Abstract
Abstract. A new system for continuous, highly resolved oceanic and atmospheric measurements of N2O, 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–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 R/V Maria S. Merian cruises. A similar record of high-resolution CO measurements was simultaneously obtained, offering, for the first time, the possibility of a comprehensive view of the distribution and emissions of these climate-relevant gases in the area studied. The relatively simple underway N2O/CO/CO2 setup is suitable for long-term deployment onboard research and commercial vessels although potential sources of drift, such as cavity temperature, and further technical improvements towards automation, still need to be addressed.
Highlights
The assessment of marine emissions of climate-relevant gases has become a critical issue in the attempt to improve our current understanding of the impacts of the ocean on atmospheric composition and chemistry and on climate
The recent development of sensitive analytical techniques based on infrared detection such as cavity ringdown spectroscopy (CRDS), enhanced integrated cavity output spectroscopy (ICOS) and off-axis integrated cavity output spectroscopy (OA-ICOS) has enabled the detection of a number of trace gases in nanomolar concentrations and opened the possibility of obtaining continuous measurements for a variety of applications, including soil and atmospheric monitoring (Baer et al, 2002; Kasyutich et al, 2003; Maddaloni et al, 2006) as well as for surveys of the surface ocean onboard research vessels (Becker et al, 2012; Grefe and Kaiser, 2013) and ships of opportunity (Gülzow et al, 2011, 2013)
The device is based upon the OA-ICOS technology which combines the use of a high-finesse optical cavity with a continuous-wave, narrow-band laser aligned in an off-axis configuration (Fig. 1)
Summary
The assessment of marine emissions of climate-relevant gases has become a critical issue in the attempt to improve our current understanding of the impacts of the ocean on atmospheric composition and chemistry and on climate. The recent development of sensitive analytical techniques based on infrared detection such as cavity ringdown spectroscopy (CRDS), enhanced integrated cavity output spectroscopy (ICOS) and off-axis integrated cavity output spectroscopy (OA-ICOS) has enabled the detection of a number of trace gases in nanomolar concentrations and opened the possibility of obtaining continuous measurements for a variety of applications, including soil and atmospheric monitoring (Baer et al, 2002; Kasyutich et al, 2003; Maddaloni et al, 2006) as well as for surveys of the surface ocean onboard research vessels (Becker et al, 2012; Grefe and Kaiser, 2013) and ships of opportunity (Gülzow et al, 2011, 2013) This represents a great improvement compared to conventional detection techniques (mainly based upon gas chromatography) due to the strongly increased temporal resolution that can be achieved. It was employed during two oceanographic campaigns in the equatorial Atlantic Ocean to test its performance and to explore its potential for autonomous deployment onboard voluntary observing ships (VOS)
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