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<p>The COVID-19 (coronavirus disease 2019) European lockdowns have led to a significant reduction in the emissions of primary pollutants such as NO (nitric oxide) and NO<span class="inline-formula">₂</span> (nitrogen dioxide). As most photochemical processes are related to nitrogen oxide (<span class="inline-formula">NO_<i>x</i>≡</span> NO <span class="inline-formula">+</span> NO<span class="inline-formula">₂</span>) chemistry, this event has presented an exceptional opportunity to investigate its effects on air quality and secondary pollutants, such as tropospheric ozone (O<span class="inline-formula">₃</span>). In this study, we present the effects of the COVID-19 lockdown on atmospheric trace gas concentrations, net ozone production rates (NOPRs) and the dominant chemical regime throughout the troposphere based on three different research aircraft campaigns across Europe. These are the UTOPIHAN (Upper Tropospheric Ozone: Processes Involving HO<span class="inline-formula">_<i>x</i></span> and NO<span class="inline-formula">_<i>x</i></span>) campaigns in 2003 and 2004, the HOOVER (HO<span class="inline-formula">_<i>x</i></span> over Europe) campaigns in 2006 and 2007, and the BLUESKY campaign in 2020, the latter performed during the COVID-19 lockdown. We present in situ observations and simulation results from the ECHAM5 (fifth-generation European Centre Hamburg general circulation model, version 5.3.02)/MESSy2 (second-generation Modular Earth Submodel System, version 2.54.0) Atmospheric Chemistry (EMAC), model which allows for scenario calculations with business-as-usual emissions during the BLUESKY campaign, referred to as the “no-lockdown scenario”. We show that the COVID-19 lockdown reduced NO and NO<span class="inline-formula">₂</span> mixing ratios in the upper troposphere by around 55 % compared to the no-lockdown scenario due to reduced air traffic. O<span class="inline-formula">₃</span> production and loss terms reflected this reduction with a deceleration in O<span class="inline-formula">₃</span> cycling due to reduced mixing ratios of NO<span class="inline-formula">_<i>x</i></span>, while NOPRs were largely unaffected. We also study the role of methyl peroxyradicals forming HCHO (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">α</mi><mrow class="chem"><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">3</mn></msub><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="4e7ef59e1cf19627b1205622895a9813"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-6151-2022-ie00001.svg" width="34pt" height="12pt" src="acp-22-6151-2022-ie00001.png"/></svg:svg></span></span>) to show that the COVID-19 lockdown shifted the chemistry in the upper-troposphere–tropopause region to a NO<span class="inline-formula">_<i>x</i></span>-limited regime during BLUESKY. In comparison, we find a volatile organic compound (VOC)-limited regime to be dominant during UTOPIHAN.</p>