Reply on RC1

Abstract

Column ozone variability has important implications for surface photochemistry and climate. Ice-core nitrate isotopes are suspected to be influenced by column ozone variability and δ15N(NO3-) has been sought to serve as a proxy of column ozone variability. In this study, we examined the ability of ice-core nitrate isotopes to reflect column ozone variability by measuring δ15N(NO3-) and Δ17O(NO3-) in a shallow ice core drilled at the South Pole. The ice core covers the period of 1944 to 2005, and during this period δ15N(NO3-) was of large annual variability ((59.2 ± 29.3) ‰), but with no apparent response to the Antarctic ozone hole. Utilizing a snow photochemical model, we estimated 6.9 ‰ additional enrichments in δ15N(NO3-) could be caused by the development of the ozone hole. But this enrichment is nevertheless small and masked by the effects of snow accumulation rate variability in addition to that of the slightly increased snow accumulation rate at the South Pole over the same period of the ozone hole. The Δ17O(NO3-) record displays a decreasing trend by ~ 3.4 ‰ since 1976. This magnitude of change can’t be caused by enhanced post-depositional processing owing to the ozone hole. Instead, the Δ17O(NO3-) decrease was more likely due to the proposed decreases in O3 / HOx ratio in the extratropical Southern Hemisphere. Our results suggest ice-core δ15N(NO3-) is more sensitive to snow accumulation rate than to column ozone, but at sites with relatively constant snow accumulation rate, information of column ozone variability embedded in δ15N(NO3-) should be retrievable. In comparison with the South Pole, up to 21 ‰ additional δ15N(NO3-) enrichments can be caused by the ozone hole at Dome A and the signal would be possibly detected if where snow accumulation rate has stayed relatively constant.