Nitrous oxide in the surface layer of the tropical North Atlantic Ocean
along a west to east transect
Sylvia Walter, Hermann W. Bange, and Douglas W. R. Wallace
Forschungsbereich Marine Biogeochemie, Leibniz-Institut fu¨r Meereswissenschaften (IFM-GEOMAR), Kiel, Germany
Received 10 March 2004; accepted 16 July 2004; published 5 October 2004.
[1] Nitrous oxide (N2O) was measured during the first
German SOLAS (Surface Ocean – Lower Atmosphere
Study) cruise in the tropical North Atlantic Ocean on board
R/V Meteor during October/November 2002. About 900
atmospheric and dissolved N2O measurements were
performed with a semi-continuous GC-ECD system
equipped with a seawater-gas equilibrator. Surface waters
along the main transect at 10
N showed no distinct
longitudinal gradient. Instead, N2O saturations were highly
variable ranging from 97% to 118% (in the Guinea Dome
Area, 11
N, 24
W). When approaching the continental shelf
of West Africa, N2O surface saturations went up to 113%.
N2O saturations in the region of the equatorial upwelling (at
0–1.5
N, 23.5–26
W) were correlated with decreasing sea
surface temperatures and showed saturations up to 109%.
The overall mean N2O saturation was 104 ± 4% indicating
that the tropical North Atlantic Ocean is a net source of
atmospheric N2O. INDEX TERMS: 4820 Oceanography:
Biological and Chemical: Gases; 0312 Atmospheric Composition
and Structure: Air/sea constituent fluxes (3339, 4504); 0322
Atmospheric Composition and Structure: Constituent sources and
sinks. Citation: Walter, S., H. W. Bange, and D. W. R. Wallace
(2004), Nitrous oxide in the surface layer of the tropical North
Atlantic Ocean along a west to east transect, Geophys. Res. Lett.,
31, L23S07, doi:10.1029/2004GL019937.
1. Introduction
[2] Nitrous oxide (N2O) is an important atmospheric
trace gas because it influences, directly and indirectly, the
Earth’s climate to a significant degree: In the troposphere, it
acts as a greenhouse gas with a relatively long atmospheric
lifetime [Intergovernmental Panel on Climate Change
(IPCC), 2001] whereas in the stratosphere it is the major
source for nitric oxide radicals, which are involved in one of
the main ozone reaction cycles [World Meteorological
Organization, 2003]. Published source estimates indicate
that the world’s oceans play a major role in the global
budget of atmospheric nitrous oxide [IPCC, 2001]. Gener-
ally, oligotrophic areas seem to be near equilibrium with the
atmosphere, whereas coastal and equatorial upwelling areas
show enhanced N2O concentrations [Nevison et al., 1995;
Suntharalingam and Sarmiento, 2000]. Here we present
about 900 measurements of dissolved and atmospheric N2O
during the first German SOLAS (Surface Ocean – Lower
Atmosphere Study) cruise. It is the first high-resolution data
set of N2O in the tropical North Atlantic Ocean along a
West to East transect and it is complementary to previous
N2O measurements of Oudot et al. [1990, 2002] and Weiss
et al. [1992].
[3] The cruise took place on board R/V Meteor (expedi-
tion no. M55) from Willemstad (Curac¸ao, Netherl. Antilles)
to Douala (Cameroon) from 12 October to 17 November
2002. The cruise track consisted of two main transects:
(i) The West to East transect along 10–12
N covering the
oligotrophic tropical North Atlantic Ocean and the conti-
nental shelf area of the West African coast off Guinea
Bissau and (ii) a shorter West to East transect along the
equatorial upwelling (Figure 1).
2. Method
[4] N2O was determined with a gas chromatograph
equipped with an electron capture detector. Further details
of the analysis system are described in Bange et al. [1996].
A series of measurements of atmospheric N2O and N2O in
seawater-equilibrated air followed by two standards was
repeated every 50 min. Mixtures of N2O in synthetic air
were used to obtain two-point calibration curves. The
mixtures used contained 311.7 ± 0.1 and 346.5 ± 0.2 ppb
N2O, respectively. These are gravimetrically prepared gas
mixtures (Deuste Steininger GmbH, Mu¨hlhausen Germany)
and have been calibrated against the NOAA (National
Oceanic and Atmospheric Administration, Boulder, Co.)
standard scale in the laboratories of the Air Chemistry
Division of Max Planck Institute for Chemistry Mainz,
Germany. The precision, calculated as the ratio of the
standard deviation of the atmospheric measurements and
the mean atmospheric mixing ratio, was 0.8%.
[5] Seawater was pumped continuously from a depth of
4 m into a shower-type equilibrator developed by R. F.
Weiss (Scripps Institution of Oceanography, La Jolla, Ca.).
N2O concentrations (C, in nmol L

1) were calculated by
applying the solubility equation of Weiss and Price [1980]:
C ¼ b T ; Sð Þx0 P;
where x0 is the measured N2O dry mole fraction, P is the
atmospheric pressure, and b is the solubility coefficient,
which is a function of the water temperature (T) and salinity
(S). Time series of seawater temperature (SST), salinity,
wind speed, and atmospheric pressure were obtained from
the ship’s records. Differences between the seawater
temperature at the seawater intake and the continuously
recorded water temperature in the equilibrator were
corrected:
Cw ¼ C b Teq


=b SSTð Þ
with b(SST) and b(Teq) representing the N2O solubility at
seawater temperature and water temperature inside the
GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L23S07, doi:10.1029/2004GL019937, 2004
Copyright 2004 by the American Geophysical Union.
0094-8276/04/2004GL019937$05.00
L23S07 1 of 4

equilibrator at the time of the measurement, respectively.
N2O saturations (Sat) in % (i.e., 100% = equilibrium) were
calculated as follows:
Sat ¼ 100Cw=Ca
where Ca is the equilibrium concentration of dissolved N2O
based on the actual measurement of ambient air (see above).
The mean relative errors of the N2O concentrations and
saturations were calculated to be 1.2% and 1.6%,
respectively (details of the error propagation computation
are given by Bange et al. [2001]).
3. Results and Discussion
[6] The mean atmospheric N2O dry mole fraction was
318 ± 3 ppb. Due to the seasonal northward shift of the
Intertropical Convergence Zone to about 10
N, the origin of
the air masses sampled during the cruise were from both the
northern and the southern hemisphere. 4-days air mass back
trajectories (provided by the German Weather Service,
Offenbach, Germany) indicated that air masses sampled at
latitudes south of 7
N originated from the southern hemi-
sphere. Based on this classification we computed mean N2O
values for northern and southern hemisphere air masses of
319 ± 3 ppb and 317 ± 2 ppb, respectively. The observed
atmospheric values are in agreement with N2O measure-
ments at the baseline monitoring stations Ragged Point,
Barbados and Cape Grim, Tasmania. Monthly mean values
were 317 ppb (Cape Grim) and 318 ppb (Ragged Point) for
October/November 2002. These values were taken from the
Advanced Global Atmospheric Gases Experiment
(AGAGE) data set (updated version from November
2003) [Prinn et al., 2000]. AGAGE data are available
from the anonymous ftp site ftp://cdiac.esd.ornl.edu
(subdirectory pub/ale_gage_Agage/Agage/gc-md/monthly)
at the Carbon Dioxide Information Analysis Center in
Oak Ridge, Tennessee.
[7] N2O saturations along the main cruise track ranged
from 97% to 118% and the SST was generally between 27
and 30
C (Figure 2). Since the main cruise track was
located between the eastward flowing North Equatorial
Countercurrent (NECC) and the westward flowing North
Equatorial Current (NEC) [Stramma and Schott, 1999], we
crossed several times meandering waters of different origins
causing a high variability of the N2O saturation: Low N2O
saturations of about 100% observed around 24 Oct., 27–
28 Oct., and 2 Nov. were generally associated with decreases
in salinity (Figure 2). This results from the retroflection of the
North Brazil Current, which advects Amazon plume waters
(with low N2O, see below) eastward into the NECC
[Fratantoni andGlickson, 2002]. Freshwater influences were
observed twice: First, at around 50
E (19 Oct., Figure 2)
when we crossed the northern boundary of the Amazon
river plume (minimum salinity 32.14) and second, on the
continental shelf off West Africa where we measured a drop
in salinity down to 31.30 (5–6 Nov., Figure 2). N2O
saturations were not enhanced in the Amazon River plume,
whereas an increase in N2O saturations up to 113% were
observed on the West African shelf. The low N2O satura-
tions in the Amazon River plume were attributed to the fact
that N2O-rich waters from the Amazon River are N2O-
depleted because of outgasing to the atmosphere and
mixing with near-equilibrium oceanic waters while distrib-
uted to the North [Oudot et al., 2002]. The high N2O
saturations on the continental African shelf might result
from N2O-rich riverine waters or groundwater seepage, but
not from coastal upwelling as indicated by the uniform
SSTs. N2O saturations up to 118% were observed in the
area of the Guinea Dome at 11
N, 24
E (3–4 Nov.,
Figure 2) which is well-known for pronounced Ekman
upwelling [Siedler et al., 1992; Signorini et al., 1999]. In
the equatorial region (0–1.5
N, 28–30 Oct., Figure 2)
SSTs dropped well below 27
C and were associated with
enhanced N2O saturations (up to 109%). We found a good
Figure 1. Cruise track of M55 in October–November
2002. N2O measurements were started 17 October and were
finished 14 November. Areas of special interest discussed in
the text are marked.
Figure 2. Salinity, seasurface temperature (SST), N2O
saturation, wind speed in 10 m height (u10), and N2O flux
density during M55. Area of special interest discussed in
the text are marked (see Figure 1): A, equatorial upwelling;
B, Guinea Dome; C, shelf off West Africa (water depths
<200 m).
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