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Nitrous oxide in the ocean and the marine atmosphere
Oeocllintica et Cosmochimica Acta,1663,To127,rr. 949to 055.Peraumon PressLtd. Printed inNortllarn Ireland
Nitrous oxide in the ocean and the marine atmosphere H. CRAIG and L. I. GORDON Department of Earth Sciences and Scripps Institution University
of California, (Received
of Oceanography La Jolla, California
20 Mcw&
1963)
Abgtr&--Nitrous oxide has been found in surface and deep ocetm waters of the S. Pacific. The observed concentrations are lower than expected for solubility equilibrium with concentraLower atmospheric concentrations tions previously reported in the continent&l &tmosphere. than previous continental data have been found in N. Pacific air. These variations indicate that X,0 should be an important tracer gas for atmospheric studies.
THE presence of trace amounts of N20 in the atmosphere has been established from studies of the solar spectrum (ADEL, 1951 and references therein); the most recent data indicate a concentration of the order of 0.5 ppm by volume (ppmv), if one assumes the gas to be uniformly distributed with altitude. The direct analytical determination of N20 is very difficult because N20, being isosteric with CO,, has almost identical physical properties including vapor pressure, and, in addition, the two gases have identical molecular masses of 44, 45, and 46. Thus N20 cannot be separated from CO2 by fractional distillation, is not readily distinguishable from CO2 in the mass spectrometer, and its almost total lack of chemical activity precludes a direct chemical analysis. SLOBOD and KROGH (1950) analysed concentrates from fractional condensation of air in liquid nitrogen, using the mass spectrometer after treating the gases with solid reagents to remove the CO,. Working with residual gas samples of about 3 microliters (1) STP they found a mean value of O-5 ppmv for the N20 concentration in nine air samples from Texas and one from Wyoming. They give no details of their observed peak ratios, and, as they point out, their condensation traps were not constructed to prevent loss of solid particles entrained in the air stream; thus it is not possible to evaluate the accuracy of their measurements. The little information available on the geochemistry of N20 has been summarized by HUTCHINSOW(1950) and a detailed study of N20 production rates in soils has been made by ARNOLD (1954). N20 is certainly produced in soils by bacterial activity; an additional production may take place in the atmosphere by the reaction of nitrogen and ozone, and the gas is destroyed in the atmosphere by photochemical dissociation and/or reaction with oxygen atoms. If the production of N,O in soils is the dominant source of this gas in the atmosphere, N,O should be a very important gas for atmospheric studies, as the source function should show a pronounced peak in the continental latitudes of the northern hemisphere and a smaller peak at low latitudes in the southern hemisphere. Thus if the time constant, relat,ive to photochemical destruction, is comparable to cross-hemispheric mixing times, the southern hemisphere air may contain much less N,O than the northern because of its smaller land area. Moreover, the southern oceans would be expected to contain smaller amou~lts of the gas than surface waters of the northern oceans, so that the gas may be a good tracer for the oceans.
H. CRAIGand L.I.
950
GORDON
It is evident that the geochemical cycle of N,O is of interest in many connections if accurate analytical work can be done. We have developed a gas chromatographic method for the separation of N,O and CO, and the quantitative measurement of N,O in gas samples. Using this method, we have established the presence of N,O in surface and deep ocean waters, and in marine air. Sea water $~~~~~~ Dissolved gases were extracted from sea water sa,mples using an a’ll-glass high vacuum line operated aboard ship. The samples were collected and processed on 70’5 0
"
20
30
40 1
50
60 till1
I
l,l'l'-
0
0
IO
0 0
I
0 0 0
0
I
I
0 0
0
0 ---
0
5-
0
0 LATITUDE
tSOUTHl
Fig. 1. Location, depth, and group combinations, of South Pacific samples analysed for N,O. Station longitudes: at 64-2”s. 166%‘; at 60*2”S, 171.5”W; at 59.3’5, 169.5”W; between 20 and 4O”S, a direct track from 38.5%, 164”W, to 24.5”5,15~j~W; at 4-4 to 5.?“S, 149.5”W. Surface samples shown were taken at 10 m.
the research vessel ARGO by the senior author, during the Scripps Institution’s Expedition Monsoon, from t,he Antarctic circle to San Diego (January-April, 1961). Two-liter water samples were collected in plastic Van Dorn hydrographic bottles, The and about 1250 ml of each sample were let into the vacuum line for extraction. filled sample bottles were suspended on gimbals to prevent agitation, and each sample was processed within 15 minutes after arriving at the surface. The sea water was acidified in vacuum with 2 ml of concentrated H,PO4, boiled, stirred, and pumped with a Toepler pump which collected the gases in a 300 ml sample tube closed by a silicone-greased stopcock and stored with several cm of mercury above the bore. Laboratory tests established that the procedure collected more than 99.95Oh of nitrogen and oxygen, and of CO,, which comes off more slowly because of its chemistry; thus it was certain that all the N,O was collected, Water
Nitrous oxide in t;he ocean and the marine atmosphere
951
volumes were measured volumetrically to about 0.20/o. Total processing time for each sample was about 1 hour. Figure 1 shows the location and depth of the 2% samples, all in the South Pacific, in which N,O was measured. In order to obtain a sufficient quantity of gas to analyse, the CO,--N,O fractions were combined into three groups as shown in the figure; thus only three analyses could be made. In the laboratory? each gas sample was dried over CO, slush traps, two 10% fractions were split off for other analyses, and the remaining gas was cycled through a liquid N, trap to remove the CO,-N,O fraction. These fractions were then combined into the three groups shown in Fig. 1, so that each group sample contained on the order of 500 ml of CO,, and (as it turned out) about 2A of N,O. Each of the group samples was condensed into a flask containing NaOH on solid suspension (Ascarite), where the CO, was removed by reaction. The residual gas was dried in a slush trap, condensed into a breakoffski, a,nd transferred to the gas Blank tests were run with 500 ml of pure CO, prepared from ~hromatograph. reagent CaCO, acidified in vacuum with H,PO,, and with no gas; these showed that no detectable N,O was produced by this treatment. The samples were analysed on a Beckman GC-1 gas chromatograph using an eight-foot silica gel column (28-200 mesh) at room temperature, with He carrier gas at a pressure of 40 pounds and a flow rate of 83 ml/min. The thermal conductivity filament was operated at 350 mA for maximum sensitivity, and the instrument was packed in fibre glass for temperature stability of the detector. Samples were condensed into a U-tube which could be switched into the helium flow. Peak areas were read with a mechanical integrator unit. The N,O sensitivity, found by calibration with 33 samples of pure gas ranging from 3.6 to 27 iz (NTP), was 2.6 5 O-2 counts/& taking one full sweep of the integrator pen as 10 units. Calibration was done by partitioning gas volumes in calibrated glassware, and the indicated error represents the limits of the observed scatter. The counting rate was linearly proportional to sample volume with a zero intercept at zero volume. RetBention times and resolution of the two gases, at the 2% 2 sample level, are shown in Fig. 2; complete resolution was obtained at these volumes and no other peaks were ever observed in our experiments. The lower part of Fig. 2 shows the actual recorder trace for Group I after CO, removal. The small CO, peak represents a residual CO, volume of 0.4 L, showing that the NaOH treatment reduced the CO, concentration by 106. The CO, peaks in the other groups were even smaller. The results obtained for the three groups of sea water samples are given in TabIe 1. The data are subject to an uncertainty of about 125% due to random integrator counting errors from instrumental drift, and an additional 18% absolute uncertainty from calibration errors. The mean value for the N20 content of sea water is 0.2 ppmv (NTP) from the present data, with an uncertainty probably less than 30%. ,Warine air samples Four 5-1. flasks of marine air from the North Pacific were available for analysis_ A small amount of the air had been taken for CO, analysis previously, so that only the ratio N,O/CO, could be measured. The samples were passed through a liquid
H.
952
CRAIG
and L.1,
GORDON
N, trap for removal of the CO,-N,O fractions, and the fractions were then combined into two samples, each representing 10 1. of air from the two locations shown in Table 1. The trap had a fritted glass filter in the outlet arm, and experiments had established that it removed all condensible gas from the air. However, as the Aasks were pumped out, the N,O partial pressure in the air stream decreased until at some
5--
I
LAB
MIXTURE
hip
O-
I\
Ih
I
I
I
I
CO,
I
I
5-
MONSOON GROUP
I
Ih 4
v
SEA
WATER
I AFTER
CO*
NoOH
SAMPLES
TREATMENT
I
I
I
I
I
I
I
I
I
I
I
I
20
22
24
26
28
30
32
34
36
38
40
42
AFTER
IhiJECTlON
TIME
(MINUTESI
Fig. 2. Gas chromatograms. Top: synthetic mixture, 281 each of X,0 and CO,, showing
resolution and retention ocean samples after
times.
Uottom:
chrometogram
from Group I of
NaOH treatment for removal of CO,.
point it became equal to its vapor pressure over Iiquid N,; consideration of the vapor pressure equation for N,O indicates that not more than 5% of the gas could have been lost by this effect. The NzO-CO2 peaks were measured directly on the condensed fractions. Calibration for CO, at the 3 ml level gave a counting rate of 2.51 counts/A (NTP). Some loss of precision was experienced due to forward tailing of the much larger CO, peak, so that about 40% of the NeO peak had to be estimated. The CO2 concentration in the original air samples was 317 ppmv, as measured by C. D. KEELING with the infra-red analyser; the resulting figures for N,O concentration
Nitrous oxide in the ocean and the marine atmosphere
953
are shown in Table 1. An additional uncertainty of about 10% affects the atmospheric data because of partial loss of the peak; the digerence in the two figures is not signi~~ant, and the mean value of 0.3 ppmv is probably accnrate to -&3+f. Table 1, N,O analyses of sea water and marine air. Estimated accuracy is -&25?; random instrumental errors, and an additional &8% uncertainty in absolute amounts due to calibration errors No. of
samples
Mean T”C.
Total N._,O fomd (A, KTI’)
Group I
9
3.5
L-73
0.18
Group II
9
1.3
I.56
0.16
Group III
10
9.7
2-77
0.27
MARINEAIR 28”N 122”M7 ~4~15~61)
Total CO,
(ml NTP)
Total N,O
(n NTP)
%0/C% t %A
N,O/A.ir(ppmv) -
.--
2.96
2-4
O*RO
@45
S48
3.1
1.00
0,32
32%5”N, 118QYW (4/17P)
In Tabie 1 we show the weighted mean temperatures for the three groups of sea water samples. The atmospheric concentrations of N,O required for equilibrium with the observed sea water concentrations can be calculated from data on the solubilit8y of NsO in NaCl solutions (MARKHAMaud KOBE, 1941), assuming sea water to be au 0.97 l% NaCI solution. The atmospheric concentrations required for equilibrium with Groups I, II, and III, are respectively O-15, O-12, and 0.30, at the mean water temperatures of these groups. These values are considerably lower than the N. American data of SLOBOD and KROGH(1950), and for Groups I and 11 the required values are even lower than our data on North Pacific marine air. The time constant for N,O destruction in the atmosphere has been estimated in and DONDES(1954) assumed a steady state relative to productwo ways. HAEETECK tion from N, and OS, and calculated the number of N20 molecules produced in photo~hemi~al equilibrium with 0, and 0. They assumed biological production was insignificant, so that the photochemioal production rate equals the decomposition rate at steady state, Using our value of O-3ppmv N,O in the atmosphere, their estimated rate gives an atmospheric rehbxation time of 1300 years. On the other hand, GOODYand WALSHAW(1953) estimated the atmospherio decomposition rate directly, and their value yields an atmospheric time constant of about 2-5 years. The Goody-Walahaw value was calculated assuming the atmosphere to be rapidly mixed, with ~onsta~ltN,O concentration, up to 40 km, so that the photochemical decomposition rate (which increases strongly with altitude) has its mean
954
H. CRAIGand L. I. GORDON
value at about 18 km. If we separate the stratosphere and troposphere into two well-mixed reservoirs, with time constants relative to mixing of about 5 and 20 years respectively, the tropospheric time constant, relative to photochemical destruction, is about 15 years, and the total mean tropospheric lifetime of N,O is about 8.5 years. With our assumption, the stratospheric concentration will be less than 0.1 ppmv, if photochemical production is unimportant. With either the Goody-Walshaw model or ours, the tropospheric time constant will be only a few years, rather than the very long time given by HARTECK and DONDES (1954), and variations over land and sea, and between hemispheres, should be observable. Our data indicate that Antarctic water and deep water of the S. Pacific obtain N,O from air with about 0.15 ppmv NzO, and that variations of a factor of 2 or 3 occur in the atmosphere of the world, with higher concentrations in the northern hemisphere. These data therefore support, a very short atmospheric time constant of the order of a few years, comparable with cross-hemispheric mixing times estimated by various workers. They further indicate that biological production in soils is very likely the predominant source of atmospheric N,O, probably two orders of magnitude more important than atmospheric production. The N,O concentration in Group III waters is significantly higher than in Antarctic and deep S. Pacific waters. If the S. Pacific atmospheric concentration is indeed about 0.15 ppmv as indicated, the higher value in Group III may reflect actual production of N,O by marine bacteria during nitrification, or nitrate reduction and denitrification, in equatorial and temperate zone surface and intermediate waters. ARKOLD (1954) has found evidence for both mechanisms in soils. This problem merits further investigation because of possible implications for the nitrogen cycle in the oceans. Using our mean value of O-2 ppmv in the oceans, and assuming 0.3 ppmv as an atmospheric average, the N,O world inventory, in moles/cm2 of earth surface, is 2.2 x 1OV in the oceans, and 10.6 x 1O-6 in the atmosphere, so that about 17% of the total N,O is in the sea. Conclusions In this paper we report (a) development of an unequivocal method for analysis of N,O, (b) identification of this gas as a constituent of sea water, and (c) indications from the sea water data, and two atmospheric data of rather poor precision, that the atmospheric concentrations are lower over the sea, and in the S. hemisphere, t,han over N. America. Some consequences of these findings for the geochemistry of N,O, and for oceanography, have been pointed out. We emphasize, however, that this paper is by no means intended to be a complete geochemical study, but is a report on the method and the identification in sea water. With present sensitivity, it is necessary to extract the gases from about 10 1. of sea water in order to make a good measurement. It is clear that further understanding of the geochemistry and oceanography of N,O requires a further development in sensitivity by about an order of magnitude, before detailed geochemical research is undertaken. We are continuing our work on this method. Finally, we wish to point out that the unequivocal demonstration that N,O is
Nitrous
oxide
in the ocean and the marine
atmosphere
955
produced predominantly in soils would be of great importance for extra-terrestrial “geochemistry,” as the presence or absence of this gas in the spectra of planetary atmospheres may then yield important evidence on the presence of life and condensed moisture. It seems probable to us that the relative% importance of photochemical and bacterial production on the earth can indeed be determined by geochemical studies, so that such applications to extra-terrestrial studies can be made. AcknowZedgementC?-The atmospheric samples were collected and analysed for CO, by T. HARRIS and L. WATERMAN; we are grateful to them and to C. D. KEELING for making them available. We wish to thank NORMAN ANDERSON for the hydrographic work, and FRED DIXON for general assistance, aboard the ARGO during Expedition Monsoon. This research was supported by the Office of Naval Research and the National Science Foundation. REFERENCES AI)EL A. (1951) Science 113,624. ARNOLD P. W. (1954) J. Soil Sci. 5, 116. GOODY R. M. and WALSHAW C. D. (1953) Quart. J. R. Met. Sot. 79, 496. HARTECK P. and DONDES S. (1954) I%ys. Rev. 95, 320. HIJTCHINSON G. E. (1953) The Earth as a Planet (ed. G. P. Kuiper) (University Chicago, 1953), Chap. 8, p. 402. MARKHART A. E. and KOBE K. A. (1941) J. Amer. CiLem. Sot. 63, 449. SLOBOD R. L. and KROCH M. E. (1950) J. Amer. Chem. Sot. 72, 1175.
of Chicago Press,
Nitrous oxide in the ocean and the marine atmosphere H. CRAIG and L. I. GORDON Department of Earth Sciences and Scripps Institution University
of California, (Received
of Oceanography La Jolla, California
20 Mcw&
1963)
Abgtr&--Nitrous oxide has been found in surface and deep ocetm waters of the S. Pacific. The observed concentrations are lower than expected for solubility equilibrium with concentraLower atmospheric concentrations tions previously reported in the continent&l &tmosphere. than previous continental data have been found in N. Pacific air. These variations indicate that X,0 should be an important tracer gas for atmospheric studies.
THE presence of trace amounts of N20 in the atmosphere has been established from studies of the solar spectrum (ADEL, 1951 and references therein); the most recent data indicate a concentration of the order of 0.5 ppm by volume (ppmv), if one assumes the gas to be uniformly distributed with altitude. The direct analytical determination of N20 is very difficult because N20, being isosteric with CO,, has almost identical physical properties including vapor pressure, and, in addition, the two gases have identical molecular masses of 44, 45, and 46. Thus N20 cannot be separated from CO2 by fractional distillation, is not readily distinguishable from CO2 in the mass spectrometer, and its almost total lack of chemical activity precludes a direct chemical analysis. SLOBOD and KROGH (1950) analysed concentrates from fractional condensation of air in liquid nitrogen, using the mass spectrometer after treating the gases with solid reagents to remove the CO,. Working with residual gas samples of about 3 microliters (1) STP they found a mean value of O-5 ppmv for the N20 concentration in nine air samples from Texas and one from Wyoming. They give no details of their observed peak ratios, and, as they point out, their condensation traps were not constructed to prevent loss of solid particles entrained in the air stream; thus it is not possible to evaluate the accuracy of their measurements. The little information available on the geochemistry of N20 has been summarized by HUTCHINSOW(1950) and a detailed study of N20 production rates in soils has been made by ARNOLD (1954). N20 is certainly produced in soils by bacterial activity; an additional production may take place in the atmosphere by the reaction of nitrogen and ozone, and the gas is destroyed in the atmosphere by photochemical dissociation and/or reaction with oxygen atoms. If the production of N,O in soils is the dominant source of this gas in the atmosphere, N,O should be a very important gas for atmospheric studies, as the source function should show a pronounced peak in the continental latitudes of the northern hemisphere and a smaller peak at low latitudes in the southern hemisphere. Thus if the time constant, relat,ive to photochemical destruction, is comparable to cross-hemispheric mixing times, the southern hemisphere air may contain much less N,O than the northern because of its smaller land area. Moreover, the southern oceans would be expected to contain smaller amou~lts of the gas than surface waters of the northern oceans, so that the gas may be a good tracer for the oceans.
H. CRAIGand L.I.
950
GORDON
It is evident that the geochemical cycle of N,O is of interest in many connections if accurate analytical work can be done. We have developed a gas chromatographic method for the separation of N,O and CO, and the quantitative measurement of N,O in gas samples. Using this method, we have established the presence of N,O in surface and deep ocean waters, and in marine air. Sea water $~~~~~~ Dissolved gases were extracted from sea water sa,mples using an a’ll-glass high vacuum line operated aboard ship. The samples were collected and processed on 70’5 0
"
20
30
40 1
50
60 till1
I
l,l'l'-
0
0
IO
0 0
I
0 0 0
0
I
I
0 0
0
0 ---
0
5-
0
0 LATITUDE
tSOUTHl
Fig. 1. Location, depth, and group combinations, of South Pacific samples analysed for N,O. Station longitudes: at 64-2”s. 166%‘; at 60*2”S, 171.5”W; at 59.3’5, 169.5”W; between 20 and 4O”S, a direct track from 38.5%, 164”W, to 24.5”5,15~j~W; at 4-4 to 5.?“S, 149.5”W. Surface samples shown were taken at 10 m.
the research vessel ARGO by the senior author, during the Scripps Institution’s Expedition Monsoon, from t,he Antarctic circle to San Diego (January-April, 1961). Two-liter water samples were collected in plastic Van Dorn hydrographic bottles, The and about 1250 ml of each sample were let into the vacuum line for extraction. filled sample bottles were suspended on gimbals to prevent agitation, and each sample was processed within 15 minutes after arriving at the surface. The sea water was acidified in vacuum with 2 ml of concentrated H,PO4, boiled, stirred, and pumped with a Toepler pump which collected the gases in a 300 ml sample tube closed by a silicone-greased stopcock and stored with several cm of mercury above the bore. Laboratory tests established that the procedure collected more than 99.95Oh of nitrogen and oxygen, and of CO,, which comes off more slowly because of its chemistry; thus it was certain that all the N,O was collected, Water
Nitrous oxide in t;he ocean and the marine atmosphere
951
volumes were measured volumetrically to about 0.20/o. Total processing time for each sample was about 1 hour. Figure 1 shows the location and depth of the 2% samples, all in the South Pacific, in which N,O was measured. In order to obtain a sufficient quantity of gas to analyse, the CO,--N,O fractions were combined into three groups as shown in the figure; thus only three analyses could be made. In the laboratory? each gas sample was dried over CO, slush traps, two 10% fractions were split off for other analyses, and the remaining gas was cycled through a liquid N, trap to remove the CO,-N,O fraction. These fractions were then combined into the three groups shown in Fig. 1, so that each group sample contained on the order of 500 ml of CO,, and (as it turned out) about 2A of N,O. Each of the group samples was condensed into a flask containing NaOH on solid suspension (Ascarite), where the CO, was removed by reaction. The residual gas was dried in a slush trap, condensed into a breakoffski, a,nd transferred to the gas Blank tests were run with 500 ml of pure CO, prepared from ~hromatograph. reagent CaCO, acidified in vacuum with H,PO,, and with no gas; these showed that no detectable N,O was produced by this treatment. The samples were analysed on a Beckman GC-1 gas chromatograph using an eight-foot silica gel column (28-200 mesh) at room temperature, with He carrier gas at a pressure of 40 pounds and a flow rate of 83 ml/min. The thermal conductivity filament was operated at 350 mA for maximum sensitivity, and the instrument was packed in fibre glass for temperature stability of the detector. Samples were condensed into a U-tube which could be switched into the helium flow. Peak areas were read with a mechanical integrator unit. The N,O sensitivity, found by calibration with 33 samples of pure gas ranging from 3.6 to 27 iz (NTP), was 2.6 5 O-2 counts/& taking one full sweep of the integrator pen as 10 units. Calibration was done by partitioning gas volumes in calibrated glassware, and the indicated error represents the limits of the observed scatter. The counting rate was linearly proportional to sample volume with a zero intercept at zero volume. RetBention times and resolution of the two gases, at the 2% 2 sample level, are shown in Fig. 2; complete resolution was obtained at these volumes and no other peaks were ever observed in our experiments. The lower part of Fig. 2 shows the actual recorder trace for Group I after CO, removal. The small CO, peak represents a residual CO, volume of 0.4 L, showing that the NaOH treatment reduced the CO, concentration by 106. The CO, peaks in the other groups were even smaller. The results obtained for the three groups of sea water samples are given in TabIe 1. The data are subject to an uncertainty of about 125% due to random integrator counting errors from instrumental drift, and an additional 18% absolute uncertainty from calibration errors. The mean value for the N20 content of sea water is 0.2 ppmv (NTP) from the present data, with an uncertainty probably less than 30%. ,Warine air samples Four 5-1. flasks of marine air from the North Pacific were available for analysis_ A small amount of the air had been taken for CO, analysis previously, so that only the ratio N,O/CO, could be measured. The samples were passed through a liquid
H.
952
CRAIG
and L.1,
GORDON
N, trap for removal of the CO,-N,O fractions, and the fractions were then combined into two samples, each representing 10 1. of air from the two locations shown in Table 1. The trap had a fritted glass filter in the outlet arm, and experiments had established that it removed all condensible gas from the air. However, as the Aasks were pumped out, the N,O partial pressure in the air stream decreased until at some
5--
I
LAB
MIXTURE
hip
O-
I\
Ih
I
I
I
I
CO,
I
I
5-
MONSOON GROUP
I
Ih 4
v
SEA
WATER
I AFTER
CO*
NoOH
SAMPLES
TREATMENT
I
I
I
I
I
I
I
I
I
I
I
I
20
22
24
26
28
30
32
34
36
38
40
42
AFTER
IhiJECTlON
TIME
(MINUTESI
Fig. 2. Gas chromatograms. Top: synthetic mixture, 281 each of X,0 and CO,, showing
resolution and retention ocean samples after
times.
Uottom:
chrometogram
from Group I of
NaOH treatment for removal of CO,.
point it became equal to its vapor pressure over Iiquid N,; consideration of the vapor pressure equation for N,O indicates that not more than 5% of the gas could have been lost by this effect. The NzO-CO2 peaks were measured directly on the condensed fractions. Calibration for CO, at the 3 ml level gave a counting rate of 2.51 counts/A (NTP). Some loss of precision was experienced due to forward tailing of the much larger CO, peak, so that about 40% of the NeO peak had to be estimated. The CO2 concentration in the original air samples was 317 ppmv, as measured by C. D. KEELING with the infra-red analyser; the resulting figures for N,O concentration
Nitrous oxide in the ocean and the marine atmosphere
953
are shown in Table 1. An additional uncertainty of about 10% affects the atmospheric data because of partial loss of the peak; the digerence in the two figures is not signi~~ant, and the mean value of 0.3 ppmv is probably accnrate to -&3+f. Table 1, N,O analyses of sea water and marine air. Estimated accuracy is -&25?; random instrumental errors, and an additional &8% uncertainty in absolute amounts due to calibration errors No. of
samples
Mean T”C.
Total N._,O fomd (A, KTI’)
Group I
9
3.5
L-73
0.18
Group II
9
1.3
I.56
0.16
Group III
10
9.7
2-77
0.27
MARINEAIR 28”N 122”M7 ~4~15~61)
Total CO,
(ml NTP)
Total N,O
(n NTP)
%0/C% t %A
N,O/A.ir(ppmv) -
.--
2.96
2-4
O*RO
@45
S48
3.1
1.00
0,32
32%5”N, 118QYW (4/17P)
In Tabie 1 we show the weighted mean temperatures for the three groups of sea water samples. The atmospheric concentrations of N,O required for equilibrium with the observed sea water concentrations can be calculated from data on the solubilit8y of NsO in NaCl solutions (MARKHAMaud KOBE, 1941), assuming sea water to be au 0.97 l% NaCI solution. The atmospheric concentrations required for equilibrium with Groups I, II, and III, are respectively O-15, O-12, and 0.30, at the mean water temperatures of these groups. These values are considerably lower than the N. American data of SLOBOD and KROGH(1950), and for Groups I and 11 the required values are even lower than our data on North Pacific marine air. The time constant for N,O destruction in the atmosphere has been estimated in and DONDES(1954) assumed a steady state relative to productwo ways. HAEETECK tion from N, and OS, and calculated the number of N20 molecules produced in photo~hemi~al equilibrium with 0, and 0. They assumed biological production was insignificant, so that the photochemioal production rate equals the decomposition rate at steady state, Using our value of O-3ppmv N,O in the atmosphere, their estimated rate gives an atmospheric rehbxation time of 1300 years. On the other hand, GOODYand WALSHAW(1953) estimated the atmospherio decomposition rate directly, and their value yields an atmospheric time constant of about 2-5 years. The Goody-Walahaw value was calculated assuming the atmosphere to be rapidly mixed, with ~onsta~ltN,O concentration, up to 40 km, so that the photochemical decomposition rate (which increases strongly with altitude) has its mean
954
H. CRAIGand L. I. GORDON
value at about 18 km. If we separate the stratosphere and troposphere into two well-mixed reservoirs, with time constants relative to mixing of about 5 and 20 years respectively, the tropospheric time constant, relative to photochemical destruction, is about 15 years, and the total mean tropospheric lifetime of N,O is about 8.5 years. With our assumption, the stratospheric concentration will be less than 0.1 ppmv, if photochemical production is unimportant. With either the Goody-Walshaw model or ours, the tropospheric time constant will be only a few years, rather than the very long time given by HARTECK and DONDES (1954), and variations over land and sea, and between hemispheres, should be observable. Our data indicate that Antarctic water and deep water of the S. Pacific obtain N,O from air with about 0.15 ppmv NzO, and that variations of a factor of 2 or 3 occur in the atmosphere of the world, with higher concentrations in the northern hemisphere. These data therefore support, a very short atmospheric time constant of the order of a few years, comparable with cross-hemispheric mixing times estimated by various workers. They further indicate that biological production in soils is very likely the predominant source of atmospheric N,O, probably two orders of magnitude more important than atmospheric production. The N,O concentration in Group III waters is significantly higher than in Antarctic and deep S. Pacific waters. If the S. Pacific atmospheric concentration is indeed about 0.15 ppmv as indicated, the higher value in Group III may reflect actual production of N,O by marine bacteria during nitrification, or nitrate reduction and denitrification, in equatorial and temperate zone surface and intermediate waters. ARKOLD (1954) has found evidence for both mechanisms in soils. This problem merits further investigation because of possible implications for the nitrogen cycle in the oceans. Using our mean value of O-2 ppmv in the oceans, and assuming 0.3 ppmv as an atmospheric average, the N,O world inventory, in moles/cm2 of earth surface, is 2.2 x 1OV in the oceans, and 10.6 x 1O-6 in the atmosphere, so that about 17% of the total N,O is in the sea. Conclusions In this paper we report (a) development of an unequivocal method for analysis of N,O, (b) identification of this gas as a constituent of sea water, and (c) indications from the sea water data, and two atmospheric data of rather poor precision, that the atmospheric concentrations are lower over the sea, and in the S. hemisphere, t,han over N. America. Some consequences of these findings for the geochemistry of N,O, and for oceanography, have been pointed out. We emphasize, however, that this paper is by no means intended to be a complete geochemical study, but is a report on the method and the identification in sea water. With present sensitivity, it is necessary to extract the gases from about 10 1. of sea water in order to make a good measurement. It is clear that further understanding of the geochemistry and oceanography of N,O requires a further development in sensitivity by about an order of magnitude, before detailed geochemical research is undertaken. We are continuing our work on this method. Finally, we wish to point out that the unequivocal demonstration that N,O is
Nitrous
oxide
in the ocean and the marine
atmosphere
955
produced predominantly in soils would be of great importance for extra-terrestrial “geochemistry,” as the presence or absence of this gas in the spectra of planetary atmospheres may then yield important evidence on the presence of life and condensed moisture. It seems probable to us that the relative% importance of photochemical and bacterial production on the earth can indeed be determined by geochemical studies, so that such applications to extra-terrestrial studies can be made. AcknowZedgementC?-The atmospheric samples were collected and analysed for CO, by T. HARRIS and L. WATERMAN; we are grateful to them and to C. D. KEELING for making them available. We wish to thank NORMAN ANDERSON for the hydrographic work, and FRED DIXON for general assistance, aboard the ARGO during Expedition Monsoon. This research was supported by the Office of Naval Research and the National Science Foundation. REFERENCES AI)EL A. (1951) Science 113,624. ARNOLD P. W. (1954) J. Soil Sci. 5, 116. GOODY R. M. and WALSHAW C. D. (1953) Quart. J. R. Met. Sot. 79, 496. HARTECK P. and DONDES S. (1954) I%ys. Rev. 95, 320. HIJTCHINSON G. E. (1953) The Earth as a Planet (ed. G. P. Kuiper) (University Chicago, 1953), Chap. 8, p. 402. MARKHART A. E. and KOBE K. A. (1941) J. Amer. CiLem. Sot. 63, 449. SLOBOD R. L. and KROCH M. E. (1950) J. Amer. Chem. Sot. 72, 1175.
of Chicago Press,