Continuous O2/Ar measurements in surface sea
water by membrane inlet mass spectrometry
Jan Kaiser and Michael L. Bender
Department of Geosciences, Princeton University, Princeton, New Jersey, USA
e-mail: [email protected]
Ar
Abstract
Introduction
Simultaneous O2 and Ar measurements in
surface seawater allow one to estimate
oceanic O2 outgassing and to probe oceanic
productivity. Previously, the sea-to-air
flux of O2 has been estimated from temperature-based
oceanic
heat
budgets
and
modeled gas fluxes, often considering only
solubility effects due to temperature changes and neglecting variations of biological
O2 production. Whereas measuring the small
net annual average air-sea O2 flux on a
global scale might be out of the scope of
the present project (Fig. 2), it is possible to determine the individual terms,
i.e. the O2 flux to the atmosphere in summer and the return flux (reoxygenation/
ventilation) in fall and winter. We want to
make the first long continuous measurements
of biological O2 supersaturation and seaair fluxes which would provide constraints
on the net productivity of ocean systems
over very large scales. Measurements will
involve a host of novel techniques, in
particular membrane inlet mass spectrometry
and O2 optodes.
Biological O2 fluxes are due to enhanced photosynthetic production that causes O2 supersaturation. In addition to that, O2 supersaturation is affected by physical processes
such as bubble entrainment in breaking waves
(Fig. 1). O2/Ar ratios allow one to partition
the outgassing into physically- and biologically-forced flux components. Ar is used to
correct for physical supersaturation, because
it is an inert gas, but has similar solubility
characteristics as O2.
Fig. 2: Modeling of atmospheric potential oxygen (APO ≈ CO2 + O2) predicts significant O2
outgassing in the tropical Pacific.
atmosphere
CO2
O2
phytoplankton
CO2
mixed layer  100 m
thermocline
Results from MIMS
Fig. 7: O2/Ar ratios of distilled water (30 ºC).
Fig. 3: O2/Ar ratios in the upper tropical
Pacific record on processes such as net
production, subduction and diapycnal mixing on
regional scales.
Faraday cup and channeltron have about equal
precision (10-3), but the O2/Ar ratios are
strongly influenced by changing temperature and
water levels in the vacuum manifold.
Fig. 8: CO2 measurements with precisions <10-2
(or 10-3 averaged over 10 min) are possible.
0
Fig. 9: Good precision on N2 isotope ratio
1
0.95
O2/Ar (Faraday cup)
O2/Ar (channeltron)
10 min mvg. avg., O2/Ar (channeltron)
10 min mvg. avg., O2/Ar (Faraday cup)
18.8
0.8
150
0.6
200
0.4
.4
© M.B. Hendricks
-8
-6
-4
-2
[O2]/[O
]sat sat)
([O2]/[Ar])/([O
2/[Ar]
2]sat
0
2
4
6
8
37.5
18.7
37.3
18.6
37.1
18.5
36.9
18.4
rel. std. deviation 
4 ºC
18.3
1.4 ‰
36.7
1.2 ‰
36.5
18.2
36.3
18.1
36.1
18.0
35.9
H2O
temperature (vacuum manifold)
17.9
17.8
08:00
12:00
16:00
20:00
00:00
time of day
35.7
04:00
08:00
35.5
16:00
12:00
Latitude at 155°W
350
The measurements involve a quadrupole mass spectrometer (QMS) coupled via a membrane inlet
(Fig. 4) to a continuous supply of surface sea
water from an underwater sampling system. We plan
to install the QMS on the NOAA vessel “Ka’
Imimoana” (Fig. 6) that is regularly servicing
the TAO mooring array in the tropical Pacific
(Fig. 5) (TAO = Tropical Ocean Atmosphere project, http://www.pmel.noaa.gov/tao/). The O2/Ar
ratios of the sample will be calibrated regularly
to a supply of air equilibrated water which will
allow us to correct for discrimination effects in
sample inlet and analyzer (cf. Figs. 7-9).
In addition to O2 and Ar (Fig. 7) , N2 and CO2
measurements are also possible (Figs. 8+9). We
300
250
200
150
N
Methods
are currently investigating whether it will be
possible to replace conventional pCO2 instruments
(showerhead equilibrators with NDIR detection of
CO2 in the equilibrated headspace) by the membrane inlet mass spectrometer. Main issues here
are CO2 and N2O production from O2 and residual
carbon in the ion source and from N2 + O2 ion
molecule reactions, respectively.
100
rel. std. deviation  6.8 ‰
50
Another scope of the project is to investigate
whether the mass-spectrometric measurements can
be used not only to determine O2/Ar ratios, but
also the gas concentrations themselves. The
currently achieved reproducibility suggests that
this may be possible. Intercalibrations of the
mass spectrometer measurements are going to
comprise
Winkler
titrations,
oxygen
optode
measurements and isotope dilution techniques.
0
08:00
12:00
16:00
20:00
00:00
04:00
time of day
08:00
12:00
16:00
146
145
144
143
142
14
141
140
139
rel. std. deviation  4.6 ‰
138
08:00
12:00
16:00
20:00
00:00
time of day
04:00
08:00
12:00
16:00
© NOAA
References
© A. Milligan
© NOAA
Gruber N, Gloor M, Fan SM, Sarmiento JL (2001):
Air-sea flux of oxygen estimated from bulk
data:
Implications
for
the
marine
and
atmospheric oxy-gen cycles, Global Biogeochem.
Cycles, 15, 783-803.
Kana TM, Darkangelo C, Hunt MD, Oldham JB, Bennett GE, Cornwell JC (1994): Membrane inlet
mass spectrometer for rapid high-precision
determina-tion
of
N2,
O2,
and
Ar
in
environmental
water
samples,
Analytical
Chemistry, 66, 4166-4170.
O2/Ar (channeltron)
100
O2/Ar (Faraday cup)
Depth (m)
measurements opens up the possibility of
calibrations via isotope dilution.
1
50
Gruber et al. (2001)
Ar
O2