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 (Delta O-17(NO3-), wherein Delta O-17 approximate to delta O-17 - (0.52 x delta O-18)) in snow at Summit, Greenland, in an effort to enable quantitative reconstructions of paleoatmospheric oxidant concentrations from ice core Delta O-17(NO3-). Measurements of Delta O-17(NO3-) from a snowpit at Summit are compared to calculations from an atmospheric chemical box model. Measured values of Delta O-17(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 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 Delta O-17(NO3-) may result from several influences not accounted for by our box model, including nonzero Delta O-17 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 Delta O-17(NO3-) is most sensitive to changes in the ratio of [O-3]/([HO2] + [RO2]) in summer.