04 May 2023
 | 04 May 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

What controls ozone sensitivity in the upper tropical troposphere?

Clara M. Nussbaumer, Horst Fischer, Jos Lelieveld, and Andrea Pozzer

Abstract. Ozone is after water vapor, the second most important contributor to the radiative energy budget of the upper troposphere (UT). Therefore, observing and understanding the processes contributing to ozone production are important for monitoring the progression of climate change. Nitrogen oxides (NOx ≡ NO + NO2) and volatile organic compounds (VOC) are two main tropospheric precursors to ozone formation. Depending on their abundances, ozone production can be sensitive to changes in either of these two precursors. Here, we focus on processes contributing to ozone chemistry in the upper tropical troposphere between 30° S and 30° N latitude, where changes in ozone have a relatively large impact on anthropogenic radiative forcing. Based on modeled trace gas mixing ratios and meteorological parameters simulated by the EMAC atmospheric chemistry – general circulation model, we analyze a variety of commonly applied metrics including ozone production rates (P(O3)), the formaldehyde (HCHO) to NO2 ratio and the share of methyl peroxyradicals (CH3O2) forming HCHO (α(CH3O2)), for their ability to describe the chemical regime. We show that the distribution of trace gases in the tropical UT is strongly influenced by the varying locations of deep convection throughout the year, and we observe peak values for NOx and P(O3) over the continental areas of South America and Africa where lightning is frequent. We find that P(O3) and its response to NO is unsuitable for determining the dominant regime in the upper troposphere. Instead, α(CH3O2) and the HCHO / NO2 ratio in combination with ambient NO levels perform well as metrics to indicate whether NOx or VOC sensitivity is prevalent. A sensitivity study with halving, doubling and excluding lightning NOx demonstrates that lightning and its distribution in the tropics are the major determinants of the chemical regimes and ozone formation in the upper tropical troposphere.

Clara M. Nussbaumer et al.

Status: open (until 29 Jun 2023)

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  • RC1: 'Comment on egusphere-2023-816', Anonymous Referee #1, 25 May 2023 reply

Clara M. Nussbaumer et al.

Clara M. Nussbaumer et al.


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Short summary
Ozone is a greenhouse gas and contributes to the earth’s radiative energy budget and therefore to global warming. This effect is largest in the upper troposphere. In this study, we investigate the processes controlling ozone formation and the sensitivity to its precursors in the upper tropical troposphere based on model simulations by the ECHAM5/MESSy2 Atmospheric Chemistry (EMAC) model. We find that NOx emissions from lightning most importantly affect ozone chemistry at these altitudes.