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<p>The South Coast Air Basin (SoCAB), which includes the city of Los Angeles and is home to more than 15 million people, frequently experiences ozone (O<span class="inline-formula">₃</span>) levels that exceed ambient air quality standards. While strict regulation of O<span class="inline-formula">₃</span> precursors has dramatically improved air quality over the past 50 years, the region has seen limited improvement in O<span class="inline-formula">₃</span> over the past decade despite continued reductions in precursor emissions. One contributing factor to the recent lack of improvement is a gradual transition of the underlying photochemical environment from a VOC-limited regime (where VOC denotes volatile organic compound) towards an <span class="inline-formula">NO_<i>x</i></span>-limited one. The changes in human activity prompted by COVID-19-related precautions in spring and summer of 2020 exacerbated these existing changes in the O<span class="inline-formula">₃</span> precursor environment. Analyses of sector-wide changes in activity indicate that emissions of <span class="inline-formula">NO_<i>x</i></span> decreased by 15 %–20 % during spring (April–May) and by 5 %–10 % during summer (June–July) relative to expected emissions for 2020, largely due to changes in mobile-source activity. Historical trend analysis from two indicators of O<span class="inline-formula">₃</span> sensitivity (the satellite <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">HCHO</mi><mo>/</mo><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="506bea995009780c42447cfa36baaaf2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-12985-2022-ie00001.svg" width="61pt" height="14pt" src="acp-22-12985-2022-ie00001.png"/></svg:svg></span></span> ratio and the O<span class="inline-formula">₃</span> weekend<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3af55808dad7e355d8e0b0b2a0272ce7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-12985-2022-ie00002.svg" width="8pt" height="14pt" src="acp-22-12985-2022-ie00002.png"/></svg:svg></span></span>weekday ratio) revealed that spring of 2020 was the first year on record to be on average <span class="inline-formula">NO_<i>x</i></span>-limited, while the “transitional” character of recent summers became <span class="inline-formula">NO_<i>x</i></span>-limited due to COVID-19-related <span class="inline-formula">NO_<i>x</i></span> reductions in 2020. Model simulations performed with baseline and COVID-19-adjusted emissions capture this change to an <span class="inline-formula">NO_<i>x</i></span>-limited environment and suggest that COVID-19-related emission reductions were responsible for a 0–2 ppb decrease in O<span class="inline-formula">₃</span> over the study period. Reaching <span class="inline-formula">NO_<i>x</i></span>-limited territory is an important regulatory milestone, and this study suggests that deep reductions in <span class="inline-formula">NO_<i>x</i></span> emissions (in excess of those observed in this study) would be an effective pathway toward long-term O<span class="inline-formula">₃</span> reductions.</p>