Summit, Greenland snowpit nitrate D17O
| Abstract: We quantify controls on seasonal changes of the oxygen isotopic anomaly of nitrate (D17O(NO3-)), where D17O ≈ d17O - (0.52 x d18O) in snow at summit Greenland in an effort to enable quantitative reconstructions of paleoatmospheric oxidant concentrations from ice core D17O(NO3-). Measurements of D17O(NO3-) from a snowpit at Summit are compared to calculations from an atmospheric chemical box model. Measured values of D17O(NO3-) covering three seasonal cycles (Jan - Dec 2000, Jul 2003 - Mar 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 D17O(NO3-) may result from several influences not accounted for by our box model, including: Non-zero D17O 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 D17O(NO3-) is most sensitive to changes in the ratio of [O3]/([HO2]+[RO2]) in summer. | |
|
|
|
|
|
|
Figure 3: (a) 2001 and 2006 snowpit d18O(H2O) versus depth, and (b) 2001 and 2006 snowpit and box model D17O(NO3-) versus time. Pints indicate measurements of d18O(H2O) and D17O(NO3-), while the solid line indicated modeled D17O(NO3-). Gray squares represent measurements for the 2001 snowpit, while black triangles and white circles represent measurements for the 2006 snowpit taken 1 m laterally apart. Vertical error bars indicate the 2s error of measurements. Horizontal error bars in panel (b) encompass potential dating errors. |
|
|
| |
| Figure 1: Chemistry of nitrate formation in box model calculations. Two oxidations steps are included: NOx cycling and NO2 oxidation to HNO3. | |
|
|
|
| Figure 4. Sensitivity of D17O(NO3-) to various oxidation pathways. (Colors correspond to oxidation pathways in Figure 1 above.) Annual mean D17O(NO3-) is most sensitive to changes in the ratio of [O3]/([HO2]+[RO2]) in summer. | |
| People: | |
| Shelley Kunasek | |
| References: | |
| Kunasek, S. A., B. Alexander, M. G. Hastings, E. J. Steig, D. J. Gleason, and J. C. Jarvis (2008), Measurements and modeling of D17O of nitrate in a snowpit from Summit, Greenland, J. Geophys. Res. 113, D24302. (.pdf) | |
| Funding: | |
| NSF-ANT 0538049 | |
