Ozone Formation Sensitivity to Precursors and Lightning in the Tropical Troposphere Based on Airborne Observations

Tropospheric ozone (O₃) is an important greenhouse gas that is also hazardous to human health. The formation of O₃ is sensitive to the levels of its precursors NO_x (≡NO + NO₂) and peroxy radicals, for example, 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 O₃ levels today and in the future, and thus air quality and climate. In this study, we investigate O₃ sensitivity in the tropical troposphere based on in situ observations of NO, HO₂ and O₃ from four research aircraft campaigns between 2015 and 2023. These are 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 EMAC atmospheric chemistry—climate model. We use the metric α(CH₃O₂) together with NO to investigate the O₃ formation sensitivity. We show that O₃ formation is generally NO_x-sensitive in the lower and middle tropical troposphere and is in a transition regime in the upper troposphere. By distinguishing observations impacted by lightning or not we show that NO from lightning is the most important driver of O₃ sensitivity in the tropics. NO_x-sensitive chemistry predominates in regions without lightning impact, with α(CH₃O₂) ranging between 0.56 and 0.82 and observed average O₃ levels between 35 and 55 ppbv. Areas affected by lightning exhibit strongly VOC-sensitive O₃ chemistry with α(CH₃O₂) of about 1 and average O₃ levels between 55 and 80 ppbv.