A new method for continuous measurements of oceanic and atmospheric Nsub2/subO, CO and COsub2/sub: performance of off-axis integrated cavity output spectroscopy (OA-ICOS) coupled to non-dispersive infrared detection (NDIR)

摘要

A new system for continuous, highly resolved oceanic and atmosphericmeasurements of N₂O, CO and CO₂ is described. The system is basedupon off-axis integrated cavity output spectroscopy (OA-ICOS) and anon-dispersive infrared analyzer (NDIR), both coupled to a Weiss-typeequilibrator. Performance of the combined setup was evaluated by testing itsprecision, accuracy, long-term stability, linearity and response time.Furthermore, the setup was tested during two oceanographic campaigns in theequatorial Atlantic Ocean in order to explore its potential for autonomousdeployment onboard voluntary observing ships (VOS). Improved equilibratorresponse times for N₂O (2.5 min) and CO (45 min) were achieved incomparison to response times from similar chamber designs used by previousstudies. High stability of the OA-ICOS analyzer was demonstrated by lowoptimal integration times of 2 and 4 min for N₂O and CO respectively,as well as detection limits of < 40 ppt and precision better than0.3 ppb Hz–1/2. Results from a direct comparison of the methodpresented here and well-established discrete methods for oceanic N₂Oand CO₂ measurements showed very good consistency. The favorableagreement between underway atmospheric N₂O, CO and CO₂measurements and monthly means at Ascension Island (7.96° S14.4° W) further suggests a reliable operation of the underwaysetup in the field. The potential of the system as an improved platform formeasurements of trace gases was explored by using continuous N₂O andCO₂ data to characterize the development of the seasonal equatorialupwelling in the Atlantic Ocean during two R/V Maria S. Merian cruises. Asimilar record of high-resolution CO measurements was simultaneouslyobtained, offering, for the first time, the possibility of a comprehensive viewof the distribution and emissions of these climate-relevant gases in thearea studied. The relatively simple underway N₂O/CO/CO₂ setup is suitablefor long-term deployment onboard research and commercial vesselsalthough potential sources of drift, such as cavity temperature, and furthertechnical improvements towards automation, still need to be addressed.