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Federal Register / Vol. 86, No. 7 / Tuesday, January 12, 2021 / Proposed Rules
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flows in those valleys lead to recirculation of pollutants, and the Sierra crest tends to block flow further east; both of these enhance ozone concentrations. The area is mainly rural, with generally low NOX emissions and relatively high VOC emissions, so that ozone formation there is expected to be NOX-limited.74 The recirculation and the lack of NOX emissions prevents the removal of ozone through the NOX
titration process. This allows carryover of pollution from the previous day, leading to high ozone values that persist through the night at the start of the following morning, unlike the typical pattern for areas with ozone caused by locally generated emissions.75
The modeling and the modeled attainment demonstration are described in Chapter XII of the 2018 Western Nevada County Ozone Plan and in more detail in Appendix E, which provides a description of model input preparation procedures and various model configuration options. Appendix F of the 2018 Western Nevada County Ozone Plan provides the coordinates of the modeling domain and thoroughly describes the development of the modeling emissions inventory, including its chemical speciation, its spatial and temporal allocation, its temperature dependence, and quality assurance procedures. The modeling analysis uses version 5 of the Community Multiscale Air Quality CMAQ photochemical model developed by the EPA, using the 2007
version of the Statewide Air Pollution Research Center SAPRC07 chemical mechanism. The CMAQ modeling domain covers most of California, nested within a domain covering the entire state. To prepare meteorological inputs for CMAQ, CARB used the Weather and Research Forecasting model version 3.6 WRF from the National Center for Atmospheric Research. The WRF domain covers the entire state of California, nested within a domain covering most of the western United States. The modeling used inputs prepared from routinely available meteorological and air quality data 74 Ozone is generally NO -limited in rural areas X
and downwind suburban areas. See pages 24 and 38 of CARB Staff Report and also Chapter 2.1 Ozone Chemistry, Final Ozone NAAQS Regulatory Impact Analysis, March 2008, EPA Office of Air Quality Planning and Standards, available at https www3.epa.gov/ttnecas1/regdata/RIAs/452_
R_08_003.pdf. The term NOX-limited can mean either that reducing NOX emissions decrease ozone as opposed to increasing it; or that reducing NOX
is much more effective at decreasing ozone than is reducing VOC. As discussed below and on page 42
of CARB Staff Report, ozone in Western Nevada County are decreased by reducing NOX emissions.
75 2018 Western Nevada Ozone Plan, page H16.

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collected during 2012. Those data cover May through September, a period that spans the period of highest ozone concentrations in Western Nevada County. The Modeling Guidance recognizes both CMAQ and WRF as technically sound, state-of-the-art models. The areal extent and the horizontal and vertical resolution used in these models is adequate for modeling Western Nevada County ozone.
The WRF meteorological model results and performance statistics are described in Appendix E.76 The performance evaluation focuses on a smaller area than the full domain but encompassing the Western Nevada County nonattainment area and the greater Sacramento area, with special attention on the winds for high ozone days. There is a slight overprediction of wind speeds and underprediction of temperatures in the eastern portion of the nonattainment area, but overall, modeled wind speed, wind direction, and temperature all track observations very well, as shown in scatter and time series plots. The modeling replicates some important meteorological features such as the upslope-downslope flows in the Sierra Nevada foothills, and the Schulz eddy known to occur in the greater Sacramento area. The 2018
Western Nevada County Ozone Plan states that the bias and error are relatively small and are comparable to those seen in previous meteorological modeling of central California and cited in the Plan. In summary, the 2018
Western Nevada County Ozone Plans meteorological modeling performance statistics appear satisfactory.
Ozone model performance statistics are described in the 2018 Western Nevada County Ozone Plan at Appendix E.77 Appendix E includes tables of statistics recommended in the Modeling Guidance for 8-hour and 1-hour daily maximum ozone concentrations.
Predicted concentrations have a small negative bias underprediction of 4.1
ppb.78 This compares well to the range of 2.7 to 10.8 ppb seen in a previous modeling exercise for central California that is cited in the Plan; bias and error are both at the low end of those seen in a comparative study of 69 modeling 76 Appendix E, section 3.2, E17; also, refer to supplemental figures S.1S.11, E48.
77 Appendix E, section 5.2, E32; also, refer to supplemental figures S.12S.16, E55.
78 Because only the relative response to emissions changes RRF from the modeling is used, the underprediction of absolute ozone concentrations does not mean that future concentrations will be underestimated.

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exercises.79 The Plans supplemental figures with hourly time series show good performance; although some individual daily ozone peaks are missed in May and September, there are days for which the modeled highest concentration is close to the value of the highest observed concentration. This supports the adequacy of the model for use in the attainment demonstration.
As noted in the 2018 Western Nevada County Ozone Plans modeling protocol, the Modeling Guidance recognizes that limited time and resources can constrain the extent of the diagnostic and dynamic evaluation of model performance undertaken.80 The Plan describes a dynamic evaluation 81 in which model predictions of ozone concentrations for weekdays and weekends were compared to each other and to observed concentrations. This evaluation provides useful information on how well the model simulates the effect of emissions changes, since NOX
emissions are lower on weekends than on weekdays, but the days are otherwise similar. The modeled ozone decreased in response to the weekend NOX
reductions, which matches the observed decrease, and indicates that the model is simulating the chemistry correctly.
The Plan also contains results of an analysis of weekday and weekend ozone concentrations during the 20002015
period. It notes a shift over the years toward lower ozone on weekends, especially after 2010, showing that lower NOX emissions lead to lower ozone concentrations.82 Both the modeling and the observed weekdayweekend trends show that ozone responds to NOX emissions reductions, i.e., that ozone formation is NOXlimited. The modeled 2012 base year is also NOX-limited, with the weekdayweekend difference comparable to those seen historically. This match lends confidence to the modeling.
After accepting the model performance for the 2012 base case, CARB used the model to develop RRFs for the attainment demonstration.83 This entailed running the model with the same meteorological inputs as before, but with emissions inventories to reflect 79 Simon, H., Baker, K.R., Phillips, S, 2012, Compilation and Interpretation of Photochemical Model Performance Statistics Published Between 2006 and 2012, Atmos. Environ., 61, 124139.
doi:10.1016/j.atmosenv.2012.07.012.
80 2018 Western Nevada County Ozone Plan, Appendix H, Modeling Protocol, H31; Modeling Guidance, 63.
81 See Diagnostic Evaluation in Appendix E
section 5.2.1, E36.
82 2018 Western Nevada County Ozone Plan, Appendix E, E40.
83 Id. at 57, and Appendix H, Modeling Protocol, section 10.3, H34.

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