## 2 Materials and Methods
### Evaluation Approach
#### 2.5.1 Dynamic Evaluation Methodology  
For the dynamic evaluation, we follow the EPA modeling guidance procedures [14] for projecting ozone concentrations in an ozone model attainment demonstration. This approach was used in the SCAQMD's AQMP [25] to define ozone attainment VOC/NOx emission control strategies. The 2020 COVID Case represents the future-year emissions scenario, and 2019 Base Case represents the base year. Typically, the future year for a model attainment demonstration may be five or more years in the future (or a decade or more for the SoCAB extreme ozone NAA), but the abrupt emissions reductions due to COVID-19 restrictions mimic a potential future-year scenario and has the unique attribute of having observations for the comparison. In a typical model attainment demonstration, the meteorology is the same for the future and base years because the future-year meteorology is unknown. For this analysis, however, the meteorology is different between 2019 and 2020, although the June-July 2019 and 2020 modeling periods were selected to have similar meteorology. The EPA recommends using model estimates in a relative rather than absolute sense to reduce potential model bias effects for making future-year ozone projections. Fractional changes in air pollutant concentrations between the model future-year and model-base-year are calculated; these ratios are called relative response factors (RRFs). The RRFs are multiplied by base-year-observed ozone concentrations to predict future-year ozone concentrations. The base-year observations are the average of three-years of design values (DVB; defined below), and the future-year ozone design value (DVF) projection formula is as follows:  
\[\text{DVF}_{\text{i}}=\text{RRF}_{\text{i}}\times\text{DVB}_{\text{i}}, \tag{1}\]  
where \(\text{DVF}_{\text{i}}\) is the estimated design value for the future year in which attainment is required at monitoring site i; \(\text{RRF}_{\text{i}}\) is the relative response factor at monitoring site i; and \(\text{DVB}_{\text{i}}\) is the base design value at monitoring site i. We calculate RRFs as one would do for projecting future-year ozone design values, only instead of using modeling results for a base- and future-year emissions scenario with the same meteorological inputs, we use the modeling results for the 2019 Base Case and 2020 COVID Case that have different meteorology. We also calculate observation-based RRFs using the 2019 and 2020 ozone observations in a consistent fashion. We do not use the design values in our analysis because we only model 2 months, which is not long enough to define design values, therefore we only focus on the RRFs. There are additional considerations for the RRF calculations regarding which days to use in the RRF calculation, and EPA guidance has evolved over the years. We follow the current EPA recommendation, which is to calculate RRFs based on the 10 highest modeled days in the base year. This is to reflect the fact that design values are based on the three-year average 4 th high observed MDA8 ozone values. The EPA recommends \"_selecting a set of modeled days that are likely to encompass a range of values that are somewhat higher than and somewhat lower than the 4 th high value... this balances the desire to have enough days in the RRF to generate a robust calculation, but not so many days that the RRF does not represent days with concentrations near the observed design values_\" [14]. We follow EPA recommendations to evaluate CMAQ using the standard regulatory procedures so that the results are relevant to the regulatory community. The EPA describes an additional basis for their guidance and reports that \"_model response to decreasing emissions is generally most stable when the base ozonepredictions are highest. The greater model response at higher concentrations is likely due to more \"controllable\" ozone at higher concentrations...