The metric α(CH3O2) indicates ozone formation sensitivity towards NOx and VOCs in the global troposphere

While stratospheric ozone (O₃) is essential to life on Earth, tropospheric ozone can have adverse effects. At the surface, it contributes to air pollution and impacts human health. Further, it is the third most important anthropogenic greenhouse gas enhancing global warming and climate change, and its radiative forcing efficiency is largest in the upper troposphere. Therefore, it is imperative to investigate ozone formation and its sensitivity to the precursor gases nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Commonly used metrics apply to the planetary boundary layer but fail to identify O₃ formation sensitivity at higher altitudes. We introduce the new metric α(CH₃O₂) to indicate O₃ sensitivity, which represents the share of methyl peroxy radicals (CH₃O₂) forming O₃ through the reaction with NO in competition with the reaction with HO₂ terminating the catalytic (O₃ forming) HO_x cycle. We demonstrate the versatility and applicability of α(CH₃O₂) by investigating a number of studies, including several based on data from stationary field measurements, aircraft observations and model simulations in various locations around the globe, across all altitudes from the surface to the upper troposphere and over a time period of the past 20 years. We identify where O₃ chemistry is sensitive to NO_x or VOCs in the global troposphere considering a wide range of ambient conditions.