An intercomparison of oceanic methane and nitrous oxide measurements

Abstract

Large-scale climatic forcing is impacting oceanic biogeochemical cycles and is expected to influence the watercolumn distribution of trace gases, including methane and nitrous oxide. Our ability as a scientific community to evaluate changes in the water-column inventories of methane and nitrous oxide depends largely on our capacity to obtain robust and accurate concentration measurements that can be validated across different laboratory groups. This study represents the first formal international intercomparison of oceanic methane and nitrous oxide measurements whereby participating laboratories received batches of seawater samples from the subtropical Pacific Ocean and the Baltic Sea. Additionally, compressed gas standards from the same calibration scale were distributed to the majority of participating laboratories to improve the analytical accuracy of the gas measurements. The computations used by each laboratory toPublished by Copernicus Publications on behalf of the European Geosciences Union. 5892 S. T. Wilson et al.: An intercomparison of oceanic methane and nitrous oxide measurements derive the dissolved gas concentrations were also evaluated for inconsistencies (e.g., pressure and temperature corrections, solubility constants). The results from the intercomparison and intercalibration provided invaluable insights into methane and nitrous oxide measurements. It was observed that analyses of seawater samples with the lowest concentrations of methane and nitrous oxide had the lowest precisions. In comparison, while the analytical precision for samples with the highest concentrations of trace gases was better, the variability between the different laboratories was higher: 36 % for methane and 27 % for nitrous oxide. In addition, the comparison of different batches of seawater samples with methane and nitrous oxide concentrations that ranged over an order of magnitude revealed the ramifications of different calibration procedures for each trace gas. Finally, this study builds upon the intercomparison results to develop recommendations for improving oceanic methane and nitrous oxide measurements, with the aim of precluding future analytical discrepancies between laboratories.

Nitrous oxide (N<sub>2</sub>O) and methane (CH<sub>4</sub>) in rivers and estuaries of northwestern Borneo

“…These comparably high relative errors most probably resulted from the long storage time (6-7 months after sampling) for some of the samples. The higher mean measurement error of the CH 4 samples (compared to the N 2 O measurements) was attributed to the fact that CH 4 samples are more sensitive to storage time than N 2 O samples (Wilson et al, 2018).…”

A Harmonized Nitrous Oxide (N2O) Ocean Observation Network for the 21st Century

“…An important next step was the fundamental work on N 2 O solubility in seawater (Weiss and Price, 1980), which promoted the development of equilibration techniques for high-resolution surveys of the surface ocean (Weiss et al, 1992) (see section "Continuous Surface Measurements") and water column N 2 O (Butler et al, 1989;Butler and Elkins, 1991). Today, GC-ECD analysis, coupled to headspace equilibration or purge-and-trap techniques, is used by the majority of laboratories worldwide for quantifying dissolved N 2 O in discrete seawater samples (Wilson et al, 2018). Even so, mass spectrometric analysis of N 2 O is becoming increasingly wide-spread (Capelle et al, 2015;Babbin et al, 2017;Bourbonnais et al, 2017) and may become increasingly important in the future.…”

“…An inter-laboratory comparison of oceanic N 2 O measurements was recently conducted by the Scientific Committee on Oceanic Research (SCOR) international Working Group (WG) 143 2 . Discrete water samples from the subtropical Pacific Ocean and the Baltic Sea were distributed to participating laboratories (Wilson et al, 2018) for a comparison of accuracy and precision. The samples represented a range of N 2 O concentrations, from low concentrations in the oligotrophic open ocean to high concentrations in highly productive and suboxic coastal waters.…”

“…The samples represented a range of N 2 O concentrations, from low concentrations in the oligotrophic open ocean to high concentrations in highly productive and suboxic coastal waters. Recommendations arising from the inter-comparison include (Wilson et al, 2018):…”

River inflow dominates methane emissions in an Arctic coastal system

“…Standard curves were prepared using a Praxair certified gas standard tank (±5% accuracy for CH4 and N2O) and airequilibrated water samples were run daily to verify the accuracy of the system. The laboratory has been involved in international intercalibration exercises for oceanic CH4 and N2O measurements (Wilson et al, 2018). Duplicate samples were collected and duplicates with poor precision, as well as samples with clear evidence of air contamination were eliminated from the data set.…”