We report in this paper the first measurements of the isotopic anomaly of oxygen in Arctic atmospheric inorganic nitrate. Data and samples were collected at Alert, Nunavut, Canada (82°30' N, 62°19' W) in spring 2004. Focusing on the polar sunrise period, characterized by the occurrence of severe boundary layer ozone depletion events (ODEs), our data show a significant correlation between the evolution of atmospheric ozone (O₃) mixing ratios and Δ¹⁷O in nitrate Δ¹⁷O(NO^−₃)). This relationship can be expressed as: Δ¹⁷O(NO^−₃)/‰=0.15 O₃/ (nmol mol^−1) + 28.6, with R²=0.70 (n=12), for Δ¹⁷O(NO^−₃) ranging between 29 and 34. To quantitatively interpret this relationship, we derive from mechanisms at play in the arctic boundary layer isotopic mass-balance equations, which depend on the concentrations of reactive species and their isotopic characteristics. Changes in the relative importance of O₃, RO₂ and BrO in the oxidation of NO_x during ODEs, and the large isotopic anomalies that O₃ and BrO carry, are the driving force for the high variability in the measured Δ¹⁷O(NO^−₃). BrONO₂ hydrolysis is found to be the major source of nitrate in the arctic boundary layer, in agreement with recent modeling studies. In addition, the isotopic fingerprint of the activity of ozone in a relatively stable compound appears somewhat promising in the perspective of using the isotopic composition of nitrate embedded in polar ice-cores as a paleo-indicator of the atmospheric ozone level that may yield an indirect proxy for the oxidative power of past atmospheres.