## 3 Results
### Impacts of the COVID-19 Emissions Reductions on Ozone  
Next, we present the CMAQ-predicted impacts of the COVID-19 emissions reductions on ozone concentrations. The left column of Figure 10a shows the 1 June-31 July period-average, period-maximum, and period-minimum MDA8 ozone for the 2020 COVID scenario in the upper, middle, and lower panels, respectively. Average MDA8 ozone greater than 70 ppb occurs in the western region of San Bernardino County and extends a little into eastern Los Angeles County. The right column of Figure 10a shows the 1 June-31 July period-average, period-maximum, and period-minimum of the daily differences between 2020 COVID and 2020 BAU MDA8 ozone (i.e., the impacts of the COVID-19 emissions reductions) in the upper, middle, and lower panels, respectively. Grey represents minimal differences; cool shades represent ozone decreases and warm shades represent ozone increases. The period-maximum plot shows that in Los Angeles County and part of San Bernardino County ozone increases of greater than 2 ppb occurred at some time during the period. The period-minimum plot shows that throughout most of the domain (with a notable exception of the southeast quadrant of Los Angeles County) ozone decreases occurred and decreases of at least 1.5 ppb were widespread. The period-average difference plot domain-wide maximum is 0.81 ppb and the domain-wide minimum is \(-\)1.33 ppb. The average ozone response to the \(\sim\)13% NOx reductions clearly delineate the average extent of the modeled VOC-sensitive versus NOx-sensitive chemical regimes within the SoCAB. Figure 10b provides a closer look at the period-average CMAQ-predicted impacts of the COVID-19 emission reductions throughout the SoCAB, with monitor locations indicated. The area of ozone increases encompasses the southeastern quadrant of Los Angeles County and stretches into San Bernardino and Orange Counties. The area of ozone decreases occurs over a larger area that includes Santa Clarita Valley, most of San Bernardino, Riverside, and southern Orange Counties. Although the area of ozone decreases is larger than the area of ozone increases, a vast majority of the population resides in the area of ozone increases. Table 3 reports the CMAQ-predicted period-average, period-maximum, and period-minimum MDA8 ozone differences at the SoCAB monitors (refer to Figure 8 for the locations of the monitors). Monitors with higher ozone on average in the 2020 COVID Case compared to the 2020 BAU Case are shown in red, and those with lower ozone are shown in blue. All monitors have higher ozone for at least one day in the June 1--July 31 period, with an ozone increase indicating that the NOx disp benefits are not just limited to the SoCAB central urban areas. The maximum ozone increase at any monitor is at Glendora (2.77 ppb);  
Figure 9: CMAQ derived RFFs: (**a**) 2020 COVID/2019 Base; (**b**) 2020 COVID/2020 BAU. surprisingly, the monitoring site with the fourth highest ozone increase is Crestline at Lake Gregory, far downwind and in the San Bernardino Mountains, which is a site where ozone formation is usually more NO\({}_{\chi}\)-sensitive. For the period-minimum differences, all monitors have lower ozone for at least one day in the June 1-July 31 period. The largest ozone decreases with reductions of over \(-2\) ppb occur at sites that tend to be the furthest downwind, such as Banning Airport (\(-2.3\) ppb). Even at monitoring sites within central Los Angeles County where ozone formation is more VOC-sensitive, there is at least one day during the June-July 2020 episode where ozone is reduced due to the NO\({}_{\chi}\) controls. Figure 10: 1 June–31 July impacts of the COVID-19 emissions reductions on ozone, estimated by CMAQ: (**a**) Period-average, period-maximum, and period-minimum MDA8 ozone spatial maps for 2020 COVID and 2020 COVID minus 2020 BAU; (**b**) Close up of period-average 2020 COVID minus 2020 BAU for SoCAB.