Tropospheric ozone (O3) is an important greenhouse gas that is also hazardous to human health. O3 is formed photochemically from nitrogen dioxide (NO2) (with oxygen and sunlight), which in turn is generated through oxidation of nitric oxide (NO) by peroxy radicals (HO2 or RO2). The formation of O3 can be sensitive to the levels of its precursors NOx (≡ NO + NO2) 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 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, 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(CH3O2) together with NO to show that O3 formation chemistry is generally NOx-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 O3 sensitivity in the tropics. Areas affected by lightning exhibit strongly VOC-sensitive O3 chemistry, whereas NOx-sensitive chemistry predominates in regions without lightning impact.

Hydroxyl (OH) and hydroperoxyl (HO2) drive the atmosphere’s oxidation of gases emitted from Earth’s surface and the formation and aging of aerosol particles. Thus, understanding OH and HO2 chemistry is essential for examining the impact of human activity on future atmospheric composition and climate. Using the OH and HO2 dataset collected with the Penn State Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) during nine aircraft missions over the past 20 years, we compare observed OH and HO2 to that modeled using the same near-explicit photochemical box model. In general, the agreement is well within the uncertainties of the observations and models, even when the model is constrained only with a common data set of simultaneous measurements. However, in regions influenced by anthropogenic or biogenic volatile organic compounds, the model chemical mechanism and size of the data set of constraining measurements do matter. In cleaner regions, the differences between observed and modeled OH and HO2 found in previous studies generally remain and do not appear to be systematic, indicating that the differences are driven by measurement issues for ATHOS and/or other instruments. Thus, these comparisons indicate that the oxidation chemistry in most of the free troposphere is understood to as well as current measurements can determine. The focus of future research needs to be on regions rich in volatile organic compounds, where observed-to-modeled differences are more persistent, and on improving measurement consistency.

Lightning plays a major role in tropospheric oxidation, and its role on modulating tropospheric chemistry was thought to be emissions of nitrogen oxides (NOx). Recent field and laboratory measurements demonstrate that lightning generates extremely large amounts of oxidants, including hydrogen oxides (HOx) and O3. We here implement the lightning-produced oxidants in a global chemical transport model to examine its global impact on tropospheric composition. We find that lightning-produced oxidants can increase global mass weighted OH by 0.3-10%, and affect CO, O3, and reactive nitrogen substantially, depending on the emission strength of oxidants from lightning. Our work highlights the importance and uncertainties of lightning-produced oxidants, as well as the need for rethinking the role of lightning in tropospheric oxidation chemistry.