What controls ozone sensitivity in the upper tropical troposphere?

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

Ozone is an 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 (NOinline-formulaxinline-formula≡ NO inline-formula+ NOinline-formula2) and volatile organic compounds (VOCs) 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 30inline-formula S and 30inline-formula 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 ECHAM5/MESSy2 Atmospheric Chemistry (EMAC) general circulation model, we analyze a variety of commonly applied metrics including ozone production rates (P(Oinline-formula3)), the formaldehyde (HCHO) to NOinline-formula2 ratio and the share of methyl peroxy radicals (CHinline-formula3Oinline-formula2) forming HCHO (inline-formulaα(CHinline-formula3Oinline-formula2)) 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 NOinline-formulax and P(Oinline-formula3) over the continental areas of South America and Africa where lightning is frequent. We find that P(Oinline-formula3) and its response to NO is unsuitable for determining the dominant regime in the upper troposphere. Instead, inline-formulaα(CHinline-formula3Oinline-formula2) and the inline-formula M20inlinescrollmathml chem normal HCHO / chem normal NO normal 2 61pt14ptsvg-formulamathimge83e61cda919f4d1b2f7c90126486483 acp-23-12651-2023-ie00001.svg61pt14ptacp-23-12651-2023-ie00001.png ratio in combination with ambient NO levels perform well as metrics to indicate whether NOinline-formulax or VOC sensitivity is prevalent. We show that effectively only the knowledge of the availability of NO and HOinline-formula2 is required to adequately represent Oinline-formula3 precursors and its sensitivity towards them. A sensitivity study with halving, doubling and excluding lightning NOinline-formulax 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.



Nussbaumer, Clara M. / Fischer, Horst / Lelieveld, Jos / et al: What controls ozone sensitivity in the upper tropical troposphere?. 2023. Copernicus Publications.


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