Description:
<jats:p>We quantify controls on seasonal changes of the oxygen isotope anomaly of nitrate (Δ<jats:sup>17</jats:sup>O(NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>), wherein Δ<jats:sup>17</jats:sup>O ≈ <jats:italic>δ</jats:italic><jats:sup>17</jats:sup>O − (0.52 × <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O)) in snow at Summit, Greenland, in an effort to enable quantitative reconstructions of paleoatmospheric oxidant concentrations from ice core Δ<jats:sup>17</jats:sup>O(NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>). Measurements of Δ<jats:sup>17</jats:sup>O(NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) from a snowpit at Summit are compared to calculations from an atmospheric chemical box model. Measured values of Δ<jats:sup>17</jats:sup>O(NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) covering three seasonal cycles (January–December 2000, July 2003–March 2006) range from 22.4‰ in summertime to 33.7‰ in wintertime, while model results show a larger range (18.9–31.5‰). Agreement between observed and modeled results is excellent for winter, when O<jats:sub>3</jats:sub> oxidation of nitrogen oxides dominates nitrate production (winter averages agree within 0.3‰). The 2–7‰ discrepancy between summertime box model results and measurements of Δ<jats:sup>17</jats:sup>O(NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) may result from several influences not accounted for by our box model, including nonzero Δ<jats:sup>17</jats:sup>O of OH over polar regions, stratospheric influence on surface O<jats:sub>3</jats:sub> at Summit, participation of BrO in nitrate production, and tropospheric transport of nitrate. A box model sensitivity study shows that annual mean Δ<jats:sup>17</jats:sup>O(NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) is most sensitive to changes in the ratio of [O<jats:sub>3</jats:sub>]/([HO<jats:sub>2</jats:sub>] + [RO<jats:sub>2</jats:sub>]) in summer.</jats:p>