| Summary: | Ozone depletion events (ODEs) in the springtime of the Arctic have been frequently observed since the early 1980s, and the correlation between the ozone mixing ratio during the ODEs and the nitrogen oxides (NOx) concentration is still unclear. In the present study, the role of the background level of NOx in ODEs was investigated by using a box model implementing a chemical reaction mechanism containing 49 chemical species and 141 related reactions. A concentration sensitivity analysis was also applied to discover the dependence of the ozone mixing ratio during the ODEs on each constituent of the initial air composition. The simulation results showed that a critical value of the NOx background level exists, with which the ozone depletion rate is independent of the initial concentration of NOx, and the critical value was found to be approximately 55 ppt (ppt = part per trillion, 10−12 mol/mol) in the present study. The concentration sensitivity analysis also showed that the existence of NOx has a two-sided impact on the depletion of ozone, depending on the initial amount of NOx. With a low background level of NOx (less than 55 ppt), the increase of the initial NOx can advance the ozone depletion. On the contrary, with a high initial NOx level (more than 55 ppt), NOx would delay the consumption of ozone during the ODEs.
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