We report the first measurements of the oxygen isotope anomaly of atmospheric inorganic nitrate from the Arctic. Nitrate samples and complementary data were collected at Alert, Nunavut, Canada (82°30 ' N, 62°19 ' W) in spring 2004. Covering the polar sunrise period, characterized by the occurrence of severe boundary layer ozone depletion events (ODEs), our data show a significant correlation between the variations of atmospheric ozone (O<sub>3</sub>) mixing ratios and Δ<sup>17</sup>O of nitrate (Δ<sup>17</sup>O(NO<sup>−</sup><sub>3</sub>)). This relationship can be expressed as: Δ<sup>17</sup>O(NO<sup>−</sup><sub>3</sub>)/‰, =(0.15±0.03)×O<sub>3</sub>/(nmol mol<sup>–1</sup>)+(29.7±0.7), with R<sup>2</sup>=0.70(n=12), for Δ<sup>17</sup>O(NO<sup>−</sup><sub>3</sub>) ranging between 29 and 35 ‰. <br></br> We derive mass-balance equations from chemical reactions operating in the Arctic boundary layer, that describe the evolution of Δ<sup>17</sup>O(NO<sup>−</sup><sub>3</sub>) as a function of the concentrations of reactive species and their isotopic characteristics. Changes in the relative importance of O<sub>3</sub>, RO<sub>2</sub> and BrO in the oxidation of NO during ODEs, and the large isotope anomalies of O<sub>3</sub> and BrO, are the driving force for the variability in the measured Δ<sup>17</sup>O(NO<sup>−</sup><sub>3</sub>) . BrONO<sub>2</sub> hydrolysis is found to be a dominant source of nitrate in the Arctic boundary layer, in agreement with recent modeling studies.