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Federal Register / Vol. 86, No. 7 / Tuesday, January 12, 2021 / Proposed Rules
the expected changes between the 2012
base year and the 2020 future year.
These modeling inventories exclude emissions events which are either random and/or cannot be projected to the future . . . wildfires, and events such as the San Francisco Bay Area Chevron refinery fire. 84 The future inventories project the base year with these exclusions into the future by including the effect of economic growth and emissions control measures.
The 2018 Western Nevada County Ozone Plan carries out the attainment test procedure consistent with the Modeling Guidance. The RRF is calculated as the ratio of future to base year concentrations; these are then applied to the 2013 weighted design values for the Grass Valley monitor to arrive at a future year design value.85
Typically the RRFs would be applied to a weighted design value for 2012, the model base year,86 but in this case CARB used the somewhat higher value for 2013, considering the upward trend design values starting in 2013.87 The predicted 2020 ozone design value is 67
ppb or 0.067 ppm, well below the level of the 2008 8-hour ozone NAAQS of 0.075 ppm.
Finally, the 2018 Western Nevada County Ozone Plan modeling includes an Unmonitored Area Analysis UAA
to assess whether locations without a monitor are able to reach attainment; the standard attainment test procedure covers only locations with a monitor.88
The Modeling Guidance describes a procedure utilizing gradient adjusted spatial fields, as well as the EPA
software used to carry it out.89 This procedure uses a form of interpolation, combining monitored concentrations and modeled gradients modeled changes in concentration with distance from a monitor to estimate future concentrations at locations without a
monitor. The 2018 Western Nevada County Ozone Plan describes a UAA
carried out using software developed by CARB and implemented in R, 90 using a procedure virtually the same as that outlined in the Modeling Guidance. The Plan states that the 2020 results show concentrations below 75 ppb at all locations in the nonattainment area; it did not examine the surrounding area.
Because the results are well below the 2008 ozone NAAQS level of 75 ppb, the UAA supports the demonstration that all locations in Western Nevada County will attain the NAAQS in 2020.
In addition to the formal attainment demonstration, the Plan also contains a WOE analysis within Appendix A to the CARB Staff Report. It mainly shows the long-term downward trends that continue through 2017, the latest year available prior to development of the 2018 Western Nevada County Ozone Plan. As described in the WOE, Western Nevada County has shown a general downward trend in measured ozone concentrations and number of days above the ozone NAAQS but has recently seen increases in 2017 and 2018. Atypical high ozone concentrations were observed in 2017, though CARBs staff analysis does not point to specific anthropogenic or biogenic emission increases or meteorology as likely causes for the unusual number of exceedances.
Additionally, the area may have experienced higher than normal ozone concentrations in 2018 due to wildfire impacts in the surrounding areas during the summer and fall months. Despite the recent exceptions, there are strong downward trends in emissions of ozone and of the ozone precursors NOX and VOC, both within the Western Nevada County area and in the upwind Sacramento and San Francisco Bay areas.91 These all show the substantial
air quality progress made in the Western Nevada County Area and add support to the attainment demonstration for 2020.
b. Control Strategy The control strategy for attainment of the 2008 ozone NAAQS is detailed in Chapter IV of the 2018 Western Nevada County Ozone Plan. The Plans strategy relies primarily on emissions reductions from control measures that have been adopted by the Districts and CARB prior to the submittal of the Plan. The District has adopted rules for reducing emissions from a broad scope of stationary and area sources into its RACT SIP. Additionally, a detailed description of the mobile source control programs and a comprehensive list of CARB regulations are included in Appendices B and C of the Plan. CARBs comprehensive strategy to reduce emissions from mobile sources consists of emissions standards for new vehicles, in-use programs to reduce emissions from existing vehicle and equipment fleets, cleaner fuels, and incentive programs to accelerate the penetration of the cleanest vehicles beyond that achieved by regulations alone.
As Table 2 and Table 3 show, the vast majority of emissions reductions relied upon by the Plans control strategy are from the onand off-road mobile source inventory and can be largely attributed to control measures adopted by CARB, subsequently approved by the EPA, and cited in detail in Section III.C.
Generally, the bulk of the emissions reductions on which the control strategies rely is expected to come from already-adopted measures, which are discussed in Section III.C of this document. For the 2008 ozone NAAQS, already-adopted measures are expected to achieve all of the reductions needed from the 2012 base year to attain the NAAQS in 2020.

TABLE 22012 AND 2020 NOX EMISSIONS FOR WESTERN NEVADA COUNTY
Summer planning inventory, tpd Source category
2012

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Stationary Sources
Area Sources
On-Road Mobile Sources
Other Mobile Sources

84 Id. at Appendix H, H33; and, Appendix F, Modeling Emissions Inventory, F35. To include the fires in the base year but not the future year would effectively credit the Plans control measures with eliminating emissions from the fire.
85 Id. at 57, and Appendix H, Modeling Protocol, section 10.3, H34. The combination of years used is illustrated in Appendix E, Table 1, E11.

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86 The Modeling Guidance recommends that RRFs be applied to the average of three three-year design values, for the base year and the two subsequent years. This amounts to a 5-year weighted average of individual year 4th high concentrations, centered on the base year, and so is referred to as a weighted design value.
87 2018 Western Nevada County Ozone Plan, Appendix E, E10; also Plan, 58.

PO 00000

Emissions difference from 2012 to 2020

2020

Percentage of total emissions change %
9.4
2.2
45.7
21.8

88 2018 Western Nevada County Ozone Plan, Appendix E, section 5.4, E41.
89 Modeling Guidance section 4.7, 138.
90 The R Project for Statistical Computing, https
www.r-project.org.
91 2018 Western Nevada County Ozone Plan, page 41.

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