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

Abstract

We quantify controls on seasonal changes of the oxygen isotope anomaly of nitrate (Δ 17 O(NO 3 − ), wherein Δ 17 O ≈ δ 17 O − (0.52 × δ 18 O)) in snow at Summit, Greenland, in an effort to enable quantitative reconstructions of paleoatmospheric oxidant concentrations from ice core Δ 17 O(NO 3 − ). Measurements of Δ 17 O(NO 3 − ) from a snowpit at Summit are compared to calculations from an atmospheric chemical box model. Measured values of Δ 17 O(NO 3 − ) 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 3 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 17 O(NO 3 − ) may result from several influences not accounted for by our box model, including nonzero Δ 17 O of OH over polar regions, stratospheric influence on surface O 3 at Summit, participation of BrO in nitrate production, and tropospheric transport of nitrate. A box model sensitivity study shows that annual mean Δ 17 O(NO 3 − ) is most sensitive to changes in the ratio of [O 3 ]/([HO 2 ] + [RO 2 ]) in summer.