High-Precision Measurement of N₂O Concentration in Ice Cores

Atmospheric nitrous oxide (N₂O) is a greenhouse gas and ozone-depleting substance whose emissions are substantially perturbed by current human activities. Although air trapped in polar ice cores can provide direct information about N₂O evolution, analytical precision was not previously sufficient for high temporal resolution studies. In this work, we present a highly improved analytical technique with which to study N₂O concentrations in ancient-air-trapped ice cores. We adopt a melt-refreezing method to extract air and use a gas chromatography-electron capture detector (GC-ECD) to determine N₂O concentrations. The GC conditions are optimized to improve the sensitivity for detecting N₂O. Retrapped N₂O in ice during the extraction procedure is precisely analyzed and corrected. We confirmed our results using data from the Styx Glacier ice core in Antarctica by comparing them with the results of a dry-extraction method. The precision estimated from the pooled standard deviation of replicated measurements of the Styx ice core was 1.5 ppb for ∼20 g of ice, a smaller sample of ice than was used in previous studies, showing a significant improvement in precision. Our preliminary results from the Styx Glacier ice core samples have the potential to define small N₂O variations (a few parts per billion) at centennial time scales.