hide
Free keywords:
-
Abstract:
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.
