Measurements and modeling of Δ17O of nitrate in snowpits from Summit, Greenland

We quantify controls on seasonal changes of the oxygen isotope anomaly of nitrate (Δ17O(NO3−), wherein Δ17O ≈ δ17O − (0.52 × δ18O)) in snow at Summit, Greenland, in an effort to enable quantitative reconstructions of paleoatmospheric oxidant concentrations from ice core Δ17O(NO3−). Measurements of Δ17O(NO3−) from a snowpit at Summit are compared to calculations from an atmospheric chemical box model. Measured values of Δ17O(NO3−) 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 O3 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 Δ17O(NO3−) may result from several influences not accounted for by our box model, including nonzero Δ17O of OH over polar regions, stratospheric influence on surface O3 at Summit, participation of BrO in nitrate production, and tropospheric transport of nitrate. A box model sensitivity study shows that annual mean Δ17O(NO3−) is most sensitive to changes in the ratio of [O3]/([HO2] + [RO2]) in summer.