness of controls. Through comparison of the contexts of these guidance documents, which recommended photochemical modeling, and that of section 182c2C, the commenter suggests that the 2017
Eastern Kern Ozone SIP should have included similar photochemical grid modeling to determine whether the substitute NOX emission reductions result in equivalent ozone reductions.
Response to Comment 2: In general, we agree with the commenters descriptions of the relative roles of VOC
and NOX in ozone formation and geographic differences in the ozone response to precursor control, depending on whether an area is NOXsaturated or NOX-limited. We also agree with the commenter that Congress was aware of these issues and provided for the EPA to address them under provisions of the CAA.
However, we disagree with the commenters characterization of the 2017 Eastern Kern Ozone SIP and the EPAs proposed approval. While the preamble of the EPAs proposed approval did not provide an analysis showing that NOX substitution would result in a reduction in ozone concentrations at least equivalent to the required VOC emissions reductions needed for RFP, the supporting documentation in the docket for the proposed approval, as further clarified in our response to comments herein, provides such analysis. As described below, we find that the analysis included with the modeling and control strategy in the 2017 Eastern Kern Ozone SIP adequately demonstrates that annual and cumulative NOX reductions in Eastern Kern will result in a reduction in ozone concentrations that is at least equivalent to the ozone reductions that would be achieved by VOC emission reductions alone. We therefore agree with the use of NOX
substitution in the RFP demonstration for Eastern Kern.
Under CAA section 182c2B, the RFP demonstration for a Serious ozone nonattainment area will demonstrate RFP based solely on the prescribed annual rate of VOC emission reductions.
Alternatively, under CAA section 182c2C, the demonstration may satisfy the RFP requirement based on a combination of VOC and NOX
reductions if it demonstrates that reductions of VOC and NOX would result in a reduction in ozone concentrations at least equivalent to that which would result from the amount of VOC emission reductions otherwise required. For Eastern Kern, the RFP
demonstration for milestone years 2017
and 2020 both rely on a combination of
VerDate Sep<11>2014

16:27 Jun 24, 2021

Jkt 253001

VOC reductions and NOX reductions from the RFP baseline year of 2011.
The revised RFP demonstration in the 2018 SIP Update, as corrected in the 2020 Conformity Budget Update, shows the extent to which the area is relying on NOX emissions reductions to substitute for otherwise-required VOC
reductions in milestone years 2017 and 2020. For milestone year 2017, the RFP
demonstration relies on a combination of 1.4 tons per day tpd VOC reductions and 0.4 tpd NOX reductions from the 2011 RFP baseline year rather than the otherwise-required VOC reductions of 1.6 tpd. That is, 0.4 tpd of NOX
reductions substitutes for 0.2 tpd of VOC reductions otherwise required, which represents a 2:1 ratio for substitution of NOX for VOC in RFP
milestone year 2017. This substitution of NOX reductions for VOC reductions is acceptable under CAA section 182c2C so long as the ozone concentration reductions from 2011 to 2017 in Eastern Kern under the combined VOC/NOX emissions reduction scenario are at least equivalent to that which would result under the VOC-only reduction scenario.
The same applies to milestone year 2020. For that year, the RFP
demonstration relies on a combination of 1.5 tpd VOC reductions and 3.1 tpd NOX reductions from the 2011 RFP
baseline year rather than the otherwiserequired VOC reductions of 2.3 tpd.
That is, 3.1 tpd of NOX reductions substitutes for 0.8 tpd of VOC
reductions otherwise required, which means that NOX is substituted for VOC
in RFP milestone year 2020 at roughly a 4:1 ratio. Again, this substitution of NOX reductions for VOC reductions is acceptable under CAA section 182c2C so long as the ozone concentration reductions from 2011 to 2020 in Eastern Kern under the combined VOC/NOX emissions reduction scenario are at least equivalent to that which would result under the VOC-only reduction scenario.
The 2017 Eastern Kern Ozone SIP
contains a demonstration supporting the use of NOX substitution in the Eastern Kern nonattainment area. This is based on evidence that the Eastern Kern nonattainment area is NOX-limited, and also on evidence that NOX reductions are more effective at reducing ozone than VOC reductions alone. In this notice, we use NOX-limited as meaning a situation where reducing NOX emissions decreases ozone, not that it is more effective than reducing VOC. Elsewhere, including in the 2017
Eastern Kern Ozone SIP, the term NOXlimited is sometimes used to mean the condition where NOX reductions are
PO 00000

Frm 00054

Fmt 4700

Sfmt 4700

more effective than VOC reductions at decreasing ozone.
Evidence that the Eastern Kern nonattainment area is NOX-limited is presented in Figure 14 in Appendix F of the Eastern Kern 2017 Ozone Plan.
Figure 14 and the explanatory text document weekday and weekend monitored ozone data at the Mojave monitoring site from 20002015.26 The results show that in nearly all years, weekdays with their higher NOX
emissions have increased ozone, while weekends with their lower NOX, have decreased ozone. Figure 14 includes a 1:1 line on which weekday and weekend ozone are the same.27 Of the sixteen years examined, thirteen are above the 1:1 line, indicating higher weekday ozone and NOX-limited ozone formation. All years after 2007 are above the 1:1 line. The three years i.e., 2001, 2003, and 2007 below the 1:1 line indicate slightly higher ozone from reducing NOX. However, all three of those years are in the transitional regime close to the 1:1 line; this indicates the three years have only a weak ozone response to NOX
reductions, as opposed to a disbenefit.
This data analysis is strong evidence that ozone formation is NOX-limited in the Eastern Kern nonattainment area.
The Eastern Kern 2017 Ozone Plan also included photochemical modeling results reflecting base year 2012
emissions and meteorology. The weekday-weekend analysis discussed above was repeated for modeled concentrations, which were found to be NOX-limited. 28 The degree of NOXlimitation, that is the response of ozone to NOX emissions reductions, was found to be comparable to and somewhat greater than that in the ambient data.
Given the Eastern Kern 2017 Ozone Plans usage of the term NOX-limited, the photochemical modeling also indicates that NOX reductions are more effective than VOC at reducing ozone.29
26 Eastern Kern 2017 Ozone Plan, Appendix F, F
42F43; and Appendix H, H22H23.
27 Id.
28 Eastern Kern 2017 Ozone Plan, Appendix F, Figure 14, F42.
29 The use of NO -limited in the 2017 Eastern X
Kern Ozone SIP is mainly consistent with NOX
reductions being more effective than VOC
reductions, i.e., NOX-limited in a relative sense rather than the strict sense of ozone decreasing with NOX reductions. See Appendix F of the Eastern Kern 2017 Ozone Plan: NOX-limited region in Figure 13, ozone formation shows a benefit to reductions in NOX emissions, while changes in ROG emissions result in only minor decreases in ozone, F40; in Figure 13, the NOX-limited region is one with isopleth lines nearly parallel to the VOC axis, indicating little change in ozone as VOC changes, and relatively large changes in ozone as NOX changes, F41; This region Eastern Kern is in close proximity to biogenic ROG emissions
E:FRFM25JNR1.SGM

25JNR1