O3 formation sensitivity to precursors and lightning in the tropical troposphere based on airborne observations

Tropospheric ozone (O) is an important greenhouse gas that is also hazardous to human health. O is formed photochemically from nitrogen dioxide (NO) (with oxygen and sunlight), which in turn is generated through oxidation of nitric oxide (NO) by peroxy radicals (HO or RO). The formation of O can be sensitive to the levels of its precursors NOx (≡ NO + NO) and peroxy radicals, e.g., generated by the oxidation of volatile organic compounds (VOCs). A better understanding of this sensitivity will show how changes in the levels of these trace gases could affect O levels today and in the future, and thus air quality and climate. In this study, we investigate O sensitivity in the tropical troposphere based on in situ observations of NO, HO and O from four research aircraft campaigns between 2015 and 2023, namely, OMO (Oxidation Mechanism Observations), ATom (Atmospheric Tomography Mission), CAFE Africa (Chemistry of the Atmosphere Field Experiment in Africa) and CAFE Brazil, in combination with simulations using the ECHAM5/MESSy2 Atmospheric Chemistry (EMAC) model. We use the metric α(CHO) together with NO to show that O formation chemistry is generally NO-sensitive in the lower and middle tropical troposphere and in a transition regime in the upper troposphere. By distinguishing observations, which are either impacted by lightning or not, we show that NO from lightning is the most important driver of O sensitivity in the tropics. Areas affected by lightning exhibit strongly VOC-sensitive O chemistry, whereas NO-sensitive chemistry predominates in regions without lightning impact.