A new method for continuous measurements of oceanic and atmospheric N<sub>2</sub>O, CO and CO<sub>2</sub>: performance of off-axis integrated cavity output spectroscopy (OA-ICOS) coupled to non-dispersive infrared detection (NDIR)

Abstract

A new system for continuous, highly-resolved oceanic and atmospheric measurements of N₂O</sub>, CO and CO₂ is described. The system is based upon off-axis integrated cavity output spectroscopy (OA-ICOS) and a non-dispersive infrared analyzer (NDIR) both coupled to a Weiss-type equilibrator. Performance of the combined setup was evaluated by testing its precision, accuracy, long-term stability, linearity and response time. Furthermore, the setup was tested during two oceanographic campaigns in the equatorial Atlantic Ocean in order to explore its potential for autonomous deployment onboard voluntary observing ships (VOS). Improved equilibrator response times for N₂O (2.5 min) and CO (45 min) were achieved in comparison to response times from similar chamber designs used by previous studies. High stability of the OA-ICOS analyzer was demonstrated by low optimal integration times of 2 and 4 min for N₂O and CO respectively, as well as detection limits of < 40 ppt and precision better than 0.3 ppb Hz^&minus;1/2. Results from a direct comparison of the method presented here and well-established discrete methods for oceanic N₂O and CO₂ measurements showed very good consistency. The favorable agreement between underway atmospheric N₂O, CO and CO₂ measurements and monthly means at Ascension Island (7.96° S 14.4° W) further suggests a reliable operation of the underway setup in the field. The potential of the system as an improved platform for measurements of trace gases was explored by using continuous N₂O and CO₂ data to characterize the development of the seasonal equatorial upwelling in the Atlantic Ocean during two <i>R/V Maria S. Merian</i> cruises. A similar record of high-resolution CO measurements was simultaneously obtained offering for the first time the possibility of a comprehensive view on the distribution and emissions of these climate relevant gases on the area. The relatively simple underway N₂O/CO/CO₂ setup is suitable for long-term deployment on board of research and commercial vessels although potential sources of drift such as cavity temperature and further technical improvements towards automation still need to be addressed

Nitrous oxide during the onset of the Atlantic cold tongue

“…Along‐track measurements of dissolved and atmospheric N 2 O were performed with an underway system based upon off‐axis‐integrated cavity output spectroscopy (OA‐ICOS) coupled to a shower‐type equilibrator, as described by Arévalo‐Martínez et al . []. Seawater was continuously drawn from about 6.5 m depth either by using the ship's supply system (centrifugal pump; MSM 18‐2) or a submersible pump (LOWARA, Xylem Inc., Germany) installed at the ship's moonpool (MSM 18‐3), and it was conducted through the equilibration chamber at a rate of 2 L min −1 .…”

“…In order to minimize interferences due to water vapor on the sample stream, the air was dried before entering the analyzer by using a cold trap (KWG Isotherm) followed by a Nafion ® membrane. Control measurements of two reference gases (mixing ratios: 360 and 746 ppb N 2 O) prepared by Deuste Steininger GmbH (Mühlhausen, Germany) and calibrated at the Max Planck Institute for Biogeochemistry (Jena, Germany) against the World Meteorological Organization (WMO) standard scale, were used to postcorrect our data for instrumental drift (maximal drift ∼1%) [see Arévalo‐Martínez et al ., ]. Data reduction and calibration procedures were done according to Arévalo‐Martínez et al .…”

N2O Emissions From the Northern Benguela Upwelling System

“…Along-track measurements of N 2 O (resolution ∼0.2 km) were conducted with an autonomous setup which combined the methods by Pierrot et al (2009) and Arévalo-Martínez et al (2013). In brief, a GO CO 2 system (General Oceanics Inc., USA) was coupled to an RMT-200 N 2 O/CO analyzer (Los Gatos Research Inc., USA), such that the VALCO valve (Vici ® ) of the former automatically switched between seawater, atmosphere, and reference gases measurement circuits.…”

“…Data reduction and calibration as well as the calculation of N 2 O concentrations followed those from Arévalo-Martínez et al (2013) and references therein. Atmospheric N 2 O data were linearly interpolated over the time of the cruises and then used for the computation of the equilibrium concentrations, saturations, and sea-air fluxes of N 2 O.…”

Extreme N&lt;sub&gt;2&lt;/sub&gt;O accumulation in the coastal oxygen minimum zone off Peru

“…Our standard gases were calibrated against NOAA primary standards at the Max Planck Institute for Biogeochemistry in Jena, Germany, if the standard gas concentrations were within the calibration range of the NOAA gases. Gases with N 2 O concentrations outside the NOAA calibration range were internally calibrated using an LGR N 2 O/CO analyser (Los Gatos Research, Mountain View, CA, USA), which was proven to have a linear response and minimal drift within the calibration range (Arévalo-Martínez et al, 2013). The N 2 O concentration in the samples was calculated according to Walter et al (2006) using the solubility function of Weiss and Price (1980 …”