Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign

Abstract. Variations in the stable oxygen isotope composition of atmospheric nitrate act as novel tools for studying oxidative processes taking place in the troposphere. They provide both qualitative and quantitative constraints on the pathways determining the fate of atmospheric nitrogen oxides (NO + NO₂ = NO_x). The unique and distinctive ¹⁷O excess (Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O) of ozone, which is transferred to NO_x via oxidation, is a particularly useful isotopic fingerprint in studies of NO_x transformations. Constraining the propagation of ¹⁷O excess within the NO_x cycle is critical in polar areas, where there exists the possibility of extending atmospheric investigations to the glacial–interglacial timescale using deep ice core records of nitrate. Here we present measurements of the comprehensive isotopic composition of atmospheric nitrate collected at Dome C (East Antarctic Plateau) during the austral summer of 2011/2012. Nitrate isotope analysis has been here combined for the first time with key precursors involved in nitrate production (NO_x, O₃, OH, HO₂, RO₂, etc.) and direct observations of the transferrable Δ¹⁷O of surface ozone, which was measured at Dome C throughout 2012 using our recently developed analytical approach. Assuming that nitrate is mainly produced in Antarctica in summer through the OH + NO₂ pathway and using concurrent measurements of OH and NO₂, we calculated a Δ¹⁷O signature for nitrate on the order of (21–22 ± 3) ‰. These values are lower than the measured values that ranged between 27 and 31 ‰. This discrepancy between expected and observed Δ¹⁷O(NO₃⁻) values suggests the existence of an unknown process that contributes significantly to the atmospheric nitrate budget over this East Antarctic region. However, systematic errors or false isotopic balance transfer functions are not totally excluded.