 |
| Titel |
Continuous measurements of nitrous oxide, carbon monoxide, methane and carbon dioxide in the surface ocean with novel laser-absorption analysers |
| VerfasserIn |
Jan Kaiser, Imke Grefe, Natalie Wager, Dorothee C. E. Bakker, Gareth A. Lee |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250084236
|
|
|
|
|
|
| Zusammenfassung |
| In recent years, improvements in spectroscopic technology have revolutionised atmospheric
trace gas research. In particular, cavity-based optical absorption analysers allow
determination of gas concentrations with high frequency, repeatability, reproducibility and
long-term stability. These qualities make them particularly suitable for autonomous
measurements on voluntary observing ships (VOS). Here, we present results from three of the
first deployments of such analysers on research ships, as a first step towards VOS
installations.
Los Gatos off-axis ICOS (Integrated Cavity Output Spectroscopy) analysers were
used to measure nitrous oxide (N2O), carbon monoxide (CO), methane (CH4) and
carbon dioxide (CO2) mixing ratios in ocean surface water during research cruises in
2010, 2011 and 2012. The analysers were coupled to an equilibrator fed by the
scientific seawater supply in the ship’s laboratories. The equilibrator measurements
were alternated with regular measurements of marine air and calibrated standard
gases.
Short-term precision for 10 s-average N2O mole fractions at an acquisition rate of 1
Hz was better than 0.2 nmol mol-1. The same value was achieved for duplicate
measurements of a standard gas analysed within 1 hour of each other. The response time to
concentration changes in water was 142-203 s, depending on the headspace flow
rate.
During the first deployment on the AMT20 cruise (Atlantic Meridional Transect,
Southampton to Punta Arenas, 12 October to 25 November 2010), we unexpectedly found the
subtropical gyres to be slightly undersaturated in N2O, implying that this region acted as a
sink for this greenhouse gas. In contrast, the equatorial region was supersaturated and
a source of nitrous oxide to the atmosphere. Mean sea-to-air fluxes were overall
small and ranged between -1.6 and 0.11 μmol m-2 d-1 (negative fluxes imply an
net uptake by the ocean). Despite the good short-term repeatability, significant
calibration drift occurred between the six-hourly calibration intervals. We have therefore
repeated the observations during the AMT22 cruise (Southampton to Punta Arenas, 10
October to 24 November 2012) and will present the results together with the 2010
data.
The second deployment during the D366 Round Britain ocean acidification
cruise (6 June to 9 July 2011) showed virtually no day-to-day drift, based on the
calibration gases and marine air analyses. Preliminary analyses of the data show that
CH4 and N2O were supersaturated in the Skagerrak region, presumably due to the
influence of Baltic Sea water, and in coastal areas. Phytoplankton blooms show
evidence of CO2 draw-down. CO is extremely supersaturated (up to 50 times),
which can be attributed to photochemical breakdown of dissolved organic carbon
compounds.
For the more recent deployments, a CO2/CH4 and an N2O/CO analyser were
successfully operated in series, off of a single equilibrator feed. No leaks or other
problems occurred during these deployments, which shows that such a configuration
would be ideal for VOS installations as part of the ICOS observational network. |
|
|
|
|
|
|