Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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High-Precision Measurement of N2O Concentration in Ice Cores Yeongjun Ryu, Jinho Ahn,* and Ji-Woong Yang School of Earth and Environmental Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul Republic of Korea S Supporting Information *
ABSTRACT: 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.
1. INTRODUCTION Nitrous oxide (N2O) is a long-lived greenhouse gas that plays important roles in climate change, nitrogen cycles, and stratospheric ozone destruction.1,2 Atmospheric N2O concentrations have continually increased since the late 19th century,3 and current rapid N2 O growth has become a global environmental concern. The recent N2O growth is considered to be mostly caused by enhanced agricultural activities with the increasing use of nitrogen fertilizer and animal manure from livestock.4−6 A large input of extra reactive nitrogen to the biosphere has perturbed global nitrogen cycles and increased N2O emissions. Other anthropogenic N2O sources include wastewater treatment, reactive nitrogen leaching to river and coastal regions, fossil fuel combustion, and industrial nylon production.7−10 However, the anthropogenic source and sink budgets are not constrained as well as the natural ones. To determine the quantitative mass balance of sources and sinks, instrumental measurements of atmospheric N2O began in the late 1970s.11 The data permit us to recognize short-term variations of N2O with a precision better than 0.1 parts per billion (ppb). However, instrumental records are too short to decipher the source and sink budgets. In this context, gases occluded in polar ice cores can provide the most direct information on the paleo-atmospheric environment and extend existing atmospheric records to the distant past. Reconstructed greenhouse gas records from ice cores revealed that the atmospheric greenhouse gas levels have increased an unprecedented amount since the Industrial Revolution.3,12−15 Recent high-resolution CO2 and CH4 ice © XXXX American Chemical Society
core records clearly show preindustrial variations of greenhouse gases on millennial to centennial time scales.12−14 However, existing N2O records do not clearly show submillennial variations, partly due to large analytical uncertainties of >3.4 ppb.15−18 The large uncertainty in the N2O analysis is attributed to its relatively small mixing ratio in the atmosphere, which is about 1000 times lower than that of CO2, and a water solubility that is around 45 times higher than that of atmospheric air, which can increase the analytical uncertainty during the air extraction procedure.19 A total of three principally different methods are used to extract air from ice core samples: crushing or milling ice at cold temperatures, known as “dry” extraction;18,20−22 sublimating ice;23 and melt−refreezing ice, known as “wet” extraction.15−17,24 Among these extraction methods, a dry extraction method with a “cheese grater” was adopted to obtain the highest-resolution greenhouse gas records for the last two millennia from the Law Dome ice core.18 The “cheese grater” is composed of a metal cylinder with a perforated plate, and the gas is extracted during the vigorous sliding and shaking motion of the cylinder, which grates ice finely. Macfarling-Meure et al. (2006) measured CO2, CH4, and N2O concentrations and used the standard deviation (1σ) of replicate measurements from 15 depths as the uncertainty. Although the results from the Law Received: October 12, 2017 Revised: December 14, 2017 Accepted: December 16, 2017
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DOI: 10.1021/acs.est.7b05250 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
radiation (VCR) fitting. In the line, the vacuum is provided by a high-efficiency vacuum pump system (High Cube 80, Pfeiffer Vacuum GmbH, Germany), which maintains the pressure at
