Influence of various criteria on identifying the springtime tropospheric ozone depletion events (ODEs)at Utqiaġvik, Arctic

Abstract. Tropospheric ozone depletion events (ODEs) occurring in the Arctic spring are a unique photochemical phenomenon in which the boundary layer ozone drops rapidly to near-zero levels. However, the criterion for identifying ODEs remains inconsistent among different studies, which may influence conclusions regarding the characteristics of ODEs. To address this issue, in this study, we applied various criteria used in previous studies to identify springtime ODEs at Utqiaġvik, Arctic (the BRW Station), based on observational data spanning 23 years (2000–2022), and investigated the influence of implementing different criteria. We compared three types of criteria: traditional methods (fixed thresholds), variability-based methods (considering the mean value and standard deviation), and machine learning methods (Isolation Forest), and we found that criteria using fixed thresholds (e.g., 10 ppbv) and relative thresholds based on monthly averaged ozone levels (0.42 times the monthly average) are more suitable for capturing ODEs at BRW compared to other criteria. Results applying these appropriate criteria reveal a significant decline in ODE occurrence frequency over the investigated 23 years, particularly in April, suggesting potential links to climate change and Arctic sea ice melting. However, implementing relative thresholds or more stringent fixed thresholds (5 and 4 ppbv) instead of the 10 ppbv threshold reveals a more significant decline in the number of ODE hours across these 23 years. Further investigation of meteorological conditions indicates that ODEs at BRW are more prevalent under northerly and northeasterly winds with moderate wind speeds (3–6 m s−1), at lower temperatures and higher pressures, while severe ODEs are more associated with lower wind speeds and temperatures below 256 K. This research highlights the importance of selecting appropriate criteria to accurately identify ODEs and contributes to a better understanding of the complex processes driving the Arctic ODEs.