An inverse model to correct for the effects of post-depositional processing on ice-core nitrate and its isotopes: model framework and applications at Summit, Greenland, and Dome C, Antarctica

Comprehensive evaluation of the effects of post-depositional processing is a prerequisite for appropriately interpreting ice-core records of nitrate concentration and isotopes. In this study, we developed an inverse model that uses archived snow/ice-core nitrate signals to reconstruct primary nitrate flux (i.e., the deposition flux of nitrate to surface snow that originates from long-range transport or stratospheric input) and its isotopes ( delta 15 N and Delta 17 O). The model was then applied to two polar sites, Summit, Greenland, and Dome C, Antarctica, using measured snowpack nitrate concentration and isotope profiles in the top few meters. At Summit, the model successfully reproduced the observed atmospheric delta 15 N(NO 3 - ) and Delta 17 O(NO 3 - ) and their seasonality. The model was also able to reasonably reproduce the observed snowpack nitrate profiles at Dome C as well as the skin layer and atmospheric delta 15 N(NO 3 - ) and Delta 17 O(NO 3 - ) at the annual scale. The calculated F pri at Summit was 6.9 x 10 - 6 kgN m 2 a - 1 , and the calculated Delta 17 O(NO 3 - ) of F pri is consistent with atmospheric observations in the Northern Hemisphere. However, the calculated delta 15 N(NO 3 - ) of F pri displays an opposite seasonal pattern to atmospheric observations in the northern mid-latitudes, but it is consistent with observations in two Arctic coastal sites. The calculated F pri at Dome C varies from 1.5 to 2.2 x 10 - 6 kgN m - 2 a - 1 , with delta 15 N(NO 3 - ) of F pri varying from 6.2 parts per thousand to 29.3 parts per thousand and Delta 17 O(NO 3 - ) of F pri varying from 48.8 parts per thousand to 52.6 parts per thousand. The calculated F pri at Dome C is close to the previous estimated stratospheric denitrification flux in Antarctica, and the high delta 15 N(NO 3 - ) and Delta 17 O(NO 3 - ) of F pri at Dome C also point towards the dominant role of stratospheric origin of primary nitrate to Dome C.