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| Titel |
Greenhouse Gases in the South Atlantic: Testing and Automation of Instrumentation for Long-Term Monitoring |
| VerfasserIn |
D. Lowry, R. Fisher, S. Sriskantharajah, M. Lanoisellé, A. Etchells, A. Manning, E. Nisbet |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028983
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| Zusammenfassung |
| Understanding ocean uptake of atmospheric CO2 by the Southern Ocean is important for
modelling of future global warming scenarios, particularly since it was recently proposed that
this sink was reducing (Le Quéré, et al., 2007). To help our understanding of this problem a
new project aims to flask sample air from 5 South Atlantic sites and set up continuous
monitoring at the 2 most accessible of these: Ascension Island and the Falklands. Flask
sample measurements will include CO2 and CH4 mixing ratios and the δ13C measurement of
both of these gases using the rapid continuous flow trace gas analysis system at Royal
Holloway, University of London (RHUL). Routine precisions are ±0.03 per mil
and ±0.05 per mil for CO2 and CH4, respectively (Fisher et al., 2006). A time
series of δ13C in CH4 was maintained for Ascension Island from 2000-2005 and a
time series for methane isotopes commenced for the Falkland Islands in autumn
2007.
To meet the continuous monitoring requirements of the new project, three Picarro G1301
CO2 / CH4 / H2O Cavity Ring Down Spectrometers (CRDS) were installed at RHUL in
October 2008 for testing, calibration and the development of an automated air inlet system
suitable for analysis of calibration gases at the remote sites. Initial testing included calibration
with NOAA calibrated and target gases, validation of the Picarro-defined H2O-correction of
CO2, and derivation of an H2O-correction for CH4. Continuing checks on the H2O correction
are made by having 2 instruments side-by-side taking air from the same inlet, but
one having a combined Nafion / Mg-perchlorate drying system that utilizes the
analysis system exhaust gas for the reverse flow through the Nafion and maintains
water-levels at 0.05% for more than 2 weeks. These instruments are connected to the same
air inlet as a GC measuring CH4 mixing ratio and a LiCor 6252 measuring CO2
mixing ratio at 30-minute and 1-minute intervals respectively. The third CRDS
instrument is connected to a separate airline and can be switched between inlets that are
within 1m of grass lawn at ground level or within 5 m of a large oak tree at canopy
level.
Flow rates vary between the internal pumps of the CRDS instruments, but within the
range 260-300 cc/min when inlet valves are fully opened. Controlling flows below 200
cc/min significantly increases stabilisation time for cylinder gases. Likewise setting outlet
pressures for NOAA and target gases at 4 psi and allowing the instrument pumps to control
flow speeds up stabilization. Currently the instruments are measuring CO2, CH4 and H2O at
5-second intervals. Precisions (1 SD) of NOAA tanks, based on the final 10 minutes of a
30-minute analysis period are better than ±0.03 ppm for CO2 and ±0.3 ppb for
CH4.
Automated inlets and automated data retrieval will be tested during spring, for
deployment on the South Atlantic islands later in 2009.
Fisher, R., Lowry, D., Wilkin, O., Sriskantharajah S. & Nisbet. E.G. (2006)
High-precision, automated stable isotope analysis of atmospheric methane and carbon
dioxide using continuous-flow isotope-ratio mass spectrometry. Rapid Comm. Mass. Spec.
20, 200-208.
Le Quéré, C., C. Rödenbeck, E. T. Buitenhuis, T. J. Conway, R. Langenfelds, A. Gomez,
C. Labuschagne, M. Ramonet, T. Nakazawa, N. Metzl, N. Gillett, and M. Heimann,
Saturation of the Southern Ocean CO2 sink due to recent climate change, Science, 316,
1735-1738, 2007. |
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