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.

Particles containing meteoric material were observed in the lower stratosphere during five aircraft research missions in recent years. Single particle laser ablation technique in a bipolar configuration was used to measure the chemical composition of particles in a size range of approximately 150 nm to 3 µm. The five aircraft missions, conducted between 2014 and 2018, cover a latitude range from 15 to 68°N. In total, more than 330 000 single particles were analyzed. A prominent fraction (more than 50 000) of the analyzed particles was characterized by strong abundances of magnesium, iron, and rare iron oxide compounds, together with sulfuric acid. This particle type was found almost exclusively in the stratosphere and is interpreted as meteoric material immersed or dissolved in stratospheric sulfuric acid particles. Below the tropopause the fraction of this particle type decreases sharply. However, small abundances were observed below 3000 m a.s.l. in the Canadian Arctic and also at the Jungfraujoch high altitude station (3600 m a.s.l.). Thus, the removal pathway by sedimentation and/or mixing into the troposphere is confirmed. Our data show that particles containing meteoric material are present in the lower stratosphere in very similar relative abundances, regardless of latitude or season. This finding suggests that the meteoric material is transported from the mesosphere into the stratosphere in the downward branch of the Brewer-Dobson-Circulation and efficiently distributed towards low latitudes by isentropic mixing. As a result, meteoric material is found in particles of the stratospheric Junge layer at all latitudes.